Passage
The tendency of a chemical species to undergo reduction during an electrochemical reaction is indicated by the standard potential E° for the reaction. As represented generally by Reaction 1, E° measures the potential Z at which element X with oxidation number N accepts N electrons and is reduced to its elemental state.X(N) + Ne⁻ → X(0) , E° = Z voltsReaction 1In a galvanic cell, E° > 0 indicates a spontaneous reaction. Therefore, chemical species with higher positive E° values are reduced more easily than those with lower E° values.Using E° as a basis of comparison, the relative thermodynamic stabilities of different chemical species involving the same element at different oxidation states can be presented graphically using a Frost diagram. For a series of compounds containing element X, a Frost diagram plots the value of NE° for each compound against the corresponding oxidation number N of element X within the species. Frost diagrams for several species containing manganese or chlorine are shown in Figure 1.
Figure 1 Frost diagrams for manganese and chlorineOn a Frost diagram, NE° is proportional to the standard Gibbs free energy ΔG° according to the relationshipΔG° = −FNE° = −nFE°Equation 1where n is the number of moles of electrons transferred during the electrochemical process and F is the Faraday constant. Free energy considerations also show that a species is prone to disproportionation if its position on the Frost diagram lies above a line connecting the points of two adjacent species, as seen for species B in Figure 2.
Figure 2 General Frost diagram for an element forming species A, B, C, and D.The slope of a line segment joining two species on a Frost diagram is equal to the standard reduction potential for the couple. A greater slope indicates a higher corresponding reduction potential. As a result, in Figure 2, the reduction potential for B to A is lower than that for D to C.
-Suppose an experiment is to be performed utilizing a reaction that reduces ClO₄⁻ to ClO₃⁻. Which experimental approach can be expected to make the reduction of ClO₄⁻ more favorable during the reaction?

Question

Passage
The tendency of a chemical species to undergo reduction during an electrochemical reaction is indicated by the standard potential E° for the reaction. As represented generally by Reaction 1, E° measures the potential Z at which element X with oxidation number N accepts N electrons and is reduced to its elemental state.X(N) + Ne⁻ → X(0) , E° = Z voltsReaction 1In a galvanic cell, E° > 0 indicates a spontaneous reaction. Therefore, chemical species with higher positive E° values are reduced more easily than those with lower E° values.Using E° as a basis of comparison, the relative thermodynamic stabilities of different chemical species involving the same element at different oxidation states can be presented graphically using a Frost diagram. For a series of compounds containing element X, a Frost diagram plots the value of NE° for each compound against the corresponding oxidation number N of element X within the species. Frost diagrams for several species containing manganese or chlorine are shown in Figure 1.

Figure 1 Frost diagrams for manganese and chlorineOn a Frost diagram, NE° is proportional to the standard Gibbs free energy ΔG° according to the relationshipΔG° = −FNE° = −nFE°Equation 1where n is the number of moles of electrons transferred during the electrochemical process and F is the Faraday constant. Free energy considerations also show that a species is prone to disproportionation if its position on the Frost diagram lies above a line connecting the points of two adjacent species, as seen for species B in Figure 2.

Figure 2 General Frost diagram for an element forming species A, B, C, and D.The slope of a line segment joining two species on a Frost diagram is equal to the standard reduction potential for the couple. A greater slope indicates a higher corresponding reduction potential. As a result, in Figure 2, the reduction potential for B to A is lower than that for D to C.
-Suppose an experiment is to be performed utilizing a reaction that reduces ClO₄⁻ to ClO₃⁻. Which experimental approach can be expected to make the reduction of ClO₄⁻ more favorable during the reaction?

Quizplus · Accepted Answer

11ecba2d_1225_6cfe_a3bf_71a4310aa7aa_MD0008_00 As stated in the passage, an NE° (the y-axis) value on a Frost diagram is proportional to the standard Gibbs free energy ΔG° according to the relationship ΔG° = −FNE° = −nFE° where n is the number of moles of electrons transferred during the electrochemical process, F is the Faraday constant, and E° is the standard reduction potential. Because reactions with a ΔG° < 0 can occur spontaneously, a redox process with a larger positive E° is more favorable than a redox process with a smaller E°. The passage states that on a Frost diagram, the slope of a line segment joining two species is equal to E° for the couple. As a result, when comparing line segments, the segment with the greatest positive slope is the more favorable reaction. Comparing the slopes of the line segments between ClO₄⁻ and ClO₃⁻ for both pH 0 and pH 14 on the Frost diagram of chlorine (Figure 1 in the passage), the slope for acidic conditions (pH 0) is greater than the slope for basic conditions (pH 14). Therefore, performing the experiment under acidic conditions will make the reduction of ClO₄⁻ more favorable during the reaction. (Choice B) There is no experimental data in the passage to support the idea that performing the reaction under neutral conditions (pH 7) would make the reduction of ClO₄⁻ more favorable. Data at pH 7 would need to be collected for comparison before any conclusion about neutral conditions could be made. (Choice C) On the Frost diagram, the slope of the line segment between ClO₄⁻ and ClO₃⁻ at pH 14 is less steep than the slope of the line segment between ClO₄⁻ and ClO₃⁻ at pH 0. This indicates that the reduction of ClO₄⁻ is less favorable under basic conditions. (Choice D) On the Frost diagram, the plot of the species in solution at pH 0 displays a distinctly different profile from the plot of the same species placed in solution at pH 14. Therefore, the pH of the reaction mixture does affect the results. Educational objective: Reactions associated with a larger positive standard reduction potential are more thermodynamically favorable. The standard reduction potential E° for an oxidation-reduction couple can be evaluated graphically from electrochemical data.

Passage The Tendency of a Chemical Species to Undergo Reduction During