Question

From each of the following pairs of substances, use data in Appendix E to choose the one that is the stronger reducing agent: $$ \begin{array}{l}{\text { (a) } \mathrm{Fe}(s) \text { or } \mathrm{Mg}(s)} \\\ {\text { (b) } \mathrm{Ca}(s) \text { or } \mathrm{Al}(s)} \\ {\text { (c) } \mathrm{H}_{2}\left(g, \text { acidic solution ) or } \mathrm{H}_{2} \mathrm{S}(g)\right.} \\ {\text { (d) } \mathrm{BrO}_{3}^{-}(a q) \text { or } \mathrm{IO}_{3}^{-}(a q)}\end{array} $$

Short answer

The stronger reducing agents in each pair are: (a) \(\mathrm{Mg}(s)\) (b) \(\mathrm{Ca}(s)\) (c) \(\mathrm{H}_{2}(g)\) (d) \(\mathrm{IO}_{3}^{-}(aq)\)

Step-by-step solution

Find the standard reduction potentials for each substance

First, we need to look for the standard reduction potentials of the given substances in Appendix E. Here are the values we need: - Fe(s): \(\mathrm{Fe^{2+}} + 2\mathrm{e^{-}} \rightarrow \mathrm{Fe}(s)\) ; E° = -0.44 V - Mg(s): \(\mathrm{Mg^{2+}} + 2\mathrm{e^{-}} \rightarrow \mathrm{Mg}(s)\) ; E° = -2.37 V - Ca(s): \(\mathrm{Ca^{2+}} + 2\mathrm{e^{-}} \rightarrow \mathrm{Ca}(s)\) ; E° = -2.87 V - Al(s): \(\mathrm{Al^{3+}} + 3\mathrm{e^{-}} \rightarrow \mathrm{Al}(s)\) ; E° = -1.66 V - H2(g): \(\mathrm{2H^+} + 2\mathrm{e^{-}} \rightarrow \mathrm{H}_{2}(g)\) ; E° = 0.00 V - H2S(g): \(\mathrm{2H^+} + \mathrm{H}_{2}\mathrm{S} \rightarrow 2\mathrm{H}_{2}\mathrm{O} + \mathrm{S}\) ; E° = +0.14 V - BrO3⁻(aq): \(\mathrm{BrO}_{3}^{-} + 6\mathrm{H^+} + 6\mathrm{e^{-}} \rightarrow \mathrm{Br^{-}} + 3\mathrm{H}_{2}\mathrm{O}\) ; E° = +1.52 V - IO3⁻(aq): \(\mathrm{IO}_{3}^{-} + 6\mathrm{H^+} + 6\mathrm{e^{-}} \rightarrow \mathrm{I^{-}} + 3\mathrm{H}_{2}\mathrm{O}\) ; E° = +0.54 V

Compare the standard reduction potentials

Now, let's compare the standard reduction potentials in each pair to determine which substance is the stronger reducing agent: - For (a), \(\mathrm{Fe}(s)\) has an E° of -0.44 V, and \(\mathrm{Mg}(s)\) has an E° of -2.37 V. Since the reduction potential of \(\mathrm{Mg}(s)\) is more negative, it is a stronger reducing agent. - For (b), \(\mathrm{Ca}(s)\) has an E° of -2.87 V, and \(\mathrm{Al}(s)\) has an E° of -1.66 V. \(\mathrm{Ca}(s)\) has a more negative standard reduction potential, making it a stronger reducing agent. - For (c), \(\mathrm{H}_{2}(g)\) has an E° of 0.00 V, while \(\mathrm{H}_{2}\mathrm{S}(g)\) has an E° of +0.14 V. Since \(\mathrm{H}_{2}(g)\) has a more negative standard reduction potential, it is a stronger reducing agent. - For (d), \(\mathrm{BrO}_{3}^{-}(aq)\) has an E° of +1.52 V, and \(\mathrm{IO}_{3}^{-}(aq)\) has an E° of +0.54 V. Here, \(\mathrm{IO}_{3}^{-}(aq)\) has a more negative standard reduction potential, making it a stronger reducing agent.

Write the answers

Based on the standard reduction potentials, the stronger reducing agents in each pair are: (a) \(\mathrm{Mg}(s)\) (b) \(\mathrm{Ca}(s)\) (c) \(\mathrm{H}_{2}(g)\) (d) \(\mathrm{IO}_{3}^{-}(aq)\)

Key concepts

Standard Reduction Potential

Understanding standard reduction potential is crucial for anyone studying redox reactions. It measures the tendency of a chemical species to acquire electrons and be reduced, expressed in volts (V). Each half-reaction in a redox process has an associated standard reduction potential, which is tabulated under standard conditions: a 25°C temperature, 1M solution concentration, and a pressure of 1 atmosphere for gases.

A more negative standard reduction potential indicates a greater tendency for a substance to lose electrons and be oxidized, making it a stronger reducing agent. Conversely, a more positive potential suggests a substance is more likely to gain electrons and be reduced.

To compare different substances' reducing power, as in the exercise, you can use their standard reduction potential values. The substance with the more negative E° value is the stronger reducing agent. This concept not only aids in solving homework problems but also has practical applications, such as in the design of batteries and electroplating processes.

Electrochemical Series

The electrochemical series, also known as the activity series, is a list of elements organized according to their standard reduction potential. At the top of the series are the elements with the most positive reduction potential, considered the weakest reducing agents. As you move down the series, the reduction potential becomes more negative, and the strength of the reducing agents increases.

This series is a powerful tool for predicting the outcome of chemical reactions. It helps in determining which elements will displace others in a chemical reaction. For instance, in the textbook exercise, by referring to the electrochemical series, it becomes apparent why magnesium (Mg) is a stronger reducing agent than iron (Fe), as magnesium appears lower in the series due to its more negative standard reduction potential.

Redox Reactions

Redox reactions are a family of reactions where reduction and oxidation occur simultaneously. Reduction involves the gain of electrons, while oxidation involves the loss of electrons. To remember this, you can use the mnemonic 'OIL RIG'—Oxidation Is Loss, Reduction Is Gain.

These reactions are fundamental to numerous processes in chemistry and biology, including cellular respiration and the functioning of batteries. Understanding which substances act as reducing agents or oxidizing agents is essential. Reducing agents donate electrons and are themselves oxidized, while oxidizing agents accept electrons and are reduced. In the context of our textbook exercise, once you grasp the concept of standard reduction potentials, you can easily identify the stronger reducing agent in a pair by looking at which substance will more readily donate electrons.