AP Chemistry Unit 9 Progress Check: MCQ

The table above provides approximate S° values for several substances. Based on the information, which of the following reactions has the largest increase in entropy, ΔS° ?

A. 2C(s)+O2(g)→2CO(g)

In an experiment done at a low temperature, one hour after the reaction started, Y was produced but almost no Z was produced. At a much higher temperature, one hour after the reaction started, both Y and Z had been produced. Which of the following best explains why very little Z was produced at the lower temperature?

A. A smaller number of X molecules had enough energy to overcome the activation energy barrier for reaction 2, and very little Z was produced.

The graphs above represent the changes in concentration over time for the reactant and product of two separate reactions at 25°C. Based on these graphs, which of the following best supports the claim that reaction 1 is thermodynamically favorable but reaction 2 is not?

A. At equilibrium, for reaction 1 [Product]/[Reactant]>>1 and ΔG°rxn<0 but for reaction 2 [Product]/[Reactant]<<1 and ΔG°rxn>0ΔG°rxn>0.

The diagrams above represent physical changes for potassium. Which of the following correctly identifies the physical process and provides the correct ΔS° for the process in the indicated diagram?

A. In diagram 1, the process is freezing and ΔS°freezing is negative.

Under standard conditions, the galvanic cell shown above has a cell potential of +1.56V using the reaction given. The salt bridge contains KNO3, which allows K+ ions and NO3− ions to move in the directions indicated. If KNO3 in the salt bridge is replaced with KOH, some Zn(OH)2(s) precipitates in the Zn-Zn(NO3)2 half-cell. Which of the following best explains how the cell potential is affected as Zn(OH)2(s) starts to precipitate, and why?

A. The cell potential increases because the concentration of Zn2+(aq) decreases and Q, [Zn2+]/[Ag+]2, becomes smaller.

The use of zinc-mercury cells in hearing aids has been replaced by zinc-air cells that operate using the oxidation of Zn by O2 from the air, generating a potential of +1.60V. Table 2 provides the standard reduction potentials for the half-reactions used in zinc-mercury and zinc-air cells. Which of the following best explains the modification to the cell design that is mostly responsible for the difference in standard cell potentials for zinc-mercury and zinc-air cells?

A. The greater standard cell potential of the Zn-air cell compared to that of the zinc-mercury cell most likely results from the thermodynamically more favorable reduction of O2 compared to HgO.

Under standard conditions, a Ni-Cd galvanic cell generates a potential of +0.15V using the chemical reaction represented above. Which of the following best explains how [Ni2+]>1M affects the cell potential if this is the only change made to the operation of the galvanic cell?

A. The ratio [Cd2+]/[Ni2+][ is smaller if [Ni2+]>1M, resulting in Q<1 and Ecell>E°cell.

At 720°C, the reaction represented above has an equilibrium constant of 1.0×10−30. Which of the following mathematical expressions can be used to support the claim that the reaction is not thermodynamically favored under these conditions?

A. ΔG°rxn=−8.314×(720+273)×ln(1.0×10−30)/1,000 > 0

Based on the information given, which of the following is a major difference between the zinc-mercury cell and the lithium-iodine cell?

B. During the initial cell operation, each reaction is thermodynamically favorable, but the larger operating potential of the lithium-iodine cell indicates that its cell reaction is more thermodynamically favorable.

The operation of a hydrogen fuel cell under standard conditions relies on the chemical reaction represented above. The table provides the relevant reduction half-reactions and the standard reduction potentials. Based on the information given, which of the following equations can be used to calculate the standard reduction potential, in volts, of the half-reaction occurring at the cathode?

B. E°red(cathode)=−(−474,000/4×96,500)

Two samples containing an equal number of moles of N2(g) are kept inside separate 1-liter rigid containers. The particle diagrams above show the distribution of molecular speeds in each sample. Based on the information given, which of the following identifies the sample with the greater S°, and why?

B. Sample 1, because the N2 molecules have a larger distribution of energies compared to sample 2.

The chemical equation above represents the combustion of glucose, and the table provides the approximate standard absolute entropies, S°, for some substances. Based on the information given, which of these equations can be used to calculate an approximation of S° for H2O(g) ?

B. S°=1/6[900+209+(6×205)−(6×214)]J/(mol⋅K)

A 5.00g-sample of KOH(s) at 25.0°C was added to 100.0g of H2O(l) at room temperature inside an insulated cup calorimeter, and the contents were stirred. After all the KOH(s) dissolved, the temperature of the solution had increased. Based on the information given, which of the following best justifies the claim that the dissolution of KOH(s) is a thermodynamically favorable process?

B. The energy required to break the bonds between the ions in the solid is less than that released as the ion-dipole attractions form during solvation, thus ΔH<0. Also, the ions become more widely dispersed as KOH(s) dissolves, thus ΔS>0. Therefore, ΔG<0.

Two different electrolytic cells are constructed to electroplate two identical objects based on the half-reactions given in the table above. An object will be plated with a certain number of moles of Au in one cell, and in the other cell another object will be plated with the same number of moles of Ag in the same amount of time. Which of the following mathematical relationships can be used to determine how much more current is needed to electroplate an object with Au than to electroplate it with Ag?

B. Three times more electrons are needed to reduce Au3+(aq) to Au(s) than are needed to reduce Ag+(aq) to Ag(s).

The chemical equations above represent the main reactions that occur during the production of Fe(l) under certain conditions. The overall reaction couples reactions 1 and 2, resulting in a thermodynamically favorable process. Which of the following best explains whether or not a particle diagram could represent how the coupling of reaction 1 and reaction 2 results in ΔG°rxn<0 ?

C. A particle diagram cannot represent how the changes in energy that take place as reaction 1 occurs are more than offset by the changes in energy taking place as reaction 2 occurs, resulting in a thermodynamically favorable overall reaction.

The diagrams above illustrate the equipment used to electroplate four identical objects with silver or zinc. The table provides the conditions used. Which of the following provides the basis for the identification of the object that requires the highest current, I, to complete the electroplating?

C. Object 3, based on the coulombs of charge needed and the time of operation.

A standard galvanic cell is made using Pb-Pb(NO3)2 and Cu-Cu(NO3)2 half-cells. Which of the following modifications to the cell will cause the greatest increase in E°cell, and why? Assume all solutions are 1M.

C. Replacing the Pb-Pb(NO3)2 half-cell with an Al-Al(NO3)3 half-cell, because the oxidation of Al(s) is more thermodynamically favorable than the oxidation of Pb(s), resulting in E°cell=+2.00V.

The particle diagrams above represent NaCl(l) and its decomposition into Na(l) and Cl2(g) in an electrochemical cell after voltage is applied. Which of the following statements about the thermodynamic favorability of the decomposition of NaCl(l) is supported by the particle diagrams and why?

C. The decomposition is not thermodynamically favored because the pure elements form only after electrical energy is supplied

The galvanic cell illustrated above was constructed using a salt bridge containing KNO3. A second cell is constructed from identical half-cells but uses NaNO3 for the salt bridge. Which of the following best explains whether the initial potential of the second cell will be different from the initial potential of the first cell?

C. The initial potential of the second cell will be the same as the first cell because the ions from the salt bridge are not oxidized or reduced during cell operation.

The chemical equation and thermodynamic data for the melting of tungsten are given above. Based on this information, which of the following provides the best prediction with correct justification about whether a sample of pure tungsten will melt at 3723K?

C. The sample will melt because T > ΔH°fus/ΔS°fus.

Based on the information, which of the following statements best helps to explain whether or not the reaction is thermodynamically favored at 298K?

C. ΔG°rxn<<0 and the reaction is thermodynamically favored because the energy released when the bonds in the products are formed is greater than the energy absorbed to break the bonds in the reactants.

The hydrolysis of adenosine triphosphate (ATP) is represented by the equation above. This reaction is critically important in cellular biology, but the reaction itself proceeds at a very slow rate. Based on the information given, which of the following best explains why an enzyme (biological catalyst) is required for the reaction to occur at a faster rate?

D. Although the hydrolysis of ATP is thermodynamically favorable, without a catalyst the reaction occurs at a very slow rate because it has a large activation energy.

The reaction represented by the equation above serves as the basis for the construction of an electrochemical cell. The table gives the reduction half-reactions and their respective standard reduction potentials. Based on the overall reaction, which type of electrochemical cell was constructed, and why?

D. An electrolytic cell, because ΔG=−2×96,500×(−0.46)/1,000=89kJ/molrxn and its operation requires a potential greater than 0.46V to be supplied.

On average, after one year of operation, the potential of a lithium-iodine cell decreases by 1%-2%. Which of the following best helps to explain the cause for the decrease in cell potential?

D. Ecell=E°cell−RT/nF lnQ, and as the cell operates, Q increases.

Two identical spoons were electroplated with Ag or Cd through the use of the electrolytic cells illustrated above. A current of 5.00A was supplied to each cell for 600. seconds, and the masses of the spoons before and after the electroplating were recorded. Which of the following mathematical equations can best be used to account for the much larger increase in mass of the spoon electroplated with Ag compared with the spoon electroplated with Cd?

D. I=q/t

An ice cube at 0°C melts when placed inside a room at 22°C (295K). Based on the concepts of enthalpy, entropy, and Gibbs free energy, which of the following best explains why the process is thermodynamically favorable?

D. Melting ice requires energy, ΔH°>0, and ΔS°>0 because the motion of the H2O molecules increases as it transitions from solid to liquid. At a temperature higher than 0°C (273K), the term TΔS° is greater than ΔH°, resulting in a thermodynamically favorable process with ΔG°<0.

The chemical equation above represents the formation of SeF6(g) from its elements and the table provides the approximate values of S° for Se(s) and SeF6(g). Based on the data, which of the following mathematical expressions can be used to correctly calculate S° for F2(g) ?

D. S°=−1/3[−337−314+42]J/(mol⋅K)

Graphite, an allotrope of carbon, is converted into cubic diamond through a process that may take a billion years or longer. As illustrated above, scientists can make synthetic diamonds using a certain process in about one week. However, these synthetic diamonds have carbon atoms in a hexagonal lattice. Diamonds with a carbon atoms in a cubic lattice are not produced even though they are thermodynamically more stable than hexagonal diamond. Which of the following best justifies why the synthetic process produces hexagonal diamond and not the more thermodynamically stable cubic diamond?

D. The activation energy needed to form cubic diamond is much greater than the activation energy needed to form hexagonal diamond.

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?

D. The volumes of collected gas in each tube should not be the same.

A sample of an ideal gas at Pi is initially confined to one chamber of the apparatus represented above, and the other chamber is initially evacuated. The valve connecting the chambers is opened, and the gas expands at constant temperature to fill both chambers. Which of the following best describes ΔS for the process?

D. ΔS>0 because the gas particles become dispersed in a larger volume