topic q

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) B +68J/(molrxn⋅K) C −56J/(molrxn⋅K) D +56J/(molrxn⋅K)

A

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? 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

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? A [Zn2+]=0.5M and [Fe2+]=1M because Q<1. B [Zn2+]=1M and [Fe2+]=2M because Q>1. C [Zn2+]=2M and [Fe2+]=0.5M because Q>1. D [Zn2+]=2M and [Fe2+]=1M because Q<1.

C

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? 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

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) ? A [Cu2+]=[Mg2+]=0.10M, because equilibrium is approached at a faster rate, causing a decrease in Ecell. B [Cu2+]=[Mg2+]=2.0M, because equilibrium is approached at a slower rate, causing a decrease in Ecell. C [Cu2+]=1.0M and [Mg2+]=0.10M, because Q is smaller than 1, making Ecelldecrease. D [Cu2+]=0.10M and [Mg2+]=1.0M, because Q is greater than 1, making Ecelldecrease.

D

Which of the following particle diagrams represents a process during which the entropy of the system increases? A B C D

A

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) ? 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 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⋅K) B S°=12[−187+(2×257)−205]J/(mol⋅K) C S°=12[187+(2×257)−205]J/(mol⋅K) D S°=[−187+257−205]J/(mol⋅K)

C

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

CO(g)+2H2(g)⇄CH3OH(g)K=2.2×104at298K 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

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? 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 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? A Mass=(0.80×30.0×6096,485×107.9)g B Mass=(0.80×30.0×60×96,485107.9)g C Mass=(0.80×30.0×60×107.996,485)g D Mass=(0.80×30.0×107.996,485)g

C

CuCl2(aq)→Cu(s)+Cl2(g) 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? A I=(2×0.125×96,4853,600)A B I=(2×0.125×96,4851)A C I=(0.125×96,4853,600)A D I=(0.125×96,4851)A

A

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? 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

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? 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 Clatoms forms, entropy increases because the number of gaseous product particles is 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

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? A K=e−490,0008.314×298<<1 and at equilibrium PPOCl3>>PPCl3. B K=e8.314×298490,000>1 and at equilibrium PO2>PPOCl3. C K=e−4908.314×298<1 and at equilibrium PPOCl3<PPCl3. D K=e8.314×298490>>1 and at equilibrium PO2>>PPOCl3.

A

4Fe(s)+3O2(g)→2Fe2O3(s)ΔG°=−1500kJ/molrxn 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. Test Tube Experimental Conditions Inside the Capped Test Tube Observations 1 Dry air No visible rust on nail 2 Air and water Rust suspended in the water and on the nail 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. B The reaction is less thermodynamically favored because the Gibbs free energy of the product is greater when water is not present. C The product is not formed in measurable quantities because the equilibrium constant for the reaction when water is not present is significantly less than one. D The rate of reaction is much slower because the oxygen molecules collide with the iron surface with less energy when water is not present.

A

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? 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 be 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 will be not be thermodynamically favorable at any temperature.

A

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°rxn 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 +170J/(molrxn⋅K) B −170J/(molrxn⋅K) C +190J/(molrxn⋅K) D −190J/(molrxn⋅K)

B

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 withouthaving to use a balance? A Ecell=E°cell−RTnFlnQ. B I=qt C ΔG°=−nFE° D ΔG°=ΔH°−TΔS°

B

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 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

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? 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 5.43V because the reaction is thermodynamically favorable. C The operation of the cell requires at least 4.07V to be supplied because the reaction is not thermodynamically favorable. D The operation of the cell requires at least 2.72V to be supplied because the reaction is not thermodynamically favorable.

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 from the 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

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 2000Kand 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

Cu2+(aq)+Zn(s)→Cu(s)+Zn2+(aq) E°=+1.10V 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 +3.17V 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? A The mass of the Ag electrode in the second cell is greater than in first, resulting in a reduction potential greater than 0.80V. B The mass of the Mg electrode in the second cell is smaller than in the first, resulting in a reduction potential lower than −2.37V. C [Mg2+]>1M and [Ag+]=1M in the second cell, resulting in Q>1, and Q=1for the first cell. D [Ag+]>1M and [Mg2+]=1M in the second cell, resulting in Q<1, and Q=1for the first cell.

D

To construct the galvanic cell illustrated above, the salt bridge was prepared by soaking a piece of cotton in 5.0MNaNO3(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