Chemistry: The Central Science (13th Edition)

Published by Prentice Hall
ISBN 10: 0321910419
ISBN 13: 978-0-32191-041-7

Chapter 20 - Electrochemistry - Exercises - Page 899: 20.6b


D is strongest reducing agent while A is the weakest reducing agent.

Work Step by Step

Because for the last reaction where $D^{3+}$ is reduced to D, the reaction has a negative reduction potential $E^0$ (-1.59)therefore the reduction reaction is non-spontaneous while reverse reaction (i.e. oxidation) is spontaneous. Due to feasible oxidation, D produces $D^{3+}$ and electrons and reduce other substances easily. On the other hand, in case of first reaction reduction potential $E^0$ is positive(1.33V). In this case reduction reaction is spontaneous thus reverse reaction(i.e. oxidation) is not feasible. As oxidation is not feasible, this reaction cannot produce an electron and reduce other substances. So A is the weakest reducing agent.
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