AP CHEM FINAL
7.7 (d) Explain how the slopes of the curves at equilibrium can be used to show that the rates of the forward and reverse reactions are the same at equilibrium.
The response indicates that the curves are horizontal (slope=0) at equilibrium, so the concentrations of the reactants and product are not changing, therefore the rate of the forward reaction must be the same as the rate of the reverse reaction.
The decomposition of NaCl(l) into Na(l) and Cl2(g) is thermodynamically unfavorable. The decomposition requires the input of energy from an external source. The diagram represents an electrolytic cell that can be used to drive the decomposition reaction. Which of the following identifies a flaw in the representation?
An external source of energy is not shown.
CH4(g)+2O2(g)⇄CO2(g)+2H2O(g) ΔH°rxn=−803kJ/molrxn ΔS°rxn=−5J/(molrxn⋅K) The chemical equation above represents the exothermic reaction of CH4(g) with O2(g). Which of the following best helps to explain why the reaction is thermodynamically favored (ΔG<0) at 2000K and 1atm?
The amount of energy released when the product bonds form is much greater than the amount of energy needed to break the reactant bonds.
(a) Write the net-ionic equation for the overall reaction that takes place as the cell operates.
Cu2++Zn→Cu+Zn2+
(b) Calculate the value of E° for the cell.
E∘cell=0.34 V−(−0.76 V)=1.10 V
(e) Assume that the mass of the Cu electrode changes by x grams in a certain period of time. Write a mathematical expression for the change in mass of the Zn electrode during the same period of time.
x g of Cu×1 mol Cu63.55 g Cu×1 mol Zn1 mol Cu×65.38 g Zn1 mol Zn=g of Zn Change in mass of Zn electrode =−65.3863.55x g
H(g)+Cl(g)→HCl(g) The formation of HCl(g) from its atoms is represented by the equation above. Which of the following best explains why the reaction is thermodynamically favored?
ΔG<0 because although entropy decreases because the number of gaseous product particles is less than the number of gaseous reactant particles, energy is released as the bond between the H and Cl atoms forms.
6.6 2H2S(g)+3O2(g)→2H2O(l)+2SO2(g)ΔH°=−1120kJ/molrxn Based on the reaction represented by the chemical equation shown above, what is the amount of heat released when 4.00mol of H2S(g) reacts with 9.00mol of O2(g)? A −560kJ B −1120kJ C −2240kJ D −3360kJ
C −2240kJ
6.8 The enthalpy change for the reaction 2Al(s)+Fe2O3(s)→2Fe(s)+Al2O3(s)2Al(s)+Fe2O3(s)→2Fe(s)+Al2O3(s) is −860kJ/mol−860kJ/mol. Based on the standard enthalpies of formation ΔH∘fΔHf° provided in the table, what is the approximate ΔH∘fΔHf° for Fe2O3(s)Fe2O3(s) ? A +2540kJ/mol+2540kJ/mol B −2540kJ/mol−2540kJ/mol C +820kJ/mol+820kJ/mol D −820kJ/mol−820kJ/mol
C
6.2 Manganese dioxide, MnO2(s)MnO2(s), is an insoluble substance that acts as a catalyst for the decomposition reaction. On the diagram above, draw a curve to represent the reaction as it occurs in the presence of MnO2(s)MnO2(s).
Catalyst drawn lower then original curve
7.11 Ba(IO3)2(s) ⇄ Ba2+(aq) + 2 IO3−(aq) Ksp=4×10−9 According to the information about the dissolution of Ba(IO3)2(s) shown above, the correct value of S, the molar solubility of Ba(IO3)2(s), can be calculated using the following mathematical relationships?
D 4S3=4×10−9 M
7.14 The table above lists the equilibrium constants and changes in thermodynamic properties for the dissolution of FeCO3 and MnCO3 at 25°C. The two particle diagrams below represent saturated solutions of each compound at equilibrium. Which of the following explains which of the properties listed in the table is best represented by the particle diagram?
D The particle diagrams best represent that the molar solubility is greater for FeCO3 compared to MnCO3.
The particle diagrams above represent a change in physical state that occurred after heating two separate solid samples of a diatomic element. Which of the following best compares the relative magnitude of ΔS° and gives the sign for the entropy change undergone by each sample, and why?
The entropy values for both samples are positive and ΔS°sample2>ΔS°sample1 , because the change in the spatial distribution of the molecules was greater for sample 2 than for sample 1.
7.12 AgCl(s) ⇄ Ag+(aq) + Cl−(aq) Ksp=1.8×10−10 Shown above is information about the dissolution of AgCl(s) in water at 298K. In a chemistry lab a student wants to determine the value of s, the molar solubility of AgCl, by measuring [Ag+] in a saturated solution prepared by mixing excess AgCl and distilled water. How would the results of the experiment be altered if the student mixed excess AgCl with tap water (in which [Cl−]=0.010M) instead of distilled water and the student did not account for the Cl− in the tap water?
B The value obtained for Ksp would be too small because less AgCl(s) would dissolve because of the common ion effect due to the Cl−(aq) already in the water.
7.14 At 298K, NH4NO3 readily dissolves in water, suggesting that the change in free energy (ΔG) favors the dissolution process. However, when NH4NO3 dissolves in water, the temperature of the water decreases. The particulate diagram above attempts to provide a microscopic view of the dissolution of NH4NO3(s) considering both the change in enthalpy (ΔH) and the change in entropy (ΔS). Which of the following explains what the particle diagram is able to illustrate and why?
C The particle diagram is able to illustrate that entropy increases when NH4NO3(s) dissolves in water because the ions disperse in solution.
7.13 Solution pH [Ag+]eq 7.00 1.41×10−4 8.00 1.41×10−4 9.00 1.37×10−4 10.00 1.00×10−4 11.00 1.96×10−5 12.00 2.00×10−6 13.00 2.00×10−7 A student investigates the effects of pH on the solubility of AgOH(s), which dissolves in water according to the equation AgOH(s) ⇄ Ag+(aq) + OH−(aq). The value for Ksp for AgOH is 2.0×10−8 at 298K. The student places the same mass of AgOH(s) into 50.0mL of different solutions with specific pH values and measures the concentration of Ag+ ions in each solution after equilibrium is reached. Based on the data in the table, what can be concluded about the solubility of AgOH?
D AgOH is less soluble at higher pH because higher concentrations of OH−(aq) shift the solubility equilibrium toward solid AgOH .
7.10 2NO(g)+Cl2(g)⇄2NOCl(g) Kc=2000 A mixture of NO(g) and Cl2(g) is placed in a previously evacuated container and allowed to reach equilibrium according to the chemical equation shown above. When the system reaches equilibrium, the reactants and products have the concentrations listed in the following table. Species Concentration (M) NO(g) 0.050 Cl2(g) 0.050 NOCl(g) 0.50 Which of the following is true if the volume of the container is decreased by one half?
D Q=1000 , and the reaction will proceed toward products.
7.12 CuBr(s) ⇄ Cu+(aq) + Br−(aq) Shown above is the chemical equation for the dissolution of the slightly soluble salt CuBr(s). Its Ksp value in pure water was experimentally determined. CuBr was found to be much less soluble in a 0.001MNaBr solution than in pure water. Which of the following correctly explains the decrease in solubility of CuBr in 0.001 M NaBr?
D The presence of additional Br− ions already in the solution means equilibrium will be reached when much less CuBr has dissolved.
The table above provides the half-reactions and standard reduction potentials needed to determine if an Al-air fuel cell can be constructed. Which of the following equations best supports the possibility of obtaining electrical energy from such a cell?
E°cell=[+0.40−(−2.31)]V and ΔG°rxn=−(12×96,500×2.71)/1,000kJ
The diagram above shows the apparatus used for the electrolysis of CuCl2(aq) according to the chemical equation given. Which of the following mathematical expressions can be used to calculate the current required to produce 0.125mol of Cu(s) in 1 hour?
I=(2×0.125×96,485/3,600)A
The reaction in which H2O(l) is decomposed into H2(g) and O2(g) is thermodynamically unfavorable (ΔG°>0). However, an electrolytic cell, such as the one represented above, can be used to make the reaction occur. Which of the following identifies a flaw in the representation?
The equation for the reaction is not correctly balanced.
(a) Using the information in the table above, calculate the value of ΔS° for the reaction.
The response gives the both the following set up and calculation: ΔS∘=ΣS∘ products−ΣS∘ reactants ΔS∘=2(240)−[2(211)+205]=−147 J/(K molrxn)
7.3 CaCO3(s)⇄CaO(s)+CO2(g) When heated strongly, solid calcium carbonate decomposes to produce solid calcium oxide and carbon dioxide gas, as represented by the equation above. A 2.0mol sample of CaCO3(s) is placed in a rigid 100.L reaction vessel from which all the air has been evacuated. The vessel is heated to 898°C at which time the pressure of CO2(g) in the vessel is constant at 1.00atm, while some CaCO3(s) remains in the vessel. Write the expression for Kp, the equilibrium constant for the reaction, and determine its value at 898°C .
The response includes both of the following two criteria: Kp=PCO2 Kp=1.00 atm
The following chemical equation represents the reaction that occurs when methylamine dissolves in water to form a basic solution. CH3NH2(aq)+H2O(l)→CH3NH3+(aq)+OH−(aq)CH3NH2(aq)+H2O(l)→CH3NH3+(aq)+OH−(aq) (b) The pHpH of 2.65MCH3NH2(aq)2.65MCH3NH2(aq) is 12.54. Determine the value of KbKb for methylamine.
The response meets all of the following three criteria: the correct calculation of [OH−] : pH = 12.54 ⇒ pOH = (14.00−12.54) = 1.46 ⇒ [OH−] = 10−1.46 = 3.47×10−2 M The use of the correct expression for Kb : As methylamine reacts with water, [CH3NH3+] = [OH−] . Let x represent [OH−] , then Kb = [CH3NH3+][OH−][CH3NH2] = x22.65−x ≈ x22.65 because x<<2.65 . The correct substitution of the numbers and calculation of the value: Substituting the value of [OH−] for x , Kb = (3.47×10−2)22.65 = 4.54×10− 4
8.5 Using the symbols in the legend above, draw particles in the following beaker to represent the relative amounts of the two species, CH3NH2CH3NH2 and CH3NH+3CH3NH3+, in the solution after the first 5.00mL5.00mL of titrant had been added to the CH3NH2(aq)CH3NH2(aq) solution.
The response shows more CH3NH2 particles than CH3NH3+ particles.
Fe2+(aq)+Zn(s)→Fe(s)+Zn2+(aq)E°cell=+0.32V A galvanic cell generates a cell potential of +0.32V when operated under standard conditions according to the reaction above. Which of the following pairs of conditions are needed to construct a similar cell that generates the lowest cell potential?
[Zn2+]=2M and [Fe2+]=0.5M because Q>1 .
6.4 A student mixes 50mL50mL of 1.0MHCl1.0MHCl and 50mL50mL of 1.0MNaOH1.0MNaOH in a coffee-cup calorimeter and observes the change in temperature until the mixture reaches thermal equilibrium. The initial and final temperatures (°C)(°C) of the mixture are shown in the diagram above of the laboratory setup. Based on the results, what is the change in temperature reported with the correct number of significant figures? A 5.5°C B 5.50°C C 5.800°C D 6°C
A 5.5°C
6.9 Reaction 1: N2O4(g)→2NO2(g)N2O4(g)→2NO2(g)ΔH1=+57.9kJ Reaction 2: 2NO(g)+O2(g)→2NO2(g)2NO(g)+O2(g)→2NO2(g)ΔH2=−113.1kJ Based on the information for two different reactions given above, which of the following gives the quantities needed to calculate the enthalpy change for the reaction represented by the overall equation below? 2NO(g)+O2(g)→N2O4(g)2NO(g)+O2(g)→N2O4(g) A ΔH1+ΔH2ΔH1+ΔH2 B ΔH1+(−ΔH2)ΔH1+(−ΔH2) C (−ΔH1)+ΔH2(−ΔH1)+ΔH2 D ΔH1+(2×ΔH2)
C
The diagram above shows the experimental setup used to electroplate a small object. A current of 0.80 ampere is supplied to AgNO3(aq) for 30.0 minutes. Which of the following equations can be used to calculate the approximate mass of Ag(s) deposited on the object?
Mass=(0.80×30.0×60×107.996,485)g
8.1 In pure water, some of the molecules ionize according to the equation H2O→H++OH−. The extent of the ionization increases with temperature. The graph above shows the pH of pure water at different temperatures. Which of the following represents the variations in the pOH of pure water under the same conditions? A B C D
A
8.3 HCN(aq)+H2O(l)⇄H3O+(aq)+CN−(aq)Ka=[H3O+][CN−][HCN]=6.2×10−10 The equilibrium reaction shown above represents the partial ionization of the weak acid HCN(aq). A 0.200MHCN(aq) solution has a pH≈4.95. If 0.05g (0.010mol) of NaCN(s) is added to 100mL of 0.200MHCN(aq), which of the following explains how and why the pH of the solution changes? A The pHpH will be higher than 4.95 because adding CN−CN− will disrupt the equilibrium, resulting in an increased production of HCNHCN that decreases the concentration of H3O+H3O+. B The pHpH will be lower than 4.95 because adding CN−CN− will disrupt the equilibrium, resulting in an increased production of HCNHCN that decreases the concentration of H3O+H3O+. C The pHpH will be higher than 4.95 because CN−CN− is a strong base that can neutralize HCNHCN. D The pHpH will remain close to 4.95 because the KaKa is so small that hardly any products form.
A
8.6 The table above includes the Lewis diagrams of the conjugate acids of some weak nitrogenous bases. Based on the relative stability of the conjugate acids, which of the following bases has the largest value for Kb ? A CH3NH2 B NH2NH2 C HONH2 D ClNH2
A
8.7 A student pours a 10.0mL sample of a solution containing HC2H3O2(pKa=4.8) and NaC2H3O2 into a test tube. The student adds a few drops of bromocresol green to the test tube and observes a yellow color, which indicates that the pH of the solution is less than 3.8. Based on this result, which of the following is true about the relative concentrations of HC2H3O2 and NaC2H3O2 in the original solution? A [HC2H3O2]>[NaC2H3O2] B [HC2H3O2]=[NaC2H3O2] C [HC2H3O2]<[NaC2H3O2] D The relative concentrations cannot be determined without knowing the value of pKb for C2H3O2−.
A
A stoichiometric mixture of CO(g) and H2(g) was allowed to react in two different 2.0L rigid containers at a constant temperature of 298K. The reaction is represented by the equation above. Diagram 1 represents the uncatalyzed reaction and diagram 2 represents the catalyzed reaction one hour after the reactants were mixed. Which of the following correctly explains the experimental results represented in the particle diagrams?
Although the reaction is thermodynamically favorable because ΔG°<0 based on the value of K , only the catalyzed reaction could proceed in one hour because it has a lower activation-energy reaction pathway.
8.5 Explain why the titration curve shows only a small change in pH per volume of acid added when the total amount of acid added is about 14.0mL . Include a balanced chemical equation as part of your answer.
An explanation that, near the half equivalence point, the concentrations of the base and of its conjugate acid are approximately equal (the buffer zone). The effect on pH of adding more acid to this system is partially counteracted because the added acid reacts with the base in the solution. The balanced equation H3O+ + CH3NH2 → H2O + CH3NH3+
6.6 HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l)ΔH°=−57.1kJ/molrxn The chemical equation above represents the reaction between HCl(aq) and NaOH(aq). When equal volumes of 1.00MHCl(aq) and 1.00MNaOH(aq) are mixed, 57.1kJ of heat is released. If the experiment is repeated with 2.00MHCl(aq), how much heat would be released? A 28.6kJ28.6kJ B 57.1kJ57.1kJ C 85.7kJ85.7kJ D 114kJ114kJ
B
6.8 C(s)+H2O(g)→CO(g)+H2(g)ΔH°=+131kJ/molrxnC(s)+H2O(g)→CO(g)+H2(g)ΔH°=+131kJ/molrxn The reaction between C(s)C(s) and H2O(g)H2O(g) is represented by the balanced chemical equation above. Based on the enthalpy change of the reaction (ΔH°ΔH°) and the standard heats of formation (ΔH°fΔHf°) given in the table below, what is the approximate ΔH°fΔHf° for CO(g)CO(g) ? Substance ΔH° (kJ/mol) C(s) 0 H2O(g) −242 CO(g) ?? H2(g) 0 A −373kJ/mol−373kJ/mol B −111kJ/mol−111kJ/mol C +111kJ/mol+111kJ/mol D +373kJ/mol
B
6.4 Mass of water 50.003g Temperature of water 24.95°C Specific heat capacity for water 4.184J/g°C Mass of metal 63.546g Temperature of metal 99.95°C Specific heat capacity for metal?? Final temperature 32.80°C In an experiment to determine the specific heat of a metal, a student transferred a sample of the metal that was heated in boiling water into room-temperature water in an insulated cup. The student recorded the temperature of the water after thermal equilibrium was reached. The data are shown in the table above. Based on the data, what is the calculated heat qq absorbed by the water reported with the appropriate number of significant figures? A 1600J1600J B 1640J1640J C 1642J1642J D 1642.3J
B 1640J1640J
8.10 HF(aq)+H2O(l)⇄H3O+(aq)+F−(aq) The equation above represents the acid ionization equilibrium for HF. To prepare a buffer with pH≈3.50, 4.20g of NaF(s) should be added to 500.0mL of 0.100MHF(aq). The buffer is accidentally prepared using 90% pure NaF(s) instead of 99% pure NaF(s). Assume that the impurities in the NaF(s) samples are inert. Which of the following explains how the error affects the pH and capacity of the buffer? A The pH is slightly higher than 3.50 and it has a lower capacity for the addition of acids because less than 4.20g of NaF(s) was added. B The pH is slightly higher than 3.50 and it has a higher capacity for the addition of acids because more than 4.20g of NaF(s) was added. C The pH is slightly lower than 3.50 and it has a lower capacity for the addition of acids because less than 4.20g of NaF(s) was added. D The pH is slightly lower than 3.50 and it has a higher capacity for the addition of acids because more than 4.20g of NaF(s) was added.
C
8.4 HC2HO3(aq)+H2O(l)⇄H3O+(aq)+C2HO3−(aq)pKa=3.18at25°C The equilibrium for the acid ionization of HC2HO3 is represented by the equation above. If 10.0mL of 0.20MHC2HO3 react with 5.0mL of 0.10MNaOH, which of the following could be used to calculate the correct pH of the resulting solution? A pH=pKa+log(0.10/0.20) B pH=pKa+log(0.0050×0.100.0100×0.20) C pH=pKa+log[((0.0050×0.10)0.0150)/((0.0100×0.20)−(0.0050×0.10)0.0150)] D pH=pKa+log[((0.0050×0.10)+(0.0100×0.20)0.0150)/((0.0100×0.20)−(0.0050×0.10)0.0150)]
C
7.5 HCl(aq)+H2O(l)⇄H3O+(aq)+Cl−(aq) In 1.0MHCl(aq), HCl is nearly 100 percent dissociated, as represented by the equation above. Which of the following best helps to explain why, in 0.01MHCN(aq), less than 1 percent of HCN is dissociated?
C The equilibrium constant for the dissociation of HCN(aq) is much smaller than that for the dissociation of HCl(aq).
8.2 Which of the following is the correct mathematical expression to use to calculate the pH of a 0.10M aqueous Ba(OH)2 solution at 25°C? A pH=−log(0.10)pH=−log(0.10) B pH=−log(0.20)pH=−log(0.20) C pH=14.00−log(0.10)pH=14.00−log(0.10) D pH=14.00+log(0.20)
D
8.8HF(aq)+H2O(l)⇄H3O+(aq)+F−(aq) Ka=6.3×10−4 at 25°C The acid ionization equilibrium for HF is represented by the chemical equation above. A student claims that the pH of a solution that contains 0.100MHF(aq) and 0.100MNaF(aq) will change only slightly when small amounts of acids or bases are added. Which of the following pairs of equations can the student use to justify the claim? A HF(aq)+OH−(aq)→H2O(l)+F−(aq) and OH−(aq)+H3O+(aq)→2 H2O(l) B H3O+(aq)+OH−(aq)→2 H2O(l) and F−(aq)+H3O+(aq)→HF(aq)+H2O(l) C HF(aq)+OH−(aq)→H2O(l)+F−(aq) and F−(aq)+H2O(l)→HF(aq)+OH−(aq) D HF(aq)+OH−(aq)→H2O(l)+F−(aq) and F−(aq)+H3O+(aq)→HF(aq)+H2O(l)
D
8.5 A 25.00mL sample of a CH3NH2(aq) solution of unknown concentration is titrated with 1.84MHCl(aq) . Following is a graph that shows pH versus the volume of 1.84MHCl(aq) added during the titration. If 28.25mL of 1.84MHCl(aq) was required to reach the equivalence point, calculate the concentration of the CH3NH2(aq) solution of unknown concentration.
The correct calculation of moles of HCl(aq) added: 0.02825 L HCl(aq) solution×1.84 mol HCl(aq)1.000 L solution = 0.05198 mol HCl(aq) The correct calculation of the concentration of the CH3NH2(aq) solution: 0.05198 mol HCl(aq) for equivalence ⇒ 0.05198 mol CH3NH2 present in the sample, thus [CH3NH2]= = 0.05198 mol CH3NH20.02500 L = 2.08 M .
7.2 Explain how the slopes of the curves at equilibrium can be used to show that the rates of the forward and reverse reactions are the same at equilibrium.
The response indicates that the curves are horizontal (slope=0) at equilibrium, so the concentrations of the reactants and product are not changing, therefore the rate of the forward reaction must be the same as the rate of the reverse reaction.
7.4 (c) The experiment was repeated, but this time starting with a 4.0mol sample of CaCO3(s).On the following graph, draw a curve showing how the pressure of CO2(g) would change over time as the vessel is heated to 898°C and equilibrium is established.
The response shows a graph that starts at the origin, rises to right, and levels off (slope=0) at 1.00 atm
8.2 Which of the following statements about the pH of 0.010MHClO4 is correct? A pH=2.00pH=2.00, because [H+]=1.0×10−2M[H+]=1.0×10−2M. B pH=2.00pH=2.00, because [H+]=2.0×10−2M[H+]=2.0×10−2M. C pH>2.00pH>2.00, because HClO4HClO4 is a strong acid. D pH<2.00pH<2.00, because HClO4HClO4 is a weak acid.
A
8.4 HNO2(aq)+NH3(aq)⇄NH4+(aq)+NO2−(aq)Kc=1×106 Nitrous acid reacts with ammonia according to the balanced chemical equation shown above. If 50.mL of 0.20MHNO2(aq) and 50.mL of 0.20MNH3(aq) are mixed and allowed to reach equilibrium at 25°C, what is the approximate [NH3] at equilibrium? A 0.00010M B 0.0010M C 0.010M D 0.10M
A
8.5 A 40.0mL sample of the weak base C5H11N was titrated with 1.00MHCl at 25°C. Based on the resulting titration curve shown above, which of the following pairs provide the best estimates for the pKb and Kb of C5H11N? A pKb≈3.0 and Kb≈1×10−3 B pKb≈5.8 and Kb≈2×10−6 C pKb≈11.0 and Kb≈1×10−11 D pKb≈12.5 and Kb≈3×10−2
A
8.5 B(aq)+HCl(aq)⇄HB+(aq)+Cl−(aq) The reaction of a weak base, B, with HCl is represented by the equation above. The graph shows the titration curve for 25.0mL of an aqueous solution of B titrated with 0.100MHCl. Based on the graph, which of the following best estimates the initial concentration of the solution of the weak base? A 0.20M B 0.10M C 0.067M D 0.048M
A
7.6 Based on the equilibrium constants given above, which of the following gives the correct expression for the equilibrium constant for reaction 3?
A K3 = K2 / K1
8.5 The graph above shows the titration curve for an aqueous solution of a weak base, B, with HCl as the titrant. Based on the graph, which of the following best estimates the pKa of HB+? A 12.0 B 10.8 C 6.0 D 1.8
B
6.7 Shown above are the equation representing the decomposition of H2O2(l) and a table of bond enthalpies. On the basis of the information, which of the following is the enthalpy of decomposition of 2 mol of H2O2(l) ? A −349kJ−349kJ B −203kJ−203kJ C 203kJ203kJ D 349kJ
C
7.11 Ag2CO3(s) ⇄ 2Ag+(aq) + CO32−(aq) The chemical equation above represents the equilibrium that exists in a saturated solution of Ag2CO3. If S represents the molar solubility of Ag2CO3, which of the following mathematical expressions shows how to calculate S based on Ksp ?
D S= ^3√Ksp4
7.11 Substance Ksp Sn(OH)2 Ksp=4S3=5.45×10−27 CuCN Ksp=S2=3.47×10−20 MgF2 Ksp=4S3=5.16×10−11 NiCO3 Ksp=S2=1.42×10−7 The equilibrium constants for the dissolution (Ksp) of various substances in aqueous solution at 25°C are listed in the table above. Which of the following provides a correct comparison of the molar solubilities (S) of some of these substances based on their Ksp?
D The molar solubilities for Sn(OH)2 and MgF2 are calculated using S= √Ksp/4 and Sn(OH)2 has a lower molar solubility than NiCO3 .
7.2 A reversible reaction is represented by the equation above. The amounts of reactants and products at time 1 are shown in the particle diagram on the left. The particle diagram on the right shows the amounts of reactants and products at time 2. Based on the diagrams, what can be inferred about the relative rates of the forward and reverse reactions between time 1 and time 2?
D The rate of the forward reaction is greater than the rate of the reverse reaction.
7.9 Fe3+(aq) + SCN−(aq) ⇄ FeSCN2+(aq) Colorless Colorless Red When colorless solutions containing Fe3+(aq) ions and SCN−(aq) ions are combined, a deep-red complex ion, FeSCN2+(aq) quickly forms, as shown in the net ionic equation above. Which of the following explains the observation that adding a few additional crystals of KSCN(s) results in the red color of the solution becoming deeper?
C The added KSCN(s) dissolves, causing the reaction system to respond by producing more product to partially consume SCN−(aq) and reduce its concentration.
7.2 The particle diagram above illustrates the changes that take place when X(g) and Y(g) are placed inside a rigid container at constant temperature. Which of the following is a characteristic of a system at equilibrium that is best represented by the particle diagram?
C The particle diagram shows that after 200s there are no observable changes in the amounts of reactants and products, which is a characteristic of a system at equilibrium.
7.9 At night, an equilibrium reaction between two different nitrogen compounds generates N2O5 in the atmosphere, as represented below. Reaction 1: NO3(g)+NO2(g)⇄N2O5(g) K=2.6×10^-11 During the day, solar radiation is absorbed by NO3(g), resulting in its decomposition. Which of the following best explains whether the equilibrium concentration of N2O5(g) in the atmosphere in the daytime is different from that in the nighttime, and why?
C [N2O5] will be higher at night, because the decomposition of NO3(g) in the daytime will result in an increase in the rate of consumption of N2O5(g) to reform NO3(g) .
7.5 At night, an equilibrium reaction between two different nitrogen compounds generates N2O5 in the atmosphere, as represented below. Reaction 1: NO3(g)+NO2(g)⇄N2O5(g) K=2.6×10−11 A mixture of NO3 and a ten-fold excess of NO2 are placed inside a rigid container at constant temperature and allowed to reach equilibrium. Which of the following provides a correct comparison of the equilibrium concentrations of these chemical species, and why?
D [N2O5]<<[NO2] , because a small K value indicates that the formation of products is not favored at equilibrium.
The diagram above represents an electrolytic cell in which the reaction 2NaCl(l)→2Na(l)+Cl2(g) takes place. The table gives the relevant reduction half-reactions and the standard reduction potentials. Based on the information given, which of the following is true?
The operation of the cell requires at least 4.07V to be supplied because the reaction is not thermodynamically favorable.
6.3: A student did an experiment to determine the specific heat capacity of a metal alloy. The student put a sample of the alloy in boiling water for several minutes, then quickly transferred the alloy into a calorimeter containing water originally at 25°C25°C. The temperature of the water was monitored over time. The data are given in the graph above. (a) What is the value of ΔTΔT that the student should use to calculate the value of qq, the heat gained by the water?
The response indicates that ΔT = 1.4 C
Cu2+(aq)+Mg(s)→Cu(s)+Mg2+(aq)E°cell=+2.71V A galvanic cell generates a potential of +2.71V when [Cu2+]=[Mg2+]=1M based on the chemical reaction represented above. Which of the following provides the best justification that the given conditions can be used to decrease the cell potential (Ecell<+2.71V) ?
[Cu2+]=0.10M and [Mg2+]=1.0M , because Q is greater than 1, making Ecell decrease.
8.4 A student mixes 40.mL of 0.10MHBr(aq) with 60.mL of 0.10MKOH(aq) at 25°C. What is the [OH−] of the resulting solution? A [OH−]=0.060M[OH−]=0.060M B [OH−]=0.033M[OH−]=0.033M C [OH−]=0.020M[OH−]=0.020M D [OH−]=0.00000010M[OH−]=0.00000010M
c
The diagram above represents the gas-phase reaction of NO2 (g) to form N2O4 (g) at a certain temperature. Based on the diagram, which of the following best predicts and explains the sign of the entropy change for the reaction, ΔS°rxn ?
ΔS°rxn is negative because the number of molecules in the gas phase decreases as the reaction proceeds.
6.9 (1)2H(g)→H2(g) ΔH°1=−436kJ (2)2O(g)→O2(g)ΔH°2=−249kJ (3)2H(g)+O(g)→H2O(g) ΔH°3=−803kJ (4)H2O(g)→H2O(l) ΔH°4=−44kJ (5)H2(g)+12O2(g)→H2O(l) ΔH°f=? A 2(−ΔH°1)+(−ΔH°2)+2(ΔH°3)+2(ΔH°4)2(−ΔH°1)+(−ΔH°2)+2(ΔH°3)+2(ΔH°4) B (−ΔH°1)+12(−ΔH°2)+(ΔH°3)+(ΔH°4)(−ΔH°1)+12(−ΔH°2)+(ΔH°3)+(ΔH°4) C (−ΔH°1)+12(ΔH°2)+(ΔH°3)(−ΔH°1)+12(ΔH°2)+(ΔH°3) D (ΔH°1)+12(ΔH°2)+(ΔH°3)+(ΔH°4)
B
7.4 (b) Write the expression for Kp , the equilibrium constant for the reaction, and determine its value at 898°C .
The response includes both of the following two criteria: Kp=PCO2 Kp=1.00 atm
7.8 Another mixture of PCl3(g) , Cl2(g) , and PCl5(g) is at equilibrium at a different temperature in a different vessel. The system at equilibrium is represented by the following box on the left. Then the volume of the vessel is decreased. The box in the middle represents the system before equilibrium is reestablished at the same temperature. (f) In the box above on the right, draw an appropriate number of each type of molecule to represent a possible new equilibrium.
The response meets both of the following criteria: There are more than three PCl5 molecules and fewer than three PCl3 and fewer than three Cl2 molecules drawn in the box. There is conservation of atoms (six P atoms and thirty Cl atoms).
2Fe2O3(s)+3C(s)→4Fe(s)+3CO2(g) In a blast furnace, the reaction represented above occurs, producing Fe(s) from its ore, Fe2O3(s). The reaction is thermodynamically favorable and based on coupling the two reactions represented below. 2Fe2O3→4Fe+3O2 C+O2→CO2 Which of the following identifies a limitation in how the representations above describe a system of coupled reactions?
The values of ΔG° for each reaction are not shown.
8.6 Each particle diagram shown is a representation of an aqueous solution of one of the acids listed in the table. The molarity of the acids in the solutions is the same. Based on the information, which particle diagram best corresponds to the indicated acid?
C
The oxidation of PCl3(g) is represented by the equation above, and the table provides the approximate values of the absolute molar entropies, S°, for these substances. Based on the information given, what is the approximate ΔS° for the reaction?
−170J/(molrxn⋅K)
Based on the reduction potentials given in the table above, which of the following gives the balanced chemical equation and the correct standard cell potential for a galvanic cell involving Sc3+(aq) and Mn2+(aq) ?
2Sc3+(aq)+3Mn(s)⇄2Sc(s)+3Mn2+(aq) E°=−0.62V
6.3 A student adds 50.0g50.0g of liquid water at 25.0°C25.0°C to an insulated container fitted with a temperature probe. The student then adds 10.0g10.0g of ice at 0.0°C0.0°C, closes the container, and measures the temperature at different intervals. Part of the data is shown in the graph above. The student predicts that the temperature will continue to decrease then level out to a constant temperature. Which of the following best explains why the student's prediction is correct? A The H2OH2O molecules initially in the ice and the molecules initially in the liquid will have the same average kinetic energy. B The transfer of energy between the H2OH2O molecules in the ice and liquid water stops once all the molecules are in the liquid phase. C Once all of the H2OH2O molecules are in the liquid phase, the individual molecular speeds either increase or decrease until all the particles have the same speed. D Once all of the H2OH2O molecules are in the liquid phase, collisions between them virtually stop as they reach an equilibrium distance from their neighboring molecules.
A
8.10 HF(aq)+H2O(l)⇄H3O+(aq)+F−(aq)pKa=3.20at25°C The acid ionization equilibrium for the weak acid HF is represented by the equation above. To prepare a buffer with a pH=3.50, a student needs to mix 250.mL of 0.100MHF and 250.mL of 0.100MKF. If the student mistakenly mixes 250.mL of 0.0500MHF and 250.mL of 0.0500MKF, which of the following is the result of this error? A The buffer will have a lower capacity because of the smaller number of moles of HF and F− available to react if an acid or base is added. B The buffer will have a lower capacity because the smaller amount of HF and F− will lower the pH of the buffer, and buffers of lower pH have a lower buffer capacity. C The buffer will have a higher capacity because a larger proportion of HF and F− will ionize at lower concentrations, resulting in the neutralization of any added acid or base. D The buffer will have the same capacity because the large volume of the buffer solution dilutes any added acid or base.
A
8.9 The equilibrium for the reaction between (CH3)2NH, a weak base, and water is represented by the equation below. The table shows the pH of three solutions of (CH3)2NH(aq) at 25°C. (CH3)2NH(aq)+H2O(l)⇄(CH3)2NH2+(aq)+OH−(aq)Kb=5.4×10−4at25°C [(CH3)2NH] pH at25°C 0.050 11.69 0.10 11.85 0.20 12.01 Which of the following equations can be used to correctly calculate the pH of a solution at 25°C that is 0.100M(CH3)2NH(aq) and 0.100M(CH3)2NH2Cl(aq) ? A pH=−log(1.0×10−145.4×10−4) B pH=−log(5.4×10−41.0×10−14) C pH=14.00+(−log5.4×10−4) D pH=(−log5.4×10−4)+log(0.1000.100)
A
CS2(l)+2H2O(l)→CO2(g)+2H2S(g)ΔH°rxn=?CS2(l)+2H2O(l)→CO2(g)+2H2S(g)ΔH°rxn=? Which of the following combinations represents the individual reactions and the quantities needed to determine ΔH°ΔH° for the overall reaction represented by the chemical equation above? A CS2(l)+3O2(g)→CO2(g)+2SO2(g) ΔH°=−1075kJ 2H2O(l)+2SO2(g)→2H2S(g)+3O2(g)ΔH°=+1136kJ B CS2(l)+3O2(g)→CO2(g)+2SO2(g)ΔH°=−1075kJ H2O(l)+SO2(g)→H2S(g)+32O2(g)ΔH°=+568kJ C CS2(l)+3O2(g)→CO2(g)+2SO2(g)ΔH°=−1075kJ H2S(g)+32O2(g)→H2O(l)+SO2(g)ΔH°=−568kJ D CS2(l)+3O2(g)→CO2(g)+2SO2(g)ΔH°=−1075kJ 2H2S(g)+3O2(g)→2H2O(l)+2SO2(g)ΔH°=−1136kJ
A
7.6 H2CO3(aq) + NH3(aq) ⇄ NH4+(aq) +HCO3−(aq) K5=? The table above lists some equilibrium systems and their equilibrium constants. Which of the following identifies the correct mathematical relationship that uses the information to calculate K5?
A K5 = K1×K3 / K4
7.14 Two students prepared aqueous solutions of LiCl and measured the properties, as shown in the table above. Both students observed that the solid LiCl readily dissolved in H2O. The students drew particle diagrams to explain the changes in the enthalpy and entropy of dissolution for LiCl based on their results and observations. Based on this information, the better particle diagram was drawn by which student, and why is that diagram more accurate?
A The better particle diagram was drawn by Student 1 because when LiCl dissolves in water, it dissociates into Li+ and Cl− ions causing an increase in entropy.
7.9 At night, an equilibrium reaction between two different nitrogen compounds generates N2O5 in the atmosphere, as represented below. Reaction 1: NO3(g)+NO2(g)⇄N2O5(g) K=2.6×10−11 In the atmosphere, small water droplets are suspended in the air, forming an aerosol. N2O5(g) can form HNO3(aq) under these conditions, as shown in reaction 2, represented below. Reaction 2: N2O5(g)+H2O(l)→2HNO3(aq) Which of the following predicts the effect that the formation of HNO3(aq) will have on the equilibrium shown in reaction 1, and why?
A The equilibrium of reaction 1 will shift toward the formation of more product, because N2O5(g) is removed when it reacts to form HNO3(aq) .
7.12 CdF2(s )⇄ Cd2+(aq) + 2F−(aq) A saturated aqueous solution of CdF2 is prepared. The equilibrium in the solution is represented above. In the solution, [Cd2+]eq = 0.0585M and [F−]eq = 0.117M. Some 0.90MNaF is added to the saturated solution. Which of the following identifies the molar solubility of CdF2 in pure water and explains the effect that the addition of NaF has on this solubility?
A The molar solubility of CdF2 in pure water is 0.0585M, and adding NaF decreases this solubility because the equilibrium shifts to favor the precipitation of some CdF2 .
6.6 Mg(s)+2HCl(aq)→MgCl2(aq)+H2(g) The chemical equation shown above represents the reaction between Mg(s) and HCl(aq). When 12.15g of Mg(s) is added to 500.0mL of 4.0MHCl(aq), 95kJ of heat is released. The experiment is repeated with 24.30g of Mg(s) and 500.0mL of 4.0MHCl(aq). Which of the following gives the correct value for the amount of heat released by the reaction? A 380kJ380kJ B 190kJ190kJ C 95kJ95kJ D 48kJ
B
6.7 The oxidation of carbon monoxide can be represented by the chemical equation 2 CO(g)+O2(g)→2 CO2(g). The table above provides the average bond enthalpies for different bond types. Based on the information in the table, which of the following mathematical expressions is correct for the estimated enthalpy change for the reaction? A ΔHrxn=[2(1072kJmol)+(498kJmol)]−2(799kJmol)ΔHrxn=[2(1072kJmol)+(498kJmol)]−2(799kJmol) B ΔHrxn=[2(1072kJmol)+(498kJmol)]−4(799kJmol)ΔHrxn=[2(1072kJmol)+(498kJmol)]−4(799kJmol). C ΔHrxn=[2(799kJmol)+(142kJmol)]−4(360kJmol)ΔHrxn=[2(799kJmol)+(142kJmol)]−4(360kJmol) D ΔHrxn=[2(799kJmol)+(142kJmol)]−2(360kJmol)ΔHrxn=[2(799kJmol)+(142kJmol)]−2(360kJmol)
B
6.8 Based on the information in the table above, which of the following expressions gives the approximate ΔH°ΔH° for the reaction represented by the following balanced chemical equation? Fe2O3(s)+3CO(g)→2Fe(s)+3CO2(g)Fe2O3(s)+3CO(g)→2Fe(s)+3CO2(g) A ΔH°rxn=[(0kJ/mol)+(−394kJ/mol)]−[(−826kJ/mol)+(−111kJ/mol)]ΔH°rxn=[(0kJ/mol)+(−394kJ/mol)]−[(−826kJ/mol)+(−111kJ/mol)] B ΔH°rxn=[2(0kJ/mol)+3(−394kJ/mol)]−[(−826kJ/mol)+3(−111kJ/mol)]ΔH°rxn=[2(0kJ/mol)+3(−394kJ/mol)]−[(−826kJ/mol)+3(−111kJ/mol)] C ΔH°rxn=[(−826kJ/mol)+3(−111kJ/mol)]−[2(0kJ/mol)+3(−394kJ/mol)]ΔH°rxn=[(−826kJ/mol)+3(−111kJ/mol)]−[2(0kJ/mol)+3(−394kJ/mol)] D ΔH°rxn=[(−826kJ/mol)+(−111kJ/mol)]−[(0kJ/mol)+(−394kJ/mol)]ΔH°rxn=[(−826kJ/mol)+(−111kJ/mol)]−[(0kJ/mol)+(−394kJ/mol)]
B
8.7 H3N+CH2COOH(aq)+H2O(l)⇄H3N+CH2COO−(aq)+H3O+(aq) H3N+CH2COO−(aq)+H2O(l)⇄H2NCH2COO−(aq)+H3O+(aq) The stepwise dissociation of the amino acid glycine is represented by the chemical equations above. A student titrates a sample of glycine dissolved in dilute acid with 0.100MNaOH(aq). The data are plotted on the following graph. The figure presents a titration curve. The horizontal axis is labeled Volume of N a O H Added, in milliliters, and the numbers 0 through 75, in increments of 5, are indicated. The vertical axis is labeled p H and the numbers 0 through 14, in increments of 1, are indicated. The data represented in the graph are as follows. Note that all values are approximate. The curve begins at a volume of 0 milliliters and a p H of approximately 1.5. At 12.5 milliliters, the p H is approximately 2.3. At 20 milliliters, the p H is approximately 2.8. The curve starts to move up more sharply and is essentially vertical at 25 milliliters from a pH of approximately 5 to 7. The curve then becomes less steep. At 45 milliliters the pH is almost 10.2. At 50 milliliters around pH 11, the curve briefly becomes more steep. At 55 milliliters, the pH is 11.8. The curve ends at a volume of 70 milliliters and a pH of approximately 12.5. Based on the data, which of the following species has the highest concentration in an aqueous solution of glycine with a pH of 7? A H3N+CH2COOH(aq) B H3N+CH2COO−(aq) C H2NCH2COO−(aq) D H2NCH2COOH(aq)
B
8.8 The equilibrium for the reaction between (CH3)2NH, a weak base, and water is represented by the equation below. The table shows the pH of three solutions of (CH3)2NH(aq) at 25°C. (CH3)2NH(aq)+H2O(l)⇄(CH3)2NH2+(aq)+OH−(aq)Kb=5.4×10−4at25°C [(CH3)2NH] pH at25°C 0.050 11.69 0.10 11.85 0.20 12.01 A student mixes 100.mL of 0.200M(CH3)2NH(aq) with 100.mL of 0.200M(CH3)2NH2Cl(aq) and claims that if a small amount of strong base is added to the mixture, then the resulting change in pH of the mixture will be smaller than the change in pH that would result from adding the same amount of strong base to 200.mL of 0.200M(CH3)2NH(aq). Which of the following best explains whether or not the student's claim is correct? A The claim is correct because the mixture contains only half of the (CH3)2NH(aq) that the 0.200M(CH3)2NH(aq) contains. The lesser amount of base in the mixture makes the pH of the mixture lower than the pH of the 0.200M(CH3)2NH(aq); therefore, when a small amount of strong base is added, the change in the pH of the mixture will be smaller than the change in the pH of the 0.200M(CH3)2NH(aq). B The claim is correct because the mixture contains a significant amount of the acid (CH3)2NH2+(aq), which can react with and partially neutralize the added strong base, thereby reducing the change in pH. In 0.200M(CH3)2NH(aq), the concentration of the acid (CH3)2NH2+(aq) is very small; therefore, the added strong base is not neutralized and the change in pH will be larger. C The claim is incorrect because the mixture contains only half of the (CH3)2NH(aq) that the 0.200M(CH3)2NH(aq) contains. The lesser amount of base in the mixture makes the pH of the mixture lower than the pH of the 0.200M(CH3)2NH(aq); therefore, when a small amount of strong base is added, the change in the pH of the mixture will be larger than the change in the pH of the 0.200M(CH3)2NH(aq). D The claim is incorrect because the change in pH resulting from the addition of a small amount of strong base to the mixture and to the 0.200M(CH3)2NH(aq) will be the same given that the same amount of strong base is being added to the same volume of solution.
B
8.9 HC4H7O2(aq)+H2O(l)⇄H3O+(aq)+C4H7O2−(aq) The chemical equation above represents the acid ionization equilibrium for HC4H7O2 for which pKa=4.8. Which of the following is the best estimate for the pH of a buffer prepared by mixing 100.mL of 0.20MHC4H7O2 with 100.mL of 0.10MNaC4H7O2 ? A 1.0 B 4.5 C 4.8 D 7.0
B
7.13 CaF2(s) ⇄ Ca2+(aq) + 2F−(aq) Ksp=3.9×10−11 HF(aq) ⇄ H+(aq) + F−(aq) Kc=6.8×10−4 The dissolution of calcium fluoride is represented by the equilibrium system above at 25°C. The F− ion is produced when the weak acid HF dissociates. If solid calcium fluoride is added to equal volumes of the following solutions at 25°C, in which solution will the most calcium fluoride dissolve?
B 1 M HNO3(aq)
6.2 The following questions relate to the below information. XY2 → X + Y2 The equation above represents the decomposition of a compound XY2. The diagram below shows two reaction profiles (path one and path two) for the decomposition of XY2. Which of the following best describes the flow of heat when 1.0 mol of XY2 decomposes? A 50 kJ of heat is transferred to the surroundings. B 50 kJ of heat is transferred from the surroundings. C 100 kJ of heat is transferred to the surroundings. D 100 kJ of heat is transferred from the surroundings.
B 50 kJ of heat is transferred from the surroundings.
7.2 An equimolar mixture of X(g) and Y(g) is placed inside a rigid container at a constant temperature. The particle diagram above represents the changes that occur over time. Based on the particle diagram, which of the following best predicts whether or not the system has reached equilibrium by 300s?
B It is not possible to determine that the system has reached equilibrium by 300s because the amounts of X, Y, and XY have continued to change.
7.1 A cylinder with a moveable piston is completely filled with a small amount (100 millimoles) of liquid water at a pressure of 1.0atm and a temperature of 80∘C. All the air in the cylinder is excluded. The cylinder is placed in a water bath held at 80∘C. The piston is slowly moved out to expand the volume of the cylinder to 20L as the pressure inside the cylinder is monitored. A plot of the pressure versus volume for the system is shown in the figure above. Which of the following statements most closely indicates, with justification, the region of the curve where the equilibrium represented below occurs? H2O(l)⇄H2O(g)
B Region B, because the pressure inside the cylinder is equal to the vapor pressure of water at 80∘C when both liquid and gas phases are present.
7.13 Moles of HCl added Molar solubility of MgCO3 0 2.6×10−3 2.7×10−4 3.3×10−3 4.6×10−4 5.5×10−3 6.0×10−4 6.9×10−3 MgCO3(s) ⇄ Mg2+(aq) + CO32−(aq) A saturated solution of MgCO3 at equilibrium is represented by the equation above. Four different saturated solutions were prepared and kept at the same temperature. A given amount of HCl was added to each solution and data were collected to calculate the molar solubility of MgCO3 as shown in the table above. Which of the following can be concluded from the data?
B The molar solubility of MgCO3 increases with increasing acidity (lower pH).
7.1 In an experiment, X(g) and Y(g) were combined in a rigid container at constant temperature and allowed to react as shown in the equation above. The table provides the data collected during the experiment. Based on the data, which of the following claims is most likely correct?
B The reaction reached equilibrium between 75 minutes and 155 minutes after the reactants were combined because the concentrations of X and XY remained constant.
7.4 The diagram above represents the equilibrium between the two isomers of C2H2Cl2, and the table provides the data collected in an experiment to determine its equilibrium constant, Kc, at 490K. In a second experiment done at the same temperature, [Z]eq≈1.0 M. Which of the following is the approximate equilibrium concentration of Y in the second experiment, and why?
B [Y]eq≈1.5 M because the ratio [Z]eq[Y]eq should remain constant when the reaction is done at the same temperature.
HA(aq)+H2O(l)⇄A−(aq)+H3O+(aq) ΔG°=+35kJ/molrxn Based on the chemical equation and ΔG° given above, which of the following justifies the claim that HA(aq) is a weak acid?
Because ΔG°>>0 , Ka<<1 , and HA only partially dissociates.
6.7 An equation representing the dissociation of O2(g) and a table of bond enthalpies are shown above. Based on the information, which of the following is the enthalpy of dissociation for O2(g) ? A −641kJ/mol−641kJ/mol B −495kJ/mol−495kJ/mol C 495kJ/mol495kJ/mol D 641kJ/mol
C
8.6 B(aq)+H2O(l)⇄HB+(aq)+OH−(aq)Kb=[HB+][OH−][B] The table above provides the chemical structures for weak bases and their ionization constants, Kb. Based on the data, which of the following provides the best reason for the trend in base strengths? A The number of hydrogen atoms B The number of resonance structures C The different electronegativities of H, I, and Br D The different molar masses
C
7.5 The table above shows data for two reactions carried out in two separate evacuated 1.0-liter rigid containers at constant temperature of 298K. To each container 0.50mol of the appropriate reactants was added, and the reaction was allowed to reach equilibrium. Based on this information, which of the following correctly compares the relative concentrations of BrCl and NO present inside their respective containers at equilibrium?
C [BrCl]eq>[NO]eq because the much larger Keq for reaction 1 means that a much higher concentration of products will be present at equilibrium for reaction 1 compared with reaction 2.
8.3 The equilibrium for the reaction between (CH3)2NH, a weak base, and water is represented by the equation below. The table shows the pH of three solutions of (CH3)2NH(aq) at 25°C. (CH3)2NH(aq)+H2O(l)⇄(CH3)2NH2+(aq)+OH−(aq)Kb=5.4×10−4at25°C [(CH3)2NH] pH at25°C 0.050 11.69 0.10 11.85 0.20 12.01 Based on the information given, which of the following is true? A Solutions with a higher concentration of (CH3)2NH(CH3)2NH have a higher pOHpOH because to reach equilibrium a smaller amount of the conjugate acid (CH3)2NH2+(CH3)2NH2+ is produced. B Solutions with a higher concentration of (CH3)2NH(CH3)2NH have a higher pOHpOH because to reach equilibrium more OH−OH− is produced. C Solutions with a higher concentration of (CH3)2NH(CH3)2NH have a higher pHpH because to reach equilibrium a smaller amount of the conjugate acid (CH3)2NH2+(CH3)2NH2+ is produced. D Solutions with a higher concentration of (CH3)2NH(CH3)2NH have a higher pHpH because to reach equilibrium more OH−OH− is produced.
D
8.9 A student prepares a lactic acid-sodium lactate buffer solution by mixing 40.mL of 0.50MHC3H5O3(aq) with 200.mL of 1.0MNaC3H5O3(aq). The pKa of HC3H5O3 is 3.08. What is the pH of the resulting solution? A 2.08 B 3.08 C 3.38 D 4.08
D
6.5 The diagram above represents the melting of H2O(s). A 2.00mole sample of H2O(s) at 0°C melted, producing H2O(l) at 0°C. Based on the diagram, which of the following best describes the amount of heat required for this process and the changes that took place at the molecular level? A 3.01kJ3.01kJ of heat was absorbed to decrease the average speed of the water molecules in the liquid, which decreases the distance between molecules. B 6.02kJ6.02kJ of heat was absorbed to increase the number of hydrogen bonds between water molecules in the liquid compared to the solid. C 12.0kJ12.0kJ of heat was absorbed to decrease the polarity of the water molecules, which increases the density of the liquid compared to the solid. D 12.0kJ12.0kJ of heat was absorbed to overcome some of the hydrogen bonding forces holding the water molecules in fixed positions in the crystalline structure.
D 12.0kJ12.0kJ of heat was absorbed to overcome some of the hydrogen bonding forces holding the water molecules in fixed positions in the crystalline structure.
6.4 For an experiment, 50.0g50.0g of H2OH2O was added to a coffee-cup calorimeter, as shown in the diagram above. The initial temperature of the H2OH2O was 22.0°C22.0°C, and it absorbed 300.J300.J of heat from an object that was carefully placed inside the calorimeter. Assuming no heat is transferred to the surroundings, which of the following was the approximate temperature of the H2OH2O after thermal equilibrium was reached? Assume that the specific heat capacity of H2OH2O is 4.2J/(g⋅K)4.2J/(g⋅K). A 21.3°C21.3°C B 22.0°C22.0°C C 22.7°C22.7°C D 23.4°C
D 23.4°C
6.1 For a classroom demonstration, a chemistry teacher puts samples of two different pure solid powders in a beaker. The teacher places the beaker on a small wooden board with a wet surface, then stirs the contents of the beaker. After a short time the students observe that the bottom of the beaker is frozen to the wood surface. The teacher asks the students to make a claim about the observation and to justify their claims. Which of the following is the best claim and justification based on the students' observation? A An exothermic chemical change occurred because heat flowed from the contents of the beaker to the room. B An exothermic physical change occurred because heat flowed from the contents of the beaker and the water on the board to the room. C An endothermic physical change occurred because the freezing of water is an endothermic process. D An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.
D An endothermic chemical change occurred because the temperature of the beaker and the water on the board decreased as heat was absorbed by the reaction.
7.1 A sample of N2O4(g) is placed into an evacuated container at 373K and allowed to undergo the reversible reaction N2O4(g)⇄2NO2(g). The concentration of each species is measured over time, and the data are used to make the graph shown above. Which of the following identifies when equilibrium is first reached and provides a correct explanation?
D At 60 seconds, because [NO2] and [N2O4] remain constant, indicating that the forward and reverse reaction rates are equal.
6.3 A piece of Fe(s)Fe(s) at 25°C25°C is placed into H2O(l)H2O(l) at 75°C75°C in an insulated container. A student predicts that when thermal equilibrium is reached, the FeFe atoms, being more massive than the H2OH2O molecules, will have a higher average kinetic energy than the H2OH2O molecules. Which of the following best explains why the student's prediction is incorrect? A At thermal equilibrium, the less massive H2OH2O molecules would have a higher average kinetic energy than the FeFe atoms because they are more free to move than are the FeFe atoms. B At thermal equilibrium, the collisions between the FeFe atoms and the H2OH2O molecules would cease because the average kinetic energies of their particles would have become the same. C At thermal equilibrium, the movement of both the FeFe atoms and the H2OH2O molecules would cease; thus, the average kinetic energy of their particles would have to be the same. D At thermal equilibrium, the average kinetic energy of the FeFe atoms cannot be greater than that of the H2OH2O molecules; the average kinetic energies must be the same according to the definition of thermal equilibrium.
D At thermal equilibrium, the average kinetic energy of the FeFe atoms cannot be greater than that of the H2OH2O molecules; the average kinetic energies must be the same according to the definition of thermal equilibrium.
7.6 Reaction 1: CO(g)+3H2(g)⇄CH4(g)+H2O(g) K1=[CH4][H2O] / [CO][H2]3 Reaction 2: CO2(g)+H2(g)⇄CO(g)+H2O(g) K2=[CO][H2O] / [CO2][H2] Reaction 3: CH4(g)+2H2O(g)⇄CO2(g)+4H2(g) K3=? The chemical equations and equilibrium expressions for two reactions at the same temperature are given above. Based on the information, which of the following expressions can be used to calculate the value of K3 for reaction 3 at the same temperature?
D K3 = 1/K1 × 1/K2
6.1 Which of the following phase changes involves the transfer of heat from the surroundings to the system? A CH4(g)→CH4(l)CH4(g)→CH4(l), because CH4CH4 molecules in the gas phase must absorb energy in order to move closer together, thereby increasing the intermolecular attractions in the solid state. B CO2(g)→CO2(s)CO2(g)→CO2(s), because CO2CO2 molecules in the gas phase must absorb energy in order to move closer together, thereby increasing the intermolecular attractions in the liquid state. C H2O(l)→H2O(s)H2O(l)→H2O(s), because H2OH2O molecules in the liquid phase must absorb energy in order to create a crystalline structure with strong intermolecular attractions in the solid state. D NH3(l)→NH3(g)NH3(l)→NH3(g), because NH3NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.
D NH3(l)→NH3(g)NH3(l)→NH3(g), because NH3NH3 molecules in the liquid phase must absorb energy in order to overcome their intermolecular attractions and become free gas molecules.
7.10 2NO(g) + Br2(g ) ⇄ 2NOBr(g) Kp= (PNOBr) / 2(PNO)2(PBr2) = 28 Substance Initial Partial Pressure (torr) NO 10. Br2 2.0 NOBr 80. The system represented by the equation above is allowed to establish equilibrium. The initial pressures of the substances are given in the table. Which of the following explains what the system will do as it approaches equilibrium?
D Q=(80.)2 / (10.)2(2.0) >Kp and equilibrium will be approached by consuming NOBr because the reverse reaction is faster than the forward reaction.
7.10 Cl2(aq)+2H2O(l)⇄H3O+(aq)+Cl−(aq)+HOCl(aq) Kc= [H3O+][Cl−][HOCl] / [Cl2] = 4.8×10−4 The equilibrium reaction between Cl2(aq) and H2O(l) at 25°C is represented by the chemical equation shown above. If a solution at equilibrium at 25°C is diluted with distilled water to twice its original volume, which of the following gives the value for Qc and predicts the response by the system immediately after dilution?
D Qc=Kc4 , and the rate of the forward reaction will be greater than the rate of the reverse reaction.
6.5 A sample of CHCl3(s) was exposed to a constant source of heat for a period of time. The graph above shows the change in the temperature of the sample as heat is added. Which of the following best describes what occurs at the particle level that makes segment D longer than segment B? A The specific heat capacity of the liquid is significantly higher than that of the solid, because the particles in the liquid state need to absorb more thermal energy to increase their average speed. B The specific heat capacity of the solid is significantly higher than that of the gas, because the particles in the solid state need to absorb more thermal energy to increase their average speed. C The enthalpy of fusion is greater than the enthalpy of vaporization, because separating molecules from their bound crystalline state requires more energy than separating molecules completely from the liquid state. D The enthalpy of vaporization is greater than the enthalpy of fusion, because separating molecules completely from the liquid to form a gas requires more energy than separating molecules from their bound crystalline state to a liquid state.
D The enthalpy of vaporization is greater than the enthalpy of fusion, because separating molecules completely from the liquid to form a gas requires more energy than separating molecules from their bound crystalline state to a liquid state.
6.1 In the spring, blossoms on cherry trees can be damaged when temperatures fall below −2°C. When the forecast calls for air temperatures to be below −5°C for a few hours one night, a farmer sprays his blossoming cherry trees with water, claiming that the blossoms will be protected by the water as it freezes. Which of the following is a correct scientific justification for spraying water on the blossoms to protect them from temperatures below −2°C? A Water on the blossoms will not freeze unless the air temperature falls significantly below −5°C−5°C. B Water is a good thermal conductor that transfers heat from the cold air to the blossoms, keeping the blossoms from going below −2°C−2°C. C The freezing of water is an endothermic process; thus, water that freezes on the blossoms absorbs heat from the atmosphere, which in turn keeps the blossoms above 0°C0°C. D The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C−2°C.
D The freezing of water is an exothermic process; thus, water that freezes on the blossoms releases heat to keep the blossoms at or above −2°C−2°C.
6.3 The graphs above show Maxwell-Boltzmann distributions for one-mole samples of Ar(g)Ar(g). Graph 1 shows the distribution of particle energies at 300K300K and graph 2 shows the distribution of particle energies at 600K600K. A student predicts that if the samples are combined in an insulated container and thermal equilibrium is attained, then the most probable particle energy will be between the most probable energy shown in graph 1 and the most probable energy shown in graph 2. Which of the following is the best justification for the student's claim? A When the samples are combined, the gas particles will collide with one another, with the net effect being that the speed of the lowest energy particles decreases while the speed of the highest energy particles increases, leaving the average speed of the particles in the original samples unchanged. B When the samples are combined, the gas particles from each sample will collide with the gas particles from the other sample until every particle in the mixture has the same speed, which is between the average speed of the particles in the hotter sample and the average speed of the particles in the cooler sample. C When the samples are combined, the gas particles collide with one another until every particle in the mixture has the same kinetic energy, which is between the average kinetic energy of the particles in the hotter sample and the average kinetic energy of the particles in the cooler sample. D When the samples are combined, the gas particles will collide with one another, with the net effect being that energy will be transferred from the more energetic particles to the less energetic particles until a new distribution of energies is achieved at a temperature between 300K300K and 600K600K.
D When the samples are combined, the gas particles will collide with one another, with the net effect being that energy will be transferred from the more energetic particles to the less energetic particles until a new distribution of energies is achieved at a temperature between 300K and 600K.
6.5 A 2.00mol sample of C2H5OH undergoes the phase transition illustrated in the diagram above. The molar enthalpy of vaporization, ΔHvap, of C2H5OH is +38.6kJ/mol. Which of the following best identifies the change in enthalpy in the phase transition shown in the diagram? A +19.3kJ B +77.2kJ C −19.3kJ D −77.2kJ
D −77.2kJ
The combustion of C2H5OH is represented by the equation above and the standard entropy and enthalpy changes for the reaction are provided. When the reactants are combined at 25°C, essentially no CO2(g) or H2O(g) is produced after a few hours. Which of the diagrams above could best help explain the low yield of the reaction under these conditions, and why?
Diagram 2, because it represents a reaction with a high activation energy barrier for molecules to overcome and a very slow reaction rate, even if it is thermodynamically favorable with ΔG°<0 .
(f) For the standard cell, the Cu2+ half-cell was made with 1.0L of 1.0MCu(NO3)2 and the Zn2+ half-cell was made with 1.0L of 1.0MZn(NO3)2. The experiment was repeated, but this time the Cu2+ half-cell was made with 0.50L of 2.0MCu(NO3)2 and the Zn2+ half-cell was made with 1.0L of 1.0MZn(NO3)2. Is the cell potential for the nonstandard cell greater than, less than, or equal to the value calculated in part (b)? Justify your answer.
E=E∘−RT/nFlnQ=E∘−RT/nFln[Zn2+][Cu2+]=E∘−RT/nFln12 The natural log of a number less than one is negative, so for the nonstandard cell a negative number is subtracted from the standard potential: If , E=E∘−RT/nF(negative number) then E will be greater than E∘ . The response gives the following conclusion: The cell potential for the nonstandard cell is greater than that for the standard cell (or E is greater than E∘ ).
The vanadium-lead galvanic cell in the diagram above initially operates under standard conditions according to the net reaction shown in the table below the diagram. The cell operates for 1.0 minute at a constant known current. Which of the following mathematical relationships would be most useful for determining the change in mass of the V(s) electrode after 1.0 minute of the cell's operation without having to use a balance?
I=q/t
2POCl3(g)⇄2PCl3(g)+O2(g) ΔG°rxn=+490kJ/mol A sample of POCl3(g) is placed in a closed, rigid container at 298K and allowed to reach equilibrium according to the equation above. Based on the value for ΔG°rxn=+490kJ/mol, which of the following is true?
K=e−490/8.314×298<1 and at equilibrium PPOCl3<PPCl3 .
7.7 (b) Write the expression for Kp, the equilibrium constant for the reaction, and determine its value at 898°C .
Kp=PCO2 Kp=1.00 atm
(d) The following diagram on the left represents the copper nitrate solution in the half‑cell before the galvanic cell starts to operate. In the following diagram on the right, draw a representation of the solution in the same half‑cell after the galvanic cell has been operating for some time and is still producing a voltage.
The diagram contains one, two, or three Cu2+ ions. There is a charge balance in the diagram (the student can add K+ ions from the salt bridge or put in fewer NO3− ions).
(g) If the experiment is repeated with the Cu electrode replaced with a Cu electrode with twice the mass, will the initial cell voltage be greater than, less than, or equal to the initial cell voltage in the original experiment? Justify your answer.
The initial voltage when the mass of the Cu electrode is twice the original mass will be equal to the voltage in the original experiment. The mass and concentration of Cu(s) are not in the equation for the cell potential and do not affect it.
(c) Calculate ΔG°, in kJ/molrxn, for the cell.
The response gives the correct setup: ΔG∘=−nFE∘cell=−(2 mol)(96,485JV mol)(1.10 V) The response shows the correct result of the calculation: ΔG∘=−212,000 J/molrxn=−212 kJ/molrxn
7.7 PCl3(g) +Cl2(g )⇄PCl5(g) Kc=0.11 A 0.60mol sample of PCl3(g) and a 0.70mol sample of Cl2(g) are placed in a previously evacuated 1.0L rigid container, and the reaction represented above takes place. At equilibrium, the concentration of PCl5(g) in the container is 0.040M . (c) On the graph above, carefully draw three curves, one for each of the three gases. The curves must show how the concentration of each of the three gases changes as equilibrium is established. Label each curve with the formula of the gas.
The response meets all of the following criteria: The curve for PCl3 starts at 0.60 M and levels off at 0.56 M The curve for Cl2 starts at 0.70 M and levels off at 0.66 M The curve for PCl5 starts at 0.00 M and levels off at 0.040 M
7.7 (c) The experiment was repeated, but this time starting with a 4.0mol sample of CaCO3(s). On the following graph, draw a curve showing how the pressure of CO2(g) would change over time as the vessel is heated to 898°C and equilibrium is established
The response shows a graph that starts at the origin, rises to right, and levels off (slope=0) at 1.00 atm
6.8 The energy of the reactants is shown on the energy diagram above. On the right side of the energy diagram, draw a horizontal line segment to indicate the energy of the products. Draw a vertical line segment to indicate ΔHΔH for the reaction, and label it with the correct value.
There is a horizontal line on the right side of the diagram that is lower than the horizontal line given on the left [but it should be higher to earn the point if a positive number was calculated in part (a)]. There is a vertical line with one end at the level of the horizontal line given on the left and one end at the level of the horizontal line on the right, and it is labeled with the number calculated in part (a) (or its absolute value).
The galvanic cell shown above generates a cell potential of +3.17V when operated under standard conditions. A second galvanic cell is made from the same two metals, and the measured cell potential is +3.25V. Which of the following could be the reason for the second cell having a greater cell potential?
[Ag+]>1M and [Mg2+]=1M in the second cell, resulting in Q<1 , and Q=1 for the first cell.
N2(g)+3H2(g)⇄2NH3(g) K=5.6×105at298K ΔH°rxn=−91.8kJ/molrxn The synthesis of NH3 is represented by the equation above. Based on the equilibrium constant, K, and ΔH°rxn given above, which of the following can best be used to justify that the reaction is thermodynamically favorable at 298K and constant pressure?
ΔG°=−RTlnK<0 because K>>1
6.8 C2H4(g)+3O2(g)→2CO2(g)+2H2O(l)ΔH∘1=?C2H4(g)+3O2(g)→2CO2(g)+2H2O(l)ΔH1∘=? The combustion of C2H4(g)C2H4(g) is represented by the equation above. (a) Use the enthalpies of formation in the table below to calculate the value of ΔH∘ for the reaction
ΔH∘=ΣΔH∘f products - Delta H reactants ΔH∘=2(−394 kJ/mol)+2(−286 kJ/mol)−(52 kJ/mol)=−1412 kJ/molrxn
The reaction between AgNO3 and CaCl2 is represented by the equation above, and the table provides the approximate S° values for the reactants and products. Which of the following is the approximate ΔS° for the reaction?
−68J/(molrxn⋅K)
8.1 H2O(l)+H2O(l)⇄H3O+(aq)+OH−(aq) At 5.0°C, the value of Kw for the equilibrium shown above is 1.9×10−15 and the value of pKw is 14.73. Based on this information, which of the following is correct for pure water at this temperature? A [H3O+][H3O+] =1.9×10−15−−−−−−−−−√=1.9×10−15 B pH=−log(1.9×10−15)pH=−log(1.9×10−15) C 14.73=[H3O+]eq[OH−]eq14.73=[H3O+]eq[OH−]eq D pOH=pH+14.73
A
8.1 In pure water, some of the molecules ionize according to the equation H2O→H++OH−. The extent of the ionization increases with temperature. A student heats pure water and records the measured pH at 50°C as 6.6. Based on this information, which of the following mathematical relationships gives the pOH of pure water at 50°C? A pOH=pHpOH=pH B pOH=1pHpOH=1pH C pOH=14−pHpOH=14−pH D pOH=1×10−14pHpOH=1×10−14pH
A
8.8 CH3COOH(aq)+H2O(l)⇄H3O+(aq)+CH3COO−(aq) CH3COO−(aq)+H2O(l)⇄CH3COOH(aq)+OH−(aq) 2H2O(l)⇄H3O+(aq)+OH−(aq) A buffer solution is made up of acetic acid (CH3COOH) and sodium acetate (NaCH3COO). The major equilibria in the buffer system are represented above. Which of the following equilibria could be used to support the claim that the addition of a small amount of NaOH to the buffer will result in only a very small change in pH? A CH3COOH(aq)+CH3COO−(aq)⇄CH3COO−(aq)+CH3COOH(aq) B CH3COOH(aq)+OH−(aq)⇄CH3COO−(aq)+H2O(l) C H3O+(aq)+OH−(aq)⇄2H2O(l) D CH3COOH(aq)+H2O(l)⇄H3O+(aq)+CH3COO−(aq)
B
7.4 2A(g)+B(g)⇄2C(g) A(g) and B(g) react to form C(g), according to the balanced equation above. In an experiment, a previously evacuated rigid vessel is charged with A(g), B(g), and C(g), each with a concentration of 0.0100M. The following table shows the concentrations of the gases at equilibrium at a particular temperature. If the experiment is repeated at a higher temperature at which Kc is larger, which of the following best describes the effect of the temperature change on the concentrations of the gases at equilibrium?
C There will be a decrease in [A]eq that will be two times the decrease in [B]eq because A and B react in a 2-to-1 ratio.
8.10 HC2H3O2(aq)+H2O(l)⇄H3O+(aq)+C2H3O2−(aq) pKa=4.76 The equilibrium for the acid ionization of HC2H3O2 is represented by the equation above. A student wants to prepare a buffer with a pH of 4.76 by combining 25.00mL of 0.30MHC2H3O2 with 75.00mL of 0.10MNaC2H3O2. While preparing the buffer, the student incorrectly measures the volume of NaC2H3O2 so that the actual volume used is 76.00mL instead of 75.00mL. Based on the error, which of the following is true about the buffer prepared by the student? A The pH of the buffer will be slightly lower than 4.76 because the total volume of the buffer is 101.00mL instead of 100.00mL, and the HC2H3O2 was diluted. B The pH of the buffer will be slightly lower than 4.76 because the amount of C2H3O2− added was higher than the amount of HC2H3O2 added. C The buffer solution will have a slightly higher capacity for the addition of bases than for the addition of acids because the total volume of the buffer is 101.00mL instead of 100.00mL, and the HC2H3O2 was diluted. D The buffer solution will have a slightly higher capacity for the addition of acids than for the addition of bases because the amount of C2H3O2− added was higher than the amount of HC2H3O2 added.
D
The reaction between SO2 and O2 is represented by the chemical equation above. The table provides the approximate absolute entropies, S°, for O2(g) and SO3(g). Which of the following mathematical expressions can be used to correctly calculate S° for SO2(g) ?
S°=1/2[187+(2×257)−205]J/(mol⋅K)
6.3 The student claims that thermal equilibrium is reached at time tt. Justify the student's claim. In your justification, include a description of what occurs at the particulate level when the alloy and the water have reached thermal equilibrium.
The claim is correct because the temperature levels off at that time. At equilibrium the average kinetic energy of the atoms in the alloy is equal to the average kinetic energy of the water molecules.
Which of the following particle diagrams represents a process during which the entropy of the system increases?
The one where they're clustered together, and then spread out
The student claims that the formation of rust in test tube 2 shows that the reaction is thermodynamically favored. Which of the following justifications should the student use to explain why rust did not form in test tube 1 ?
The reaction does not occur at an observable rate when water is not present because it proceeds through a mechanism with a high activation energy.
(b) Is the reaction thermodynamically favorable at 298K? Justify your answer based on the calculation of the value and sign of ΔG°.
The response includes the following calculation. ΔG∘=ΔH∘−TΔS∘ ΔG∘=−112 kJ/molrxn−(298 K)(−0.147 kJ/(K molrxn))=−68 kJ/molrxn
6.3 In terms of what occurs at the particulate level, explain how the temperature of the water increases after the alloy sample is added.
The response indicates that the atoms in the alloy transfer kinetic energy to the molecules in the water.
6.2 2H2O2(aq)→2H2O(l)+O2(g)2H2O2(aq)→2H2O(l)+O2(g) Hydrogen peroxide, H2O2H2O2, decomposes according to the equation above. This reaction is thermodynamically favorable at room temperature. (a) A particulate representation of the reactants is shown below in the box on the left. In the box below on the right, draw the particulate representation of all the molecules that would be produced from these four reactant molecules.
The response meets both of the following criteria. The diagram shows H2O and O molecules The diagram shows atom balance.
8.5 Methylamine is a weak base with the formula CH3NH2CH3NH2. (a) In the following box, complete the Lewis electron-dot diagram for a molecule of methylamine. Show all bonding and nonbonding valence electrons
The student's drawing includes exactly 14 electrons; one pair of electrons between every two adjacent atoms plus one lone pair of electrons on the nitrogen atom. Electron pairs can be represented either by dots, or line segments.
4Fe(s)+3O2(g)⇄2Fe2O3(s) ΔH=−1,650kJ/molrxn The oxidation of Fe(s) is represented by the chemical equation above. Which of the following correctly explains whether or not the reaction is thermodynamically favorable?
There are more particles (including particles in the gas state) in the reactants than in the product, thus ΔSrxn<0 . Because ΔH is large and negative, the reaction will be thermodynamically favorable at low temperatures.
6.2 Shown below is a potential energy diagram for the uncatalyzed decomposition of H2O2(aq)H2O2(aq). ) According to the diagram, is the decomposition reaction exothermic or endothermic? Justify your answer. ( Diagram goes high to low )
This is exothermic because the difference between the products and reactants ( the enthalpy) is negative.
8.3 NH3(aq)+H2O(l)⇄NH4+(aq)+OH−(aq)Kb=1.8×10−5at25°C Initial [NH3]Equilibrium [NH4+]Equilibrium [OH−]pOH0.151.6×10−31.6×10−32.780.302.3×10−3?? NH3 is a weak base that reacts with water according to the chemical equilibrium represented above. The table provides some information for two NH3(aq) solutions of different concentration at 25°C. Which of the following is true about the more concentrated 0.30MNH3(aq), and why? A [OH−]=3.2×10−3M[OH−]=3.2×10−3M and pOH<2.78pOH<2.78, because a higher [OH−][OH−] corresponds to a lower pOHpOH for 0.30MNH3(aq)0.30MNH3(aq) compared to 0.15MNH3(aq)0.15MNH3(aq). B [OH−]=3.2×10−3M[OH−]=3.2×10−3M and pOH>2.78pOH>2.78, because a higher [OH−][OH−] corresponds to a higher pOHpOH for 0.30MNH3(aq)0.30MNH3(aq) compared to 0.15MNH3(aq)0.15MNH3(aq). C [OH−]=2.3×10−3M[OH−]=2.3×10−3M and pOH<2.78pOH<2.78, because a higher [OH−][OH−] corresponds to a lower pOHpOH for 0.30MNH3(aq)0.30MNH3(aq) compared to 0.15MNH3(aq)0.15MNH3(aq). D [OH−]=2.3×10−3M[OH−]=2.3×10−3M and pOH>2.78pOH>2.78, because a higher [OH−][OH−] corresponds to a higher pOHpOH for 0.30MNH3(aq)0.30MNH3(aq) compared to 0.15MNH3(aq)0.15MNH3(aq).
c
(c) Is the value of the equilibrium constant, K, for the reaction greater than 1, or less than 1 ? Justify your answer.
the equilibrium constant is greater than 1 because ΔG∘<0
