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.
-Based on the passage, which of the following species is LEAST likely to undergo a disproportionation 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.
-Based on the passage, which of the following species is LEAST likely to undergo a disproportionation reaction?

Quizplus · Accepted Answer

11ecba2d_1221_2731_a3bf_5799bb474739_MD0008_00 A disproportionation reaction is a redox reaction in which both the oxidation and reduction occur to atoms of the same element. Disproportionation reactions can be easily identified by assigning oxidation numbers and then comparing the oxidation number of the given element in the reactants with the oxidation number of the same element in the products. If a compound undergoes disproportionation, some of the compound molecules supply atoms of a particular element for oxidation, and other molecules of the compound supply atoms of the same element for reduction. Exam questions sometimes test required topics by presenting the concepts in an unfamiliar context using details that are not required knowledge, but which are explained within the passage. In this passage, Frost diagrams are presented as a way of predicting which chemical species are likely to undergo disproportionation. As shown in Figure 2 of the passage, a species is likely to undergo disproportionation if its position on the Frost diagram lies above a line connecting the points of two adjacent species because ΔG° < 0 with respect to the mean of the potential gradient. Accordingly, answering this question is a matter of graphical evaluation. Based on the above criteria and an examination of Figure 1, MnO₂ is least likely to undergo disproportionation because its point on the Frost diagram lies below the line connecting the points of two neighboring species. (Choices A and B) On the Frost diagram, H₃MnO₄ is positioned above a line connecting MnO₂ and HMnO₄⁻, and MnO₄³⁻ is above a line connecting MnO₂ and MnO₄²⁻. Therefore, in both cases, ΔG° < 0 relative to the mean and these species are likely to undergo a disproportionation reaction. (Choice C) The points for Mn²⁺, Mn³⁺, and MnO₂ are not perfectly linear, and Mn³⁺ lies slightly above the line connecting Mn²⁺ and MnO₂. Although disproportionation is unlikely due to the small difference, Mn³⁺ is not the least likely to undergo disproportionation. Educational objective: A disproportionation reaction is a redox reaction in which both the oxidation and the reduction occur with atoms of the same element. The tendency of a reaction toward disproportionation can be evaluated graphically.

Passage The Tendency of a Chemical Species to Undergo Reduction During