Question

s each of the following substances likely to serve as an oxidant or a reductant: (a) \(\mathrm{Ce}^{3+}(\mathrm{aq})\), (b) \(\mathrm{Ca}(\mathrm{s})\), (c) \(\mathrm{CO}_{3}^{-}(\mathrm{aq})\), (d) \(\mathrm{N}_{2} \mathrm{O}_{5}(g)\) ?

Short answer

In conclusion, (a) \(\mathrm{Ce^{3+}(aq)}\) and (d) \(\mathrm{N_2O_5(g)}\) are likely to serve as oxidants, while (b) \(\mathrm{Ca(s)}\) and (c) \(\mathrm{CO_3^{-}(aq)}\) are likely to serve as reductants.

Step-by-step solution

Look up standard reduction potentials for each substance

To understand how easily each substance can be reduced or oxidized, we will look up their reduction potentials in a standard reduction potential table (E°). The higher the value, the stronger the oxidizing agent (more likely to be reduced). The lower the value, the stronger the reducing agent (more likely to be oxidized). (a) For \(\mathrm{Ce^{3+}(aq)}\), we have the reduction reaction: $$\mathrm{Ce^{4+}(aq) + e^- \rightarrow Ce^{3+}(aq)} \quad E° = +1.74 \, V$$ (b) For \(\mathrm{Ca(s)}\), we have the reduction reaction: $$\mathrm{Ca^{2+}(aq) + 2e^- \rightarrow Ca(s)} \quad E° = -2.87 \, V$$ (c) For \(\mathrm{CO_3^{2-}(aq)}\), one possible reduction reaction is: $$\mathrm{CO_3^{2-}(aq) + 2H^+(aq) + 2e^- \rightarrow CO(g) + H_2O(l)} \quad E° = -0.52 \, V$$ (d) For \(\mathrm{N_2O_5(g)}\), one possible reduction reaction is: $$\mathrm{N_2O_5(g) + 2H^+(aq) + 2e^- \rightarrow NO_3^-(aq) + NO_2(g) + H_2O(l)} \quad E° = +1.82 \, V$$

Determine whether each substance is likely to be an oxidant or a reductant

Based on the standard reduction potentials, we can determine whether each substance is likely to act as an oxidant or a reductant. (a) \(\mathrm{Ce^{3+}(aq)}\): Since the reduction potential value is positive, it is more likely to be reduced and act as an oxidant. (b) \(\mathrm{Ca(s)}\): Since the reduction potential value is strongly negative, it is more likely to be oxidized and act as a reductant. (c) \(\mathrm{CO_3^{-}(aq)}\): Since the reduction potential value is negative, it is more likely to be oxidized and act as a reductant. However, this particular substance can act in many different reactions and, depending on its chemical environment, can have multiple roles. (d) \(\mathrm{N_2O_5(g)}\): Since the reduction potential value is positive, it is more likely to be reduced and act as an oxidant. In conclusion, (a) \(\mathrm{Ce^{3+}(aq)}\) and (d) \(\mathrm{N_2O_5(g)}\) are likely to serve as oxidants, while (b) \(\mathrm{Ca(s)}\) and (c) \(\mathrm{CO_3^{-}(aq)}\) are likely to serve as reductants.

Key concepts

Standard Reduction Potentials

Understanding standard reduction potentials is essential when studying redox reactions. These potentials, often represented as E° values, are measured under standard conditions, which include a temperature of 298K, a 1M concentration for each ion participating in the reaction, and a pressure of 1 bar for any gases involved.

The standard reduction potential of a substance indicates its tendency to gain electrons and be reduced. A more positive E° value means a substance is a better oxidant, meaning it has a greater tendency to gain electrons. Conversely, a negative E° value suggests that the substance is more likely to lose electrons, acting as a reductant in a chemical reaction.

In the context of the given exercise, we compared the E° values for various substances to determine their roles as oxidants or reductants. This method is a fundamental concept in electrochemistry and is critical to predicting the spontaneity of redox reactions.

Redox Reactions

Redox reactions are chemical processes that involve the transfer of electrons between two species. The term 'redox' is a portmanteau of reduction and oxidation. Oxidation refers to the loss of electrons, while reduction refers to the gaining of electrons. In any redox reaction, there is an oxidant which accepts electrons and a reductant which donates electrons.

Determining which compound serves as an oxidant or reductant is key to understanding the overall redox process. For instance, in the classroom exercise, we assessed substances based on their ability to act in these roles by using their standard reduction potentials as a guide. The exercise effectively illustrates that, for a balanced redox reaction to occur, there must be a substance that gets reduced (oxidant) and another that gets oxidized (reductant).

Chemical Oxidizing Agents

Chemical oxidizing agents, or simply oxidants, are substances that can accept electrons during a chemical reaction. Common examples include halogens, oxygen, and other molecules with a high affinity for electrons. The strength of an oxidizing agent depends on its tendency to gain electrons, which can be determined by its standard reduction potential.

Role in Redox Reactions

In redox reactions, oxidizing agents obtain electrons from reductants, becoming reduced themselves. The exercise highlighted oxidants like \( \mathrm{Ce^{3+}(aq)} \) and \( \mathrm{N_2O_5(g)} \) that have positive E° values, indicating their likelihood of serving as oxidants. The ability of a substance to act as an oxidizing agent is not only crucial for redox reactions but also has practical applications in areas such as energy storage, water treatment, and organic chemistry.

Electrochemical Potentials

Electrochemical potentials represent the capacity of a chemical species to undergo an electrochemical reaction, which involves electron transfer. It's a broader term that encompasses both oxidation and reduction potentials, depending on whether a species is donating or accepting electrons. The values are usually measured in volts and can be used to predict the direction of electron flow in an electrochemical cell.

For example, the exercise provides information on several substances' tendencies to gain or lose electrons, which are essentially their electrochemical potentials. This data help us understand the behavior of substances in an electrochemical context, such as in batteries where redox reactions drive the flow of electric current. Thus, knowing the electrochemical potentials is paramount in designing and working with all sorts of electrochemical devices.