unit 9
At 25°C, the equilibrium constant for the reaction represented above has a value of 1.3. At 50°C, the value of the equilibrium constant is less than 1.3. Based on this information, which of the following must be correct? A The reaction rate decreases as the temperature is increased. B The reaction is thermodynamically favorable only at temperatures above 25°C. C At 25°C, ∆G° for the reaction is positive. D At 25°C, ∆S° for the reaction is positive. E At 25°C, ∆H° for the reaction is negative.
e
Quantity that would be zero for a pure, perfect crystal at 0 K
entropy
In the electroplating of nickel, 0.200 faraday of electrical charge is passed through a solution of NiSO4. What mass of nickel is deposited? A) 2.94 g B) 5.87 g C) 11.7 g D) 58.7 g E) 294 g
b
2POCl3(g) ⇄ 2PCl3(g) + O2(g) ΔG°rxn = +490 kJ/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 = +490 kJ/mol, which of the following is true?
A. K=e−490,000/8.314×298<<1 and at equilibrium PPOCl3>>PPCl3
In galvanic cells Y and Z, which of the following takes place in half-cell 3 ? A Reduction occurs in both cell Y and cell Z. B Oxidation occurs in both cell Y and cell Z. C Reduction occurs in cell Y, and oxidation occurs in cell Z. D Oxidation occurs in cell Y, and reduction occurs in cell Z.
c
The diagram above represents an electrolytic cell in which the reaction 2 NaCl(l) → 2 Na(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? A The operation of the cell generates a potential of 1.35V because the reaction is thermodynamically favorable. B The operation of the cell generates a potential of 5.43 V because the reaction is thermodynamically favorable. C The operation of the cell requires at least 4.07 V to be supplied because the reaction is not thermodynamically favorable. D The operation of the cell requires at least 2.72 V to be supplied because the reaction is not thermodynamically favorable.
c
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)? A S° = −[−187−257+205] J mol-1K-1 B S° = 1⁄2[−187+(2×257)−205] J mol-1K-1 C S° = 1⁄2[187+(2×257)−205] J mol-1K-1 D S °= [−187+257−205] J mol-1K-1
c
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 ? a. The total number of gaseous product molecules is less than the total number of gaseous reactant molecules, thus ΔS<0 . b. The total number of gaseous product molecules is greater than the total number of gaseous reactant molecules, thus ΔS>0. c. The amount of energy released when the product bonds form is much less than the amount of energy needed to break the reactant bonds. d. The amount of energy released when the product bonds form is much greater than the amount of energy needed to break the reactant bonds.
d
The equation above represents the decomposition of a compoundXY2. The diagram below shows two reaction profiles (path one and path two) for the decomposition of XY2. The reaction is thermodynamically favorable under standard conditions at 298 K. Therefore, the value of ΔS° for the reaction must be
greater than H/298K
For which of the following processes would ΔS have a negative value? I. 2 Fe2O3(s) → 4 Fe(s) + 3 O2(g) II. Mg2+ + 2 OH- → Mg(OH)2(s) III. H2(g) + C2H4(g) → C2H6(g)
II and III only
Which of the following particle diagrams represents a process during which the entropy of the system increases?
a. molecules stacked together then 6 groups of 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? A Oxidation is occurring at the anode. B Molten Na is shown at the cathode. C An external source of energy is not shown. D The direction of the electron flow in the wires is incorrect.
c
Using the information above, which of the following is true for the decomposition of H2O2(aq)?
ΔG° < 0 and Keq > 1
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?
(0.80×30.0×60×107.996,485)g
An electric current of 1.00 ampere is passed through an aqueous solution of Ni(NO3)2. How long will it take to plate out exactly 1.00 mol of nickel metal, assuming 100 percent current efficiency? (1 faraday = 96,500Ocoulombs = 6.02 X 1023 electrons)
193,000 s
According to the standard reduction potentials given above, what is the standard cell potential for the reaction represented below?
2.46V
Cu(s) + 2 Ag+ → Cu2+ + 2 Ag(s) If the equilibrium constant for the reaction above is 3.7 x 1015, which of the following correctly describes the standard voltage, E ̊, and the standard free energy change, ∆G ̊, for this reaction? A E ̊ is positive and ∆G ̊ is negative. B E ̊ is negative and ∆G ̊ is positive. C E ̊ and ∆G ̊ are both positive. D E ̊ and ∆G ̊ are both negative. E E ̊ and ∆G ̊ are both zero.
A E ̊ is positive and ∆G ̊ is negative.
According to the information above, what is the standard reduction potential for the half-reaction M3+(aq) + 3 e- → M(s)
A. -1.66V
Which of the following equations represents a reaction for which the standard entropy change is positive (ΔS° > 0) ? a. 3O₂(g)→2O₃(g) b. 2H₂(g)+O₂(g)→2H₂O(l) c. CaCO₃(s)→CaO(s)+CO₂(g) d. I₂(g)+2K→2KI
CaCO₃(s)→CaO(s)+CO₂(g)
The decomposition of H2O2(aq) is represented by the equation above. The reaction is thermodynamically favorable. The signs of ΔG° and ΔS° for the reaction are which of the following?
G= negative, S= positive
The diagram opposite 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 0.125 mol of Cu(s) in 1 hour?
I=(2×0.125×96,4853,600)A
The elements K and Cl react directly to form the compound KCl according to the equation above. It is observed that the reaction producing KCl from its elements goes essentially to completion. Which of the following is a true statement about the thermodynamic favorability of the reaction? A The reaction is favorable and driven by an enthalpy change only. B The reaction is unfavorable and driven by an entropy change only. C The reaction is favorable and driven by both enthalpy and entropy changes. D The reaction is unfavorable due to both enthalpy and entropy changes.
a
2 H2(g) + O2(g) → 2 H2O(g) For the reaction represented above at 25°C, what are the signs of ΔH°, ΔS°, and ΔG°?
all negative
For the reaction represented opposite at 25°C, what are the signs of ΔH°, ΔS°, and ΔG°?
all negative
On the basis of the information above, the dissolution of FeF2(s) in acidic solution is A. thermodynamically favorable, because K2 > 1 B. thermodynamically favorable, because K3 > 1 C. not thermodynamically favorable, because K1 < 1 D. not thermodynamically favorable, because K3 < 1
b
The spontaneous reaction that occurs when the cell opposite operates is 2 Ag+ + Cd(s) → 2 Ag(s) + Cd2+ Which of the above occurs for the following circumstance? The salt bridge is replaced by a platinum wire. A Voltage increases. B Voltage decreases but remains above zero. C Voltage becomes zero and remains at zero D No change in voltage occurs E Direction of voltage change cannot be predicted without additional information.
c
Which of the following processes involves the greatest increase in entropy? a. SO 3 (g) + H 2 (g)--> SO 2 (g) + H 2 O(g) b. N 2 (g) + 3 H 2 (g)--> 2 NH 3 (g) c. Ag + (aq) + Cl - (aq)--> AgCl(s) d. C 2 H 2 (g) + 2 H 2 (g) --> C 2 H 6 (g) e.MgSO 3 (s)--> MgO(s) + SO 2 (g)
e. MgSO3 (s) → MgO (s) + SO2(g)
Of the following reactions, which involves the largest decrease in entropy? A. CaCO3(s) → CaO(s) + CO2(g) B. 2 CO(g) + O2(g) → 2 CO2(g) C. Pb(NO3)2(s) + 2 KI(s) → PbI2(s) + 2 KNO3(s) D. C3H8(g) + 5 O2(g) → 3 CO2(g) + 4 H2O(g) E. 4 La + 3 O → 2 La O(s) 2(g) 2 3(s)
E.
When a magnesium wire is dipped into a solution of lead (II) nitrate, a black deposit forms on the wire. Which of the following can be concluded from this observation? A The standard reduction potential, E°, for Pb2+(aq) is greater than that for Mg2+(aq). B Mg(s) is less easily oxidized than Pb(s). C An external source of potential must have been supplied. D The magnesium wire will be the cathode of a Mg/Pb cell.. E Pb(s) can spontaneously displace Mg2+(aq) from solution
a
Consider the reaction represented above at 298K. When equal volumes of N2(g) and H2(g), each at 1 atm, are mixed in a closed container at 298K, no formation of NH3(g) is observed. Which of the following best explains the observation. A. The N2(g) and the H2(g) must be mixed in a 1:3 ratio for a reaction to occur. B A high activation energy makes the forward reaction extremely slow at 298 K. C The reaction has an extremely small equilibrium constant, thus almost no product will form. D The reverse reaction has a lower activation energy than the forward reaction, so the forward reaction does not occur.
b
N2(g) + 3H2(g) ⇄ 2NH3(g) K = 5.6 × 105 at 298K ΔH°rxn = −91.8 kJ/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? A. ΔG° = −RTlnK > 0 because K >> 1 B. ΔG° = −RTlnK < 0 because K >> 1 C. ΔG° = ΔH° − TΔS° < 0 because ΔH° < 0 and ΔS° > 0 D. ΔG° = ΔH° − TΔS° > 0 because ΔH° < 0 and ΔS° < 0
b
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 ? A ΔS°rxn is negative because the number of N2O4 molecules increases as the reaction proceeds. B ΔS° is negative because the number of molecules in the gas phase decreases as the reaction proceeds C ΔS°rxn is positive because the number of N2O4 molecules increases as the reaction proceeds. D ΔS°rxn is positive because the number of molecules in the gas phase decreases as the reaction proceeds.
b
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? A +170 J mol-1K-1 B -170 J mol-1K-1 C +190 J mol-1K-1 D -190 J mol-1K-1
b
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 opposite, can be used to make the reaction occur. Which of the following identifies a flaw in the representation? A Oxidation is occurring at the anode instead of at the cathode. B The equation for the reaction is not correctly balanced. C Electrical energy is needed for the reaction to proceed. D The relative volumes of collected gas shown in each tube are incorrect.
b
The spontaneous reaction that occurs when the cell opposite operates is 2 Ag+ + Cd(s) → 2 Ag(s) + Cd2+ Which of the above occurs for the following circumstance? A 50-milliliter sample of a 2-molar Cd(NO3)2 solution is added to the left beaker. OA Voltage increases. B Voltage decreases but remains above zero. C Voltage becomes zero and remains at zero D No change in voltage occurs E Direction of voltage change cannot be predicted without additional information.
b
The spontaneous reaction that occurs when the cell opposite operates is 2 Ag+ + Cd(s) → 2 Ag(s) + Cd2+ Which of the above occurs for the following circumstance? Current is allowed to flow for 5 minutes A Voltage increases. B Voltage decreases but remains above zero. C Voltage becomes zero and remains at zero D No change in voltage occurs E Direction of voltage change cannot be predicted without additional information.
b
The spontaneous reaction that occurs when the cell opposite operates is 2 Ag+ + Cd(s) → 2 Ag(s) + Cd2+ Which of the above occurs for the following circumstance? The silver electrode is made larger. A Voltage increases. B Voltage decreases but remains above zero. C Voltage becomes zero and remains at zero D No change in voltage occurs E Direction of voltage change cannot be predicted without additional information.
b
Which of the following reactions is not thermodynamically favored at low temperatures but becomes favored as the temperature increases? A) 2 CO(g) + O2(g) ➝ 2 CO2(g) ΔH° = -566 kJ, ΔS° = -173 J/K B) 2 H2O(g) ➝ 2 H2(g) + O2(g) ΔH° = 484 kJ, ΔS°= 90.0 J/K C) 2 N2O(g) ➝ 2 N2(g) + O2(g) ΔH° = -164 kJ, ΔS° = 149 J/K D) PbCl2(s) ➝ Pb2+(aq) + 2 Cl-(aq) ΔH° = 23.4 kJ, ΔS° = -12.5 J/K
b
2 H2O2(aq) → 2 H2O(l) + O2(g) E= 0.55V O2(g) + 4 H+(aq) 4 e- → 2 H2O(l) E= 1.23 V Using the information above, what is the standard reduction potential for the half-reaction represented? A -1.78 V B -0.68 V C +0.68 V D +1.78 V
c
A cube of ice is added to some hot water in a rigid, insulated container, which is then sealed. There is no heat exchange with the surroundings. What has happened to the total energy and the total entropy when the system reaches equilibrium? a. energy remains constant, entropy remains constant b. energy remains constant, entropy decreases c. energy remains constant, entropy increases d. energy decreases, entropy increases e. energy increases, entropy decreases
c
HA(aq) + H2O(l) ⇄ A−(aq) + H3O+(aq) ΔG° = +35 kJ/molrxn Based on the chemical equation and ΔG° given above, which of the following justifies the claim that HA(aq) is a weak acid? A Because ΔG° >> 0 , Ka >> 1 , and HA completely dissociates. B Because ΔG° >> 0 , Ka >> 1 , and HA almost completely dissociates. C Because ΔG° >> 0 , Ka << 1 , and HA only partially dissociates. D Because ΔG° >> 0 , Ka << 1 , and HA does not dissociate.
c
If the half-cell containing 1.00 M Fe(NO3)2(aq) in galvanic cells Y and Z is replaced with a half-cell containing 5.00 M Fe(NO3)2(aq), what will be the effect on the cell voltage of the two galvanic cells? A The voltage will increase in both cells B The voltage will decrease in both cells. C The voltage will increase in cell Y and decrease in cell Z. D The voltage will decrease in cell Y and increase in cell Z.
c
The cell potential for the standard galvanic cell shown above is +1.56V . If AgNO3(aq)|Ag(s) is replaced with 1M Pb(NO3)2(aq) solution and a Pb electrode, which of the following describes what happens to the operation of the cell, and why? A Nothing changes because galvanic cells that have a Zn(s) electrode have a constant cell potential, E°cell , of +1.56V. B The cell stops generating a voltage because the standard reduction potentials of Pb2+ and Zn2+ are both negative. C The cell potential decreases because the reduction of Pb2+ is less thermodynamically favorable than the reduction of Ag+. D The cell potential increases because twice as many electrons are transferred between Pb2+ and Zn than between Ag+ and Zn.
c
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? A Diagram 1, because it represents a reaction that is not thermodynamically favorable with ΔG° > 0, regardless of its reaction rate. B Diagram 1, because it represents a reaction that reaches equilibrium quickly after a very small amount of the reactants is consumed. 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° < 0. D Diagram 2, because it represents a reaction that is thermodynamically favorable with ΔH° < 0, but the products formed are unstable and quickly revert to form reactants.
c
The reaction represented above goes essentially to completion. The reaction takes place in a rigid, insulated vessel that is initially at 600 K. What can be inferred about ∆S° for the reaction at 600 K? A. It must be positive, since the reaction is thermodynamically unfavorable at 600 K. B. It must be negative, since there are more moles of products than reactants. C. It must be positive, since ∆G° is negative and ∆H° is positive. D. It must be negative, since ∆G° is positive and ∆H° is positive.
c
Based on the reduction potentials given in the table opposite, which of the following gives the balanced chemical equation and the correct standard cell potential for a galvanic cell involving Sc3+ and Mn2+ ? A 2Sc3+(aq)+3Mn(s)⇄2Sc(s)+3Mn2+(aq) E°=−0.90V B 2Sc3+(aq)+3Mn(s)⇄2Sc(s)+3Mn2+(aq) E°=−0.62V C 2Sc(s)+3Mn2+(aq)⇄2Sc3+(aq)+3Mn(s) E°=+0.62V D 2Sc(s)+3Mn2+(aq)⇄2Sc3+(aq)+3Mn(s) E°=+0.90V
d
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? a. 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. B 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 not be thermodynamically favorable at any temperature. C 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 all temperatures. D 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 willnot be thermodynamically favorable at any temperature.
a
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 -68 J mol-1K-1 B +68 J mol-1K-1 C -56 J mol-1K-1 D +56J mol-1K-1
a
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? A E°cell=[+0.40−(−2.31)]V and ΔG°rxn=−(12×96,500×2.71)1,000kJ B E°cell=[+0.40−(−2.31)]V and ΔG°rxn=−(4×96,500×2.71)1,000kJ C E°cell=[−2.31−(+0.40)]V and ΔG°rxn=−(12×96,500×−2.71)1,000kJ D E°cell=[−2.31−(+0.40)]V and ΔG°rxn=−3×96,500×−2.711,000kJ
a
The reaction of iron with oxygen to form rust is represented by the equation shown above. A student cleans two iron nails and places each nail in a capped test tube. The following table gives the experimental conditions and the student's observations after one week at room temperature. 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 ?
a. 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.
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? A 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 its reactant molecules had a higher average kinetic energy. B 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. C The reaction is not thermodynamically favorable because ΔG° > 0 based on the value of K, but the addition of a catalyst improved the orientation of the reactants during collisions, allowing the catalyzed reaction to proceed in one hour. D The reaction is not thermodynamically favorable because ΔG° > 0 based on the value of K , but the catalyzed reaction could proceed in one hour because it has a lower ΔH and a higher ΔS.
b
According to the half-reactions represented above, which of the following occurs in aqueous solutions under standard conditions? Cu2+(aq) + 2 e- → Cu(s) E° = 0.34 V Cr3+(aq) + e- → Cr2+(aq) E° = -0.41 V
b
At 1.0 atm a sample of ice is heated to liquid water and then to water vapor. The heating curve is shown in the graph above. Which of the following lists the signs for the changes in enthalpy and entropy for the process corresponding to segment X, going from left to right? a. h= positive, s= negative b. h= positive, s= postive c. h= negative, s= negative d. h= negative, s= positive
b
The vanadium-lead galvanic cell in the diagram opposite initially operates under standard conditions according to the net reaction shown in the table below. 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? A Ecell=E°cell−RTnFlnQ. B I=qt C ΔG°=−nFE D ΔG°=ΔH°−TΔS°
b
A galvanic cell generates a cell potential of +0.32 V 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? A [Zn2+] = 0.5 M and [Fe2+] = 1 M because Q < 1 . B [Zn2+] = 1 M and [Fe2+] = 2 M because Q > 1 . C [Zn2+] = 2 M and [Fe2+] = 0.5 M because Q > 1 D [Zn2+] = 2 M and [Fe2+] = 1 M because Q < 1 .
c
An electrolytic cell based on the reaction represented above was constructed from zinc and copper half-cells. The observed voltage was found to be 1.00 volt instead of the standard cell potential, E°, of 1.10 volts. Which of the following could correctly account for this observation? A The copper electrode was larger than the zinc electrode. B The Zn2+ electrolyte was Zn(NO3)2, while the Cu2+ electrolyte was CuSO4. C The Zn2+ solution was more concentrated than the Cu2+ solution. D The solutions in the half-cells had different volumes. E The salt bridge contained KCl as the electrolyte.
c
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. Which of the following identifies a limitation in how the representations above describe a system of coupled reactions? A None of the equations are balanced. B The values of ΔS° for each reaction are not shown. C The values of ΔG° for each reaction are not shown. D The coupled reactions lack a common intermediate species.
c
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? A The entropy values for both samples are positive and ΔS°sample2 = ΔS°sample1 , because the changes observed for both samples required a net input of energy fromthe surroundings. B The entropy values for both samples are negative and ΔS°sample2 = ΔS°sample1 , because the changes observed for both samples released energy to the surroundings. C 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. D The entropy values for both samples are negative and ΔS°sample2 < ΔS°sample1 , because the change in the spatial distribution of the molecules was greater for sample 2 than for sample 1.
c
When water is added to a mixture of Na2O2(s) and S(s) , a redox reaction occurs, as represented by the equation below. Which of the following statements about the thermodynamic favorability of the reaction at 298 K is correct? A It is thermodynamically unfavorable. B It is thermodynamically favorable and is driven by ΔS° only. C It is thermodynamically favorable and is driven by ΔH° only. D It is thermodynamically favorable and is driven by both ΔH° and ΔS°.
c
A galvanic cell generates a potential of +2.71 V when [Cu2+] = [Mg2+] = 1 M 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 < +2.71 V) ? A [Cu2+] = [Mg2+] = 0.10 M, because equilibrium is approached at a faster rate, causing a decrease in Ecell . B [Cu2+] = [Mg2+] = 2.0 M, because equilibrium is approached at a slower rate, causing a decrease in Ecell . C [Cu2+] = 1.0 M and [Mg2+] = 0.10 M , because Q is smaller than 1, making Ecell decrease D [Cu2+] = 0.10 M and [Mg2+] = 1.0 M , because Q is greater than 1, making Ecell decrease.
d
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? A. ΔG > 0 because energy is released as the bond between the H and Cl atoms forms, and entropy increases because the number of gaseous product particles is less than the number of gaseous reactant particles. B ΔG > 0 because energy is absorbed as the bond between the H and Cl atoms forms, and entropy decreases because the number of gaseous product particles is less than the number of gaseous reactant particles. C ΔG < 0 because although energy is absorbed as the bond between the H and Cl atoms forms, entropy increases because the number of gaseous product particles is O less than the number of gaseous reactant particles. D 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.
d
The galvanic cell illustrated above generates a potential of +1.10V. For the construction of a second galvanic cell (not shown), only one modification was made: the Cu electrode has double the mass of the Cu electrode in the first cell. Which of the following correctly compares the initial E° for the second cell to that of first cell at 298K, and why? A The initial E° for the second cell is twice that of the first cell, because a larger amount of Cu(s) can be oxidized to Cu2+. B The initial E° for the second cell is twice that of the first cell, because more Cu2+ ions can be deposited on the solid Cu electrode. C The initial E° for the second cell is half that of the first cell, because the greater amount Cu(s) in the half-cell inhibits the formation of more Cu(s). D The initial E° for the second cell is the same as for the first cell, because the overall chemical reaction that occurs in the cell does not change.
d
The galvanic cell shown above generates a cell potential of +3.17 V when operated under standard conditions. A second galvanic cell is made from the same two metals, andthe measured cell potential is +3.25 V. Which of the following could be the reason for the second cell having a greater cell potential? A The mass of the Ag electrode in the second cell is greater than in first, resulting in a reduction potential greater than 0.80 V . B The mass of the Mg electrode in the second cell is smaller than in the first, resulting in a reduction potential lower than −2.37 V . C [Mg2+] > 1M and [Ag+] = 1M in the second cell, resulting in Q >1 , and Q = 1 for the first cell D [Ag+] > 1M and [Mg2+] = 1M in the second cell, resulting in Q <1 , and Q = 1 for the first cell.
d
To construct the galvanic cell illustrated above, the salt bridge was prepared by soakinga piece of cotton in 5.0M NaNO3(aq) before placing it inside the U-shaped tube filled with distilled water. If the cotton was soaked in distilled water by mistake, which of the following best explains how the operation of the cell would be affected? A The operation of the cell is not affected because neither Na+(aq) nor NO3−(aq) is involved in the redox reaction that takes place. B The operation of the cell generates a higher potential because there are fewer ions in the solution, making the reaction more thermodynamically favored. C The cell will operate for a much longer time because the flow of electrons through the circuit will eventually be reversed. D The cell would not operate because a current could not be conducted between the half-cells.
d
When solid ammonium chloride, NH4Cl(s), is added to water at 25°C, it dissolves and the temperature of the solution decreases. Which of the following is true for the values of ΔH and ΔS for the dissolving process?
h= positive, s= positive
