AP Chemistry Final Exam Review
2NO(g)+O2(g)→2NO2(g) The oxidation of NO(g) producing NO2(g) is represented by the chemical equation shown above. The initial concentrations of NO and O2 are given in the table above. The changes in the concentration of NO(g) as a function of time are shown in the following graph. Under the same conditions, one of the following graphs represents the changes in the concentration of O2(g) over the same period of time. Which graph best represents the changes in concentration of O2(g), and why?
A) Graph 1, because the rate of O2O2 consumption is half the rate at which NONO is consumed; two molecules of NONO react for each molecule of O2O2 that reacts. Correct. Based on the stoichiometry of the reaction, the rate of consumption of O2O2 is half the rate of consumption of NONO. Graph 1 shows this relationship. Related Videos 5.1: Daily Video 1 5.1: Daily Video 2
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 HFHF and F−F− available to react if an acid or base is added. Correct. Buffer capacity depends on the concentrations of the weak acid and conjugate base in solution. The lower the concentration of HFHF and F−F−, the lower the buffer capacity, because a smaller number of moles of HFHF and F−F− are available to react with any added acid or base. Related Videos 8.10: Daily Video 1
Ar(g) deviates more from ideal behavior at extremely high pressures than Ne(g) does. Which of the following is one reason for this difference?
A)The particle volume of ArAr is greater than that of NeNe. Correct. The ideal gas law assumes that the volume of gas particles is negligible compared to the volume of their container. At extremely high pressures, the particles are relatively close together and the space taken up by the gas particles is not negligible. This factor is more significant for gases composed of larger particles. Related Videos 3.6: Daily Video 1
The mass spectrum of the element Sb is most likely represented by which of the following?
B) 121: High relative abundance; 123: 60%ish relative abundance. Correct. The average atomic mass for SbSb is 121.76amu121.76amu. Using the relative abundance of the isotopes with masses of 121amu121amu and 123amu123amu, this is the predicted average atomic mass. Related Videos 1.2: Daily Video 1 1.2: Daily Video 2
Cu(s)+2AgNO3(aq)→Cu(NO3)2(aq)+2Ag(s) The reaction between solid copper and aqueous silver nitrate produces solid silver and a blue solution, as represented by the balanced equation shown above. Based on the balanced equation, which of the following identifies the oxidation and reduction half-reactions?
B) Oxidation Half-reaction = Cu(s)→Cu2+(aq)+2e− Reduction Half-reaction = Ag+(aq)+e−→Ag(s) Correct. In the oxidation half-reaction, each CuCu atom loses two electrons to form a Cu2+Cu2+ ion. In the reduction half-reaction, each Ag+Ag+ ion gains an electron to form an AgAg atom. To balance the number of electrons transferred, the reduction half-reaction is multiplied by a factor of 2, which is the coefficient for both Ag+Ag+ and Ag in the balanced equation. Related Videos 4.9: Daily Video 1
2HBr(g)+O2(g)→H2O2(g)+Br2(g) Based on a kinetics study of the reaction represented by the equation above, the following mechanism for the reaction is proposed.
B) Rate=k[HBr][O2] Correct. The first step is rate limiting; thus, the rate law is determined by the stoichiometry of the reactants of the initial slow step. The correct rate law is rate=k[HBr][O2]rate=k[HBr][O2]. Related Videos 5.8: Daily Video 1
Copper atoms and zinc atoms have the same atomic radius, 135 picometers. Based on this information, which of the following diagrams best represents an alloy containing only copper and zinc atoms?
B) Same size copper and zinc atoms. Correct. Being the same size, copper atoms and zinc atoms would form a substitutional alloy in which the atoms substitute for one another in a lattice of positive cores immersed in a sea of electrons. Related Videos 2.4: Daily Video 1
2F2(g)+2NaOH(aq)→OF2(g)+2NaF(aq)+H2O(l) A 2mol sample of F2(g) reacts with excess NaOH(aq) according to the equation above. If the reaction is repeated with excess NaOH(aq) but with 1 mol of F2(g), which of the following is correct?
B) The amount of OF2(g)OF2(g) produced is halved. Correct. The NaOH(aq)NaOH(aq) is in excess. If the amount of reactants is halved, the amount of each product is halved. The coefficients in the equation do not need to be taken into account in this case. Related Videos 4.5: Daily Video 1 4.5: Daily Video 2 4.5: Daily Video 3
The elements C and Se have the same electronegativity value, 2.55. Which of the following claims about the compound that forms from C and Se is most likely to be true?
B) The carbon-to-selenium bond is nonpolar covalent. Correct. The bonding in compounds between elements with the same (or only slightly different) electronegativity values will exhibit covalent bonding (that is, more or less equal sharing of electrons). Related Videos 2.1: Daily Video 1 2.1: Daily Video 2
HCl(aq)+NaOH(aq)→NaCl(aq)+H2O(l) A student had two dilute, colorless solutions, HCl(aq) and NaOH(aq), which were at the same temperature. The student combined the solutions, and the reaction represented above occurred. Which of the following results would be evidence that a chemical reaction took place?
B) The temperature of the reaction mixture increases. Correct. This is an exothermic reaction, and the temperature will increase. Production of heat is possible evidence that a chemical change has occurred. Related Videos 4.1: Daily Video 1
Reaction 1:N2O4(g)→2NO2(g)ΔH1=+57.9kJ Reaction 2:2NO(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)
C) (−ΔH1)+ΔH2 Correct. Hess's law states that if a reaction is carried out in a series of steps, the enthalpy change for the overall reaction equals the sum of the enthalpy changes for the individual steps. Equations (1) and (2) need to be rearranged in such a way so they can be added to give the overall equation and its enthalpy change. Equation (2) can be used as is, because it has NO(g)NO(g) and O2(g)O2(g) as reactants, as in the overall equation. However, equation (1) needs to be reversed so that N2O4(g)N2O4(g) is a product. The sign of ΔH1ΔH1 must also be reversed. Thus, the quantities needed to calculate the enthalpy for the overall reaction are −ΔH1+ΔH2−ΔH1+ΔH2. Related Videos 6.9: Daily Video 1 6.9: Daily Video 2
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?
C) An external source of energy is not shown. Correct. In an electrolytic cell, an external source of energy is necessary to make the thermodynamically unfavorable reaction occur. A battery or other generator of electrical energy should be shown connected to both electrodes, forming a complete circuit. Related Videos 9.6: Daily Video 1
The survival of aquatic organisms depends on the small amount of O2 that dissolves in H2O. The diagrams above represent possible models to explain this phenomenon. Which diagram provides the better particle representation for the solubility of O2 in H2O, and why?
C) Diagram 2, because the polar H2OH2O molecules can induce temporary dipoles on the electron clouds of O2O2 molecules. Correct. The relatively small solubility of O2O2 in water is due to dipole-induced dipole forces between the polar H2OH2O molecules and the nonpolar O2O2 molecules. The permanent dipole moment of the H2OH2O molecules induces temporary dipole moments on the electron clouds of the nonpolar O2O2 molecules. Related Videos 3.10: Daily Video 1
Two pure elements react to form a compound. One element is an alkali metal, X, and the other element is a halogen, Z. Which of the following is the most valid scientific claim that can be made about the compound?
C) It contains ionic bonds. Correct. An alkali metal will readily lose an electron and a halogen will readily gain an electron to form an ionic compound with a 1:11:1 ratio of X+:Z−X+:Z− ions. Related Videos 2.1: Daily Video 1 2.1: Daily Video 2
2HO2(g)→H2O2(g)+O2(g) The reaction represented by the chemical equation shown above occurs in Earth's atmosphere. In an experiment, [HO2] was monitored over time and the data plotted as shown in the following graph. Based on the information, which of the following is the rate law expression for the reaction?
C) Rate=k[HO2]2 Correct. The plot of 1/[HO2]1/[HO2] versus time is a straight line, which indicates that the reaction is second order with respect to HO2HO2, and the rate law expression is rate=k[HO2]2rate=k[HO2]2. Related Videos 5.3: Daily Video 1 5.3: Daily Video 2
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) ?
C) S°=1/2[187+(2×257)−205]J/(mol⋅K) Correct. Since ΔS°reaction=Σ(moles of substance×S°)products−Σ(moles of substance×S°)reactants, if x=S° for SO2(g), then −187=(2×257)−[2x+205]J/K. Solving for x, S°=1/2[187+(2×257)−205]J/(mol⋅K) for SO2(g)SO2(g). Related Videos 9.2: Daily Video 1
Fe3+(aq)Colorless+ SCN−(aq)Colorless⇄ FeSCN2+(aq)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)KSCN(s) dissolves, causing the reaction system to respond by producing more product to partially consume SCN−(aq)SCN−(aq) and reduce its concentration. Correct. By Le Chatelier's principle, disturbing the equilibrium by adding SCN−SCN− ions causes the equilibrium to shift in a way to reduce the concentration of SCN−SCN− ions by forming more red FeSCN2+FeSCN2+ ions. Related Videos 7.9: Daily Video 1 7.9: Daily Video 2 7.9: Daily Video 3
Diagram 1 above shows equimolar samples of two gases inside a container fitted with a removable barrier placed so that each gas occupies the same volume. The barrier is carefully removed as the temperature is held constant. Diagram 2 above shows the gases soon after the barrier is removed. Which statement describes the changes to the initial pressure of each gas and the final partial pressure of each gas in the mixture and also indicates the final total pressure?
C) The partial pressure of each gas in the mixture is half its initial pressure; the final total pressure is half the sum of the initial pressures of the two gases. Correct. For each gas, the partial pressure in the mixture is half its initial pressure because the volume occupied has doubled under constant nn and TT. Since Pα1V×constantPα1V×constant, Pf=12×PiPf=12×Pi for each gas. According to Dalton's law, the final total pressure (PTf)(PTf) is given by PTf=Pf,gas1+Pf,gas2=12(Pi,gas1+Pi,gas2)PTf=Pf,gas1+Pf,gas2=12(Pi,gas1+Pi,gas2). Related Videos 3.4: Daily Video 1 3.4: Daily Video 2 3.4: Daily Video 3
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 200s200s there are no observable changes in the amounts of reactants and products, which is a characteristic of a system at equilibrium. Correct. After t=200st=200s, there are no observable changes in the amounts of reactants and products, which is a characteristic of a system at equilibrium.
Which of the following best helps explain why the electronegativity of Cl is less than that of F?
C) When ClCl and FF form bonds with other atoms, the ClCl bonding electrons are more shielded from the positive ClCl nucleus than the FF bonding electrons are shielded from the positive FF nucleus. Correct. Electronegativity is based on the ability of atoms participating in bonding to attract bonding electrons to themselves. The smaller size and lower amount of shielding in FF cause it to be more electronegative than ClCl. Related Videos 1.7: Daily Video 1 1.7: Daily Video 2
Which of the following pairs of mathematical expressions can be used to correctly calculate the pH and pOH of a 0.0015MKOH(aq) solution at 25°C?
C) pH=14.00−(−log(0.0015))pH=14.00−(−log(0.0015)) and pOH=−log(0.0015) Correct. KOHKOH is a strong base that completely ionizes in aqueous solutions, producing one mole of OH−OH− ions per mole of KOHKOH dissolved. As a result, [KOH]initial=[OH−]=0.0015M[KOH]initial=[OH−]=0.0015M and pOH=−log(0.0015)pOH=−log(0.0015). At 25°C25°C, pH+pOH=14.00pH+pOH=14.00; hence, pH=14.00−(−log(0.0015))pH=14.00−(−log(0.0015)). Related Videos 8.2: Daily Video 1 8.2: Daily Video 2
How many moles of Na+ ions are in 100.mL of 0.100MNa3PO4(aq) ?
C)0.0300molCorrect. Na3PO4Na3PO4 forms three Na+Na+ ions per formula unit when it dissociates in water. 100mL100mL of 0.100MNa3PO40.100MNa3PO4 contains 100mL×0.100mol/1000mL=0.0100mol100mL×0.100mol/1000mL=0.0100mol of Na3PO4Na3PO4, which dissociates into 0.0300mol0.0300mol of Na+Na+ ions. Related Videos 3.7: Daily Video 1
If Na reacts with chlorine to form NaCl, which of the following elements reacts with Na to form an ionic compound in a one-to-one ratio, and why?
D) BrBr, because it has the same number of valence electrons as ClCl. Correct. BrBr is in the same group as ClCl and has the same number of valence electrons, namely 7. Like ClCl, BrBr can accept an electron from NaNa to become a negatively charged ion (Br−Br−). Thus, BrBr forms the compound NaBrNaBr, which is analogous to NaClNaCl. Related Videos 1.8: Daily Video 1
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=∛Ksp/4 Correct. [Ag+]=2×S[Ag+]=2×S and [CO32−]=S[CO32−]=S. Since Ksp=[Ag+]2[CO32−]Ksp=[Ag+]2[CO32−], after substituting and rearranging S=∛Ksp/4. Related Videos 7.11: Daily Video 1 7.11: Daily Video 2 7.11: Daily Video 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 Based on the information given, which of the following is true?
D) Solutions with a higher concentration of (CH3)2NH(CH3)2NH have a higher pHpH because to reach equilibrium more OH−OH− is produced. Correct. Since the pHpH was measured at the same temperature, Kb=[(CH3)2NH2+][OH−]/[(CH3)2NH]Kb=[(CH3)2NH2+][OH−]/[(CH3)2NH] and this ratio is constant. Under the same conditions, increasing [(CH3)2NH][(CH3)2NH] results in an increase in [(CH3)2NH2+][(CH3)2NH2+] and [OH−][OH−] when equilibrium is reached. Related Videos 8.3: Daily Video 1 8.3: Daily Video 2 8.3: Daily Video 3
The diagram above shows the distribution of speeds for a sample of N2(g) at 25°C. Which of the following graphs shows the distribution of speeds for a sample of O2(g) at 25°C (dashed line) ?
A) https://assets.learnosity.com/organisations/537/VH912643.g05.png Correct. Two gases at the same temperature have molecules with the same average kinetic energy. Since KE=1/2*mA*v2A=1/2*mB*v2B for two gases, AA and BB, the gas with the higher molar mass will have molecules with a lower average speed. This graph shows the O2(g)O2(g) molecules with a lower average speed than the N2(g)N2(g) molecules. Related Videos 3.5: Daily Video 1 3.5: Daily Video 2
X(g)+Y(g)⇄XY(g) 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 XX and XYXY remained constant. Correct. After 155 minutes, the concentrations of XX and XYXY did not change, indicating that the reaction had reached equilibrium.
For an experiment, 50.0g of H2O was added to a coffee-cup calorimeter, as shown in the diagram above. The initial temperature of the H2O was 22.0°C, and it absorbed 300.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 H2O after thermal equilibrium was reached? Assume that the specific heat capacity of H2O is 4.2J/(g⋅K).
D) 23.4°C Correct. Since we know that the water absorbed heat, its final temperature must be higher than 22.0°C22.0°C. Rearranging q=mcΔTq=mcΔT so the amount of the temperature increase can be determined yields ΔT=(qm×c)=(300. J50.0 g×4.2J/(g⋅K))≈300210K≈1.4K or 1.4°CΔT=(qm×c)=(300. J50.0 g×4.2J/(g⋅K))≈300210K≈1.4K or 1.4°C. The temperature increased by about 1.4°C1.4°C, so the final temperature is approximately 23.4°C23.4°C. Related Videos 6.4: Daily Video 1 6.4: Daily Video 2
A piece of Fe(s) at 25°C is placed into H2O(l) at 75°C in an insulated container. A student predicts that when thermal equilibrium is reached, the Fe atoms, being more massive than the H2O molecules, will have a higher average kinetic energy than the H2O molecules. Which of the following best explains why the student's prediction is incorrect?
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. Correct: The student's claim contradicts the actual meaning (and formal definition) of what thermal equilibrium is. Related Videos 6.3: Daily Video 1
The graphs above show Maxwell-Boltzmann distributions for one-mole samples of Ar(g). Graph 1 shows the distribution of particle energies at 300K and graph 2 shows the distribution of particle energies at 600K. 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?
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. Correct. During collisions between the molecules of the hotter sample and the cooler sample, kinetic energy is redistributed until a new distribution, different from the original sample distributions is achieved at thermal equilibrium. Related Videos 6.3: Daily Video 1
A student uses visible spectrophotometry to determine the concentration of CoCl2(aq) in a sample solution. First the student prepares a set of CoCl2(aq) solutions of known concentration. Then the student uses a spectrophotometer to determine the absorbance of each of the standard solutions at a wavelength of 510nm and constructs a standard curve. Finally, the student determines the absorbance of the sample of unknown concentration. The original solution used to make the solutions for the standard curve was prepared by dissolving 2.60g of CoCl2 (molar mass 130.g/mol) in enough water to make 100.mL of solution. What is the molar concentration of the solution?
A) 0.200M Correct. Molarity =moles of solute/volume of solution=(2.60gCoCl/2100.mL)×(1molCoCl2/130.gCoCl2)×(1000mL/1L)=0.200M Related Videos 3.7: Daily Video 1
Which of the following statements, if true, would support the claim that the NO3− ion, represented above, has three resonance structures?
A) The NO3−NO3− ion is not a polar species. Correct. If the NO3−NO3− ion did NOT have three equivalent structures, then the ion would be polar because of the unequal distribution of charge within the ion. Equal contributions of the three resonance forms to the structure of the ion result in a planar ion with three equivalent polar nitrogen-oxygen bonds. Due to the trigonal planar symmetry of the NO3−NO3− ion, the negative dipoles cancel out. Related Videos 2.6: Daily Video 1 2.6: Daily Video 2
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] Correct. The pHpH of the original solution is lower than 3.83.8, as evidenced by the yellow color after the bromocresol green is added. Since pH<pKapH<pKa, the original solution must contain primarily HC2H3O2HC2H3O2 and a lower amount of NaC2H3O2NaC2H3O2. Related Videos 8.7: Daily Video 1
Which of the following represents an electron configuration that corresponds to the valence electrons of an element for which there is an especially large jump between the second and third ionization energies? (Note: n represents a principal quantum number equal to or greater than 2.)
A) ns2 Correct. The third ionization of this element would involve removing an electron that is closer to the nucleus (in the adjacent lower shell) than are the first two electrons. This would require much more energy than the removal of the first two electrons. Related Videos 1.7: Daily Video 1 1.7: Daily Video 2
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?
A) −68J/(molrxn⋅K) Correct. ΔS°ΔS° for a reaction is calculated using the following mathematical relationship: ΔS°=Σ(moles of substance×S°)products−Σ(moles of substance×S°)reactants. Using the values provided in the table and the stoichiometric coefficients for each substance, ΔS°=[(2×96)+240]-[(2×220)+60]=−68J/(molrxn⋅K). Related Videos 9.2: Daily Video 1
The energy distribution profile (Curve C) for the Y2 molecules is shown in the graph above for the reaction X+Y2→XY2 when it is done under certain experimental conditions. Line A represents the most probable energy of the Y2 molecules, and Line B represents the activation energy. Which of the following changes should be made to the graph to explain the increase in the rate of the reaction if the only change in experimental conditions is the addition of a catalyst?
B) Line B will move to the left because a larger fraction of the Y2Y2 molecules will have the minimum energy to overcome the activation energy barrier. Correct. A catalyst can provide a different reaction path with a lower activation energy to increase the rate of a reaction. Therefore, Line B will move to the left (lower activation energy) resulting in a larger fraction of the Y2Y2 molecules having the minimum energy to overcome the activation energy barrier.
In the following diagrams, elements are represented by X and Z, which form molecular compounds with one another. Which diagram represents a molecule that has a bent molecular geometry?
B) Single bond between both Z and X, and 2 electron pairs on X. Correct. There are four electron domains around the central atom XX, and their geometry is tetrahedral, which minimizes the repulsion between the four electron pairs. The Z-X-ZZ-X-Z bond angle is approximately 109.5°109.5°, and the molecular geometry is bent. Related Videos 2.7: Daily Video 1 2.7: Daily Video 2 2.7: Daily Video 3
Two different ionic compounds each contain only copper and chlorine. Both compounds are powders, one white and one brown. An elemental analysis is performed on each powder. Which of the following questions about the compounds is most likely to be answered by the results of the analysis?
B) What is the formula unit of each compound? Correct. From the elemental analysis, the moles of copper and chlorine can be calculated, then the mole ratio of copper to chlorine can be calculated. This ratio is the formula unit of the ionic compound. Related Videos 1.3: Daily Video 1 1.3: Daily Video 2 1.3: Daily Video 3
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[Y]eq≈1.5 M because the ratio [Z]eq/[Y]eq[Z]eq/[Y]eq should remain constant when the reaction is done at the same temperature. Correct. Kc=[Z]eq/[Y]eqKc=[Z]eq/[Y]eq is constant and [Z]eq,exp1/[Y]eq,exp1=[Z]eq,exp2/[Y]eq,exp2[Z]eq,exp1/[Y]eq,exp1=[Z]eq,exp2/[Y]eq,exp2. Therefore, 0.80/1.2=1.0/[Y]eq,exp20.801.2=1.0/[Y]eq,exp2 and [Y]eq≈1.5 M[Y]eq≈1.5 M. Related Videos 7.4: Daily Video 1
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?
B) ΔHrxn=[2(1072kJmol)+(498kJmol)]−4(799kJmol) Correct. An estimate for the enthalpy change in a reaction can be made by taking the sum of the bond energies of all the bonds broken and then subtracting the sum of the bond energies of all the bonds formed. In this reaction, a total of 2 moles of C≡OC≡O bonds and 1 mole of O=OO=O bonds are broken, while a total of 4 moles of C=OC=O bonds are formed. The mathematical expression that correctly summarizes this information is ΔHrxn=[2(1072kJmol)+(498kJmol)]−4(799kJmol) Related Videos 6.7: Daily Video 1 6.7: Daily Video 2
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?
C) 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°<0ΔG°<0. Correct. Based on the values of ΔH°ΔH° and ΔS°ΔS° provided, the reaction is thermodynamically favored (ΔG°<0)(ΔG°<0) at all temperatures. Diagram 2 represents a reaction with a higher activation energy than the reaction represented in diagram 1, and this can help explain why very little product formed. At 25°C25°C, very few molecules possess enough energy to overcome the high activation energy barrier, resulting in a very slow reaction rate. Related Videos 9.4: Daily Video 1
2H2(g)+2NO(g)→N2(g)+2H2O(g) For the chemical reaction represented above, the following mechanism is proposed. Step 1:2NO(g)⇌N2O2(g)(fast equilibrium) Step 2:N2O2(g)+H2(g)→N2O(g)+H2O(g)(slow) Step 3:N2O(g)+H2(g)→N2(g)+H2O(g)(fast) Which of the following initial rate law expressions is consistent with this proposed mechanism?
C) Rate=k[H2][NO]2 Correct. The initial rate expression based on the slow step is rate=k2[N2O2][H2]rate=k2[N2O2][H2]. But intermediates like N2O2N2O2do not appear in a rate law expression because their concentrations are generally too low to measure. Because the first step is a fast equilibrium step, [N2O2][N2O2] can be found in terms of the reactants of the reaction from the ratio of the rate constants for the forward and reverse reactions in step 1. At equilibrium the rate of the forward reaction equals the rate of the reverse reaction. Using the expression kf/kr=[N2O2]/[NO]2kf/kr=[N2O2]/[NO]2, rearranging, and solving for [N2O2][N2O2] yields [N2O2]=kf/kr[NO]2[N2O2]=kf/kr[NO]2. Substituting for [N2O2][N2O2] in the rate expression yields Rate=k[H2][NO]2Rate=k[H2][NO]2 , where k=k2*kf/krk=k2*kf/kr.
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?
C) The pHpH is slightly lower than 3.50 and it has a lower capacity for the addition of acids because less than 4.20g4.20g of NaF(s)NaF(s) was added. Correct. Since the NaF(s)NaF(s) used is only 90% pure, the actual amount of NaFNaF used to prepare the buffer is less than 4.20g4.20g, the amount needed for the pHpH to be approximately 3.50. Since there is a lower concentration of F−(aq)F−(aq) the buffer will have a pHpH slightly lower than 3.50. The lower concentration of F−(aq)F−(aq) results in a lower capacity for the addition of acids because less F−(aq)F−(aq) is available for the neutralization. Related Videos 8.10: Daily Video 1
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?
C) The values of ΔG°ΔG° for each reaction are not shown. Correct. In a coupled pair of reactions, the thermodynamic favorability of each component reaction and the overall reaction is central to the understanding of how coupled reactions work. In this case the blast furnace reaction is favorable because it is a combination of the unfavorable decomposition of iron oxide into its elements (ΔG°>0)(ΔG°>0) and the favorable combustion of carbon to form carbon dioxide (ΔG°<0)(ΔG°<0). Related Videos 9.6: Daily Video 1
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) AgOHAgOH is less soluble at higher pHpH because higher concentrations of OH−(aq)OH−(aq) shift the solubility equilibrium toward solid AgOHAgOH. Correct. Higher pHpH values mean higher concentrations of OH−(aq)OH−(aq). At higher values of [OH−][OH−], the equilibrium value of [Ag+][Ag+] will be smaller because [Ag+]=Ksp/[OH−][Ag+]=Ksp/[OH−]. Therefore, AgOHAgOH is less soluble at higher pHpH. Related Videos 7.13: Daily Video 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?
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. Correct. In this demonstration, the system can be defined the contents of the beaker. As the reaction between the two solids occurs, heat is absorbed from the surroundings, which includes the beaker and the water on the wooden board. The transfer of heat from the water on the board to the system causes the temperature of the water to decrease below its freezing point. As a result, the bottom of the beaker becomes frozen to the wood surface. Therefore, the reaction is an endothermic chemical change. Related Videos 6.1: Daily Video 1 6.1: Daily Video 2 6.1: Daily Video 3
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?
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. Correct. Segment D is longer because the enthalpy of vaporization is greater than the enthalpy of fusion. More energy is needed in order for molecules to have sufficient kinetic energy to completely leave the liquid and form a gas with widely spaced-apart molecules than for molecules to leave the bound crystalline state and move around as a liquid. Related Videos 6.5: Daily Video 1 6.5: Daily Video 2
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?
D) ΔG<0ΔG<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 HH and ClCl atoms forms. Correct. Although entropy decreases, not increases, as the reaction proceeds (ΔS<0ΔS<0), the amount of energy that is released (ΔH<0ΔH<0) as the bonds between the HH and ClCl atoms form is large enough to make the reaction thermodynamically favored (ΔG<0ΔG<0). Related Videos 9.3: Daily Video 1 9.3: Daily Video 2
The cell potential for the standard galvanic cell shown above is +1.56V. If AgNO3(aq)|Ag(s) is replaced with 1MPb(NO3)2(aq) solution and a Pb electrode, which of the following describes what happens to the operation of the cell, and why?
C) The cell potential decreases because the reduction of Pb2+Pb2+ is less thermodynamically favorable than the reduction of Ag+Ag+. Correct. Pb2+Pb2+ has less tendency to gain electrons than Ag+Ag+ does because the reduction potential for Pb2+Pb2+ is negative and the reduction potential for Ag+Ag+ is positive. Half-cell reactions with negative E°E° are not thermodynamically favored as written. As a result, the cell potential becomes lower than +1.56V+1.56V. Related Videos 9.7: Daily Video 1 9.7: Daily Video 2
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?
C) The operation of the cell requires at least 4.07V4.07V to be supplied because the reaction is not thermodynamically favorable. Correct. The overall reaction represents the reduction of Na+ and the oxidation of Cl−. Regardless of the type of cell, reduction takes place at the cathode and oxidation takes place at the anode. Therefore, E°cell=E°red(cathode)−E°red(anode)=[−2.71−(1.36)]V=−4.07V, which represents the minimum potential required for the electrolysis of molten NaCl. Because ΔG°rxn=−nFE°cell and for a thermodynamically favorable process ΔG°rxn<0, an E°cell=−4.07V means that the electrolysis of molten NaCl is not thermodynamically favorable and the source of the electrical energy needed must provide a potential of 4.07V or greater. Related Videos 9.8: Daily Video 1 9.8: Daily Video 2
Which of the following is the net ionic equation for the reaction between aqueous sodium fluoride and hydrochloric acid?
D) F−(aq)+H+(aq)→HF(aq) Correct. The correct net ionic equation includes the reacting species (F−andH+)(F−andH+) and the product (HF)(HF), a soluble gas. A net ionic equation does not include the spectator ions (Na+andCl−)(Na+andCl−). Related Videos 4.2: Daily Video 1 4.2: Daily Video 2
The particle diagram above represents an aqueous solution of a weak monoprotic acid. The white circles represent H atoms. Which of the following shows the species that act as a Brønsted-Lowry base and its conjugate acid, in that order, in the solution?
D) H2O on the left and H3O on the right. Correct. In an aqueous solution of a weak acid, the water molecule behaves as a Brønsted-Lowry base. The hydronium ion formed is the conjugate acid of water. Related Videos 4.8: Daily Video 1
The electron cloud of HF is smaller than that of F2 , however, HF has a much higher boiling point than F2 has. Which of the following explains how the dispersion-force model of intermolecular attraction does not account for the unusually high boiling point of HF?
D) Liquid F2F2 has weak dispersion force attractions between its molecules, whereas liquid HFHF has both weak dispersion force attractions and hydrogen bonding interactions between its molecules. Correct. The hydrogen bonding interactions in HFHF are much stronger than the weak dispersion forces between F2F2 molecules. Related Videos 3.1: Daily Video 1 3.1: Daily Video 2 3.1: Daily Video 3