Problem 26 - Chemistry: An Atoms-Focused Approach

Question

From the table of standard reduction potentials in Appendix 6 a. Select an oxidizing agent that will oxidize \(\mathrm{Cr}(s)\) to \(\mathrm{Cr}^{3+}(a q)\) but not \(\mathrm{Cd}(s)\) to \(\mathrm{Cd}^{2+}(a q)\) b. Select a reducing agent that will reduce Br \(_{2}(\ell)\) to \(\mathrm{Br}^{-}(a q)\) but not \(\mathrm{I}_{2}(s)\) to \(1^{-}(a q).\)

Step-by-Step Solution

Answer

Question: Select appropriate oxidizing and reducing agents based on their standard reduction potentials. a) Choose an oxidizing agent that can oxidize \(\mathrm{Cr}(s)\) to \(\mathrm{Cr}^{3+}(a q)\) but not \(\mathrm{Cd}(s)\) to \(\mathrm{Cd}^{2+}(a q)\). b) Choose a reducing agent that can reduce \(\mathrm{Br}_{2}(\ell)\) to \(\mathrm{Br}^{-}(a q)\) but not \(\mathrm{I}_{2}(s)\) to \(\mathrm{I}^{-}(a q)\).

Step 1: Understanding Standard Reduction Potentials

Standard reduction potential is a measure of the tendency for a chemical species to be reduced, i.e., to gain electrons. A higher reduction potential means the species has a greater tendency to be reduced, and can act as a better oxidizing agent. Conversely, a lower reduction potential means the species has a lower tendency to be reduced and can act as a better reducing agent.

Step 2: Finding the Standard Reduction Potentials of the Species

Using the table of standard reduction potentials in Appendix 6, look up the standard reduction potentials for the given species. We will need the information for the following reactions: 1. \(\mathrm{Cr}^{3+}(a q) + 3\mathrm{e}^{-} \rightarrow \mathrm{Cr}(s)\) 2. \(\mathrm{Cd}^{2+}(a q) + 2\mathrm{e}^{-} \rightarrow \mathrm{Cd}(s)\) 3. \(\mathrm{Br}_{2}(\ell) + 2\mathrm{e}^{-} \rightarrow 2\mathrm{Br}^{-}(a q)\) 4. \(\mathrm{I}_{2}(s) + 2\mathrm{e}^{-} \rightarrow 2\mathrm{I}^{-}(a q)\)

Step 3: Select an Oxidizing Agent for Part (a)

To find an oxidizing agent that can oxidize \(\mathrm{Cr}(s)\) to \(\mathrm{Cr}^{3+}(a q)\) but not \(\mathrm{Cd}(s)\) to \(\mathrm{Cd}^{2+}(a q)\), we need to choose a species with a standard reduction potential that is greater than that of the \(\mathrm{Cr}^{3+}(a q)/\mathrm{Cr}(s)\) reaction but lesser than that of the \(\mathrm{Cd}^{2+}(a q)/\mathrm{Cd}(s)\) reaction. Using the standard reduction potentials found in Step 2, choose an appropriate oxidizing agent that fulfills this criterion.

Step 4: Select a Reducing Agent for Part (b)

To find a reducing agent that can reduce \(\mathrm{Br}_{2}(\ell)\) to \(\mathrm{Br}^{-}(a q)\) but not \(\mathrm{I}_{2}(s)\) to \(\mathrm{I}^{-}(a q)\), we need to choose a species with a standard reduction potential that is smaller than that of the \(\mathrm{Br}_{2}(\ell) / 2\mathrm{Br}^{-}(a q)\) reaction but greater than that of the \(\mathrm{I}_{2}(s) / 2\mathrm{I}^{-}(a q)\) reaction. Using the standard reduction potentials found in Step 2, choose an appropriate reducing agent that fulfills this criterion.

Key Concepts

Oxidizing Agent

An oxidizing agent is a substance that has the ability to oxidize other substances. In chemical reactions, the oxidizing agent accepts electrons and undergoes reduction. It gains electrons while causing another substance to lose them. Therefore, an effective oxidizing agent must have a high standard reduction potential. This high potential indicates a preference for gaining electrons and undergoing reduction itself.

For the exercise involving chromium (\(\mathrm{Cr}(s)\)) and cadmium (\(\mathrm{Cd}(s)\)), the oxidizing agent must be a species that can easily oxidize chromium but not cadmium. We need a species with a standard reduction potential between the (\(\mathrm{Cr}^{3+}(aq) / \mathrm{Cr}(s)\)) and (\(\mathrm{Cd}^{2+}(aq) / \mathrm{Cd}(s)\)) reactions. Chromium's reaction has a lower reduction potential compared to cadmium's, meaning it’s easier to oxidize (\(\mathrm{Cr}(s)\)) since the oxidizing agent needs less potential to do so.

• Remember, a higher reduction potential means better oxidizing power.
• The agent selected should favor reduction over the specified oxidation.

A careful balancing act is required, selecting an agent with the precise potential needed for chromium but not cadmium.

Reducing Agent

A reducing agent, contrary to an oxidizing agent, donates electrons to another substance, thus reducing that substance while getting oxidized itself. A strong reducing agent is characterized by a low (or even negative) standard reduction potential, indicating it prefers to lose electrons and undergo oxidation.

In the exercise concerning the conversion of (\(\mathrm{Br}_{2}(\ell)\)) to (\(2\mathrm{Br}^{-}(aq)\)) without affecting (\(\mathrm{I}_{2}(s)\)) to (\(2\mathrm{I}^{-}(aq)\)), the desired reducing agent should have a standard reduction potential greater than that of the (\(\mathrm{I}_{2}(s) / 2\mathrm{I}^{-}(aq)\)) reaction but lesser than the (\(\mathrm{Br}_{2}(\ell) / 2\mathrm{Br}^{-}(aq)\)) reaction. This ensures it can reduce bromine without affecting iodine.

• Reducing agents prefer losing electrons, distinguishing them through a lower reduction potential.
• They promote reduction in other species, with their own oxidation accompanying this process.

Thus, the selection of an appropriate reducing agent depends on its ability to influence one desired reaction over the other specified reaction.

Electrochemistry

Electrochemistry is an essential branch of chemistry that deals with the relationship between electrical energy and chemical changes, focusing on redox reactions (oxidation-reduction reactions). In these reactions, electrons are transferred between substances, and these processes involve voltages measured as standard reduction potentials. Such potentials reflect how readily a species is willing to be reduced.

Key to understanding electrochemistry is the use of half-reactions, where each reaction encompasses either reduction or oxidation. In the standard reduction potential tables, positive values denote substances more inclined to gain electrons (strong oxidizers), whereas negative values imply a tendency to lose electrons (strong reducers).

• It involves two parts: oxidation (loss of electrons) and reduction (gain of electrons).
• The processes are linked by the transfer and acceptance of electrons, crucial for reaction work balancing.

Comprehending these principles allows for informed decisions about which species act as oxidizing or reducing agents under given conditions, shedding light on broader chemical phenomena.