MCAT General Chemistry Question 162: Answer and Explanation

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Question: 162

12. Consider the following steps in the reaction between oxalic acid and chlorine:

I. Cl₂ + H₂O → HOCl + Cl^– + H⁺

II. H₂C₂O₄ → H⁺ + HC₂O₄^-

III. HOCl + HC₂O₄^- → H₂O + Cl^- + 2 CO₂

Which of these steps, occurring in aqueous solution, is an example of a disproportionation reaction?

A. I only
B. III only
C. I and III only
D. I, II, and III

Correct Answer: A

Explanation:

Step I is a disproportionation reaction because chlorine starts with an oxidation state of 0 in the reactants and ends up with an oxidation state of +1 in HOCl and -1 as Cl^-. In the other reactions, no element appears with different oxidation states in two different products. Therefore, only step I is a disproportionation reaction.

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