Problem 44 - Chemistry: The Central Science

Question

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

Step-by-Step Solution

Answer

In conclusion, (a) \(\mathrm{Ce}^{3+}(a q)\) is likely to serve as an oxidant, (b) \(\mathrm{Ca}(\mathrm{s})\) is likely to serve as a reductant, (c) \(\mathrm{ClO}_{3}^{-}(a q)\) is likely to serve as an oxidant, and (d) \(\mathrm{N}_{2} \mathrm{O}_{5}(g)\) is likely to serve as an oxidant.

Step 1: Firstly, we can find the oxidation state of each element in the given substances: (a) \(\mathrm{Ce}^{3+}(a q)\) - The cerium ion has an oxidation state of +3. (b) \(\mathrm{Ca}(\mathrm{s})\) - Calcium is in its elemental form, so its oxidation state is 0. (c) \(\mathrm{ClO}_{3}^{-}(a q)\) - In chlorate, chlorine has an oxidation state of +5 (as oxygen has an oxidation state of -2 and the overall charge of the ion is -1). (d) \(\mathrm{N}_{2} \mathrm{O}_{5}(g)\) - In nitrogen pentoxide, nitrogen has an oxidation state of +5 (as oxygen has an oxidation state of -2 and the overall charge of the molecule is 0). #Step 2: Identify oxidants and reductants based on oxidation state#

Now we can identify the substances as oxidants or reductants based on their oxidation state: (a) \(\mathrm{Ce}^{3+}(a q)\) - With an oxidation state of +3, it can gain one electron to become \(\mathrm{Ce}^{4+}\) (which is more stable), so it is likely to serve as an oxidant. (b) \(\mathrm{Ca}(\mathrm{s})\) - With an oxidation state of 0, it can lose two electrons to form stable \(\mathrm{Ca}^{2+}\), so it is likely to serve as a reductant. (c) \(\mathrm{ClO}_{3}^{-}(a q)\) - With an oxidation state of +5, chlorine would have to lose five electrons to be in its most stable state as \(\mathrm{Cl}^-\), making it unlikely. Alternatively, it can gain two electrons to become \(\mathrm{Cl}^{-}\), so it is likely to serve as an oxidant. (d) \(\mathrm{N}_{2} \mathrm{O}_{5}(g)\) - With an oxidation state of +5, nitrogen would either have to lose five electrons to be in its most stable state as \(\mathrm{N}^{5-}\), or gain three electrons to become the stable \(\mathrm{N}_{2}\). It is more likely to serve as an oxidant due to the oxygen atoms. In conclusion: (a) \(\mathrm{Ce}^{3+}(a q)\) - likely to serve as an oxidant. (b) \(\mathrm{Ca}(\mathrm{s})\) - likely to serve as a reductant. (c) \(\mathrm{ClO}_{3}^{-}(a q)\) - likely to serve as an oxidant. (d) \(\mathrm{N}_{2} \mathrm{O}_{5}(g)\) - likely to serve as an oxidant.

Key Concepts

Oxidation States

Understanding oxidation states is key to grasping the basics of oxidation-reduction (redox) reactions. Oxidation states, frequently referred to as oxidation numbers, provide us a way to keep track of electrons during chemical reactions, especially redox reactions. They are typically represented by integers, which can be positive, negative, or zero.

Oxidation states are assigned based on a set of rules. For example, the oxidation state of an element in its standard state, like calcium (Ca) in solid form, is always zero. For ions, the oxidation state is equal to the charge of the ion, thus cerium (Ce) in the ion (Ce^{3+}) has an oxidation state of +3.

It's important to consider that these numbers are not actual charges but rather hypothetical charges we assign for the sake of balancing electron transfer in reactions. This can be particularly useful when trying to determine which substances can act as oxidising or reducing agents, which brings us to our next concept.

Redox Agents

Redox agents, also known as oxidising and reducing agents, play crucial roles in redox reactions. Oxidising agents, or oxidants, accept electrons during a chemical reaction, thereby being reduced. Conversely, reducing agents, or reductants, donate electrons, which means they are oxidised in the process.

For instance, (Ce^{3+})(aq), when serving as an oxidising agent, could accept an electron to reach a +4 oxidation state, which represents a more chemically stable configuration for cerium. On the other side of the equation, calcium (Ca) in the solid form can donate electrons, going from an oxidation state of 0 to +2, thus serving as a reducing agent.

Assessing whether a substance will likely act as an oxidant or reductant depends on its ability to either gain or lose electrons to achieve a more stable state. For example, (ClO_{3}^{-})(aq) is predicted to act as an oxidant as it is more likely to gain electrons than lose them, going from a +5 to a -1 oxidation state.

Chemical Stability

Chemical stability refers to the propensity of a chemical substance to maintain its chemical identity rather than reacting spontaneously to form a different product. Stability is context-dependent and can be influenced by environmental factors like temperature, pressure, and the presence of other chemicals.

In redox reactions, stability can often be linked to a substance's oxidation state because elements tend to be more stable at certain oxidation levels. The term 'noble state' refers to the most stable oxidation state under standard conditions. For example, the noble state for chlorine is typically -1, and hence even though it has an oxidation state of +5 in (ClO_{3}^{-})(aq), it can still accept electrons to achieve a -1 state, showing a strong tendency to be an oxidant.

A substance's desire to achieve a noble state fuels its role as a redox agent. Understanding the preferred oxidation states can help us predict whether a substance is likely to be an oxidant or reductant, which in turn will affect the overall stability of the compounds formed in the reaction.