Question

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

Short answer

(a) Ce³⁺ is likely to serve as an oxidant, as it is more likely to gain one electron and decrease its oxidation state to Ce²⁺. (b) Solid calcium (Ca) is likely to serve as a reductant, as it would generally lose two electrons to achieve its stable configuration with an oxidation number of +2. (c) ClO₃⁻ will likely act as an oxidant, as chlorine can gain electrons, decreasing its oxidation state to form compounds such as Cl⁻ or ClO₂⁻. (d) N₂O₅ is likely to act as an oxidant, as nitrogen can gain electrons to decrease its oxidation state forming compounds like NO₂, with nitrogen in an oxidation state of +4.

Step-by-step solution

(a) Identify the role of Ce³⁺)

Because Ce³⁺ has a charge of +3, it already has a high oxidation state. To serve as a reductant, it would need to lose more electrons, but it is more likely to gain an electron and decrease its oxidation state, becoming Ce²⁺. So, Ce³⁺ will very likely act as an oxidant.

(b) Identify the role of Ca)

Calcium in its elemental state has an oxidation number of 0. In order for Ca to form a compound, it would generally lose two electrons to achieve its stable configuration (with an oxidation number of +2). Therefore, solid calcium (Ca) is likely to serve as a reductant.

(c) Identify the role of ClO₃⁻)

In ClO₃⁻, chlorine has an oxidation state of +5, which is its highest common oxidation state. Thus, ClO₃⁻ has no possibility of gaining more electrons to oxidize another substance. Oppositely, chlorine can gain electrons, decreasing its oxidation state to form compounds such as Cl⁻ or ClO₂⁻. So, ClO₃⁻ will likely act as an oxidant.

(d) Identify the role of N₂O₅)

Nitrogen in N₂O₅ has an oxidation state of +5, which is its highest possible oxidation state. Therefore, the nitrogen in N₂O₅ has no possibility of losing more electrons to oxidize another substance. Instead, it can gain electrons to decrease its oxidation state forming compounds like NO₂, with nitrogen in an oxidation state of +4. Thus, N₂O₅ is likely to act as an oxidant.

Key concepts

Oxidation and Reduction

Understanding the fundamentals of oxidation and reduction is crucial for identifying the behavior of chemical compounds in redox reactions.

Oxidation refers to the process where an atom, ion, or molecule loses electrons, resulting in an increase in its oxidation state. On the flip side, reduction involves the gain of electrons by a species, which decreases its oxidation state. A handy memory aid for this concept is 'OIL RIG': Oxidation Is Loss, Reduction Is Gain.

These two processes always occur simultaneously; when one species is oxidized, another is reduced. This relationship is essential for maintaining the balance of charge during chemical reactions. In the context of the given exercise, recognizing whether a substance tends to gain or lose electrons can reveal whether it will act as an oxidant (electron acceptor) or a reductant (electron donor).

Oxidation States

The oxidation state, also known as oxidation number, is a very telling indicator of a chemical element's behavior in a compound or ion. It's a theoretical charge that an atom would have if all bonds to atoms of different elements were fully ionic.

Oxidation states are used to keep track of electron shifts in redox reactions. For example, in the given exercise, the oxidation state of cerium, Ce³⁺, is +3, suggesting that it has lost three electrons when compared to its neutral state. These numbers can predict whether a substance is more likely to give up electrons (act as a reductant) or receive them (act as an oxidant).

When a substance is at a high oxidation state, it's less likely to lose more electrons and more apt to gain them, thus acting as an oxidant. Conversely, an element at a low oxidation state or in its elemental form will more readily lose electrons and therefore act as a reductant.

Redox Reactions

Redox reactions are the bread and butter of the interplay between oxidation and reduction processes. Standing for 'reduction-oxidation reactions', these processes involve the transfer of electrons from one substance to another.

In any redox reaction, there must be both a reductant (donates electrons) and an oxidant (accepts electrons). The reductant undergoes oxidation, and the oxidant undergoes reduction. Properly identifying these agents is vital for predicting the course and products of a reaction.

For instance, as outlined in the exercise solutions, Ca(s) is likely to act as a reductant because in its elemental state (oxidation state 0), it can easily lose electrons. Understanding this gives us important clues about the potential reactions Ca might engage in if paired with an appropriate oxidant.

Chemical Compounds Behavior

The behavior of chemical compounds in reactions, particularly redox reactions, is intricately linked to their structures, electron configurations, and existing oxidation states.

By examining the oxidation states and the surrounding chemical environment, chemists can predict whether a compound will behave as an oxidant or reductant. For example, ClO₃⁻ has chlorine at a high oxidation state of +5, which hints that it is more likely to gain electrons (be reduced) than to lose more, making it an efficient oxidant.

This conceptual understanding allows us to anticipate how substances will interact, explaining why certain compounds behave predictably under given conditions and providing a roadmap for students to follow when tasked with identifying oxidants and reductants, as shown in the exercise.