Problem 46 - Chemistry: The Central Science in SI Units, Global Edition

Question

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

Step-by-Step Solution

Answer

In summary, Ce^{3+}(aq) and ClO_{3}^{-}(aq) act as oxidants because they can gain electrons to reach their highest oxidation state (Ce^{3+} becomes Ce^{4+} and ClO_{3}^{-} becomes Cl^{+7}). In contrast, Ca(s) and N_{2}O_{5}(g) act as reductants as they can lose electrons (Ca becomes Ca^{2+} and N_{2}O_{5} gets oxidized).

Step 1: Substance (a): Ce^{3+}(aq)

In this case, Ce (Cerium) has an oxidation state of +3. It's important to notice that the maximum oxidation state for Cerium is +4. This means that Ce^{3+} is likely to gain an electron to reach its highest oxidation state and become Ce^{4+}. Therefore, Ce^{3+} will act as an oxidant.

Step 2: Substance (b): Ca(s)

In this case, Ca (Calcium) is a metal in its solid state with an oxidation state of 0. Metals typically lose electrons to form positive ions, i.e., they get oxidized. So, Ca is more likely to lose two electrons and become Ca^{2+} with an oxidation state of +2. Therefore, Ca(s) will act as a reductant.

Step 3: Substance (c): ClO_{3}^{-}(aq)

In this case, we need to determine the oxidation state of Cl (Chlorine) in the ClO_{3}^{-} ion. The sum of oxidation states in this ion is equal to -1 (charge of the ion). Let x be the oxidation state of Cl. The oxidation state of three O (Oxygen) atoms is -2 each (total -6). As the sum of oxidation states is equal to -1, we get x - 6 = -1, leading to x = +5. Since the maximum oxidation state of Cl is +7, ClO_{3}^{-} is likely to gain two electrons (get reduced) and become Cl^{+7}. Therefore, ClO_{3}^{-} will act as an oxidant.

Step 4: Substance (d): N_{2}O_{5}(g)

In this case, N_{2}O_{5} is a gaseous compound involving Nitrogen (N) and Oxygen (O). We need to determine the oxidation state of N. From the compound formula N_{2}O_{5}, it can be seen that there are two Nitrogen atoms and five Oxygen atoms. The sum of the oxidation states in this compound is equal to 0 (overall charge). Let x be the oxidation state of one N atom. The oxidation state of five O atoms is -2 each (total -10). Since there are two N atoms, the sum of oxidation states is 2x - 10 = 0, leading to x = +5. The maximum oxidation state for N is +5, which it already has in N_{2}O_{5}. Therefore, N is more likely to lose electrons (get oxidized) in this case. So, N_{2}O_{5}(g) will act as a reductant. In summary, Ce^{3+}(aq) and ClO_{3}^{-}(aq) act as oxidants, while Ca(s) and N_{2}O_{5}(g) act as reductants.

Key Concepts

Oxidation States

In the context of redox reactions, understanding oxidation states is crucial. The oxidation state, often referred to as the oxidation number, reflects the degree of oxidation of an atom in a chemical compound. It is a theoretical construct that helps determine how many electrons are lost or gained by an atom.

A few key points about oxidation states:

• They are assigned based on a set of rules that include considering the electron distribution in compounds.
• Metals in their elemental form generally have an oxidation state of 0, as shown in Calcium (\( ext{Ca}(s) \)) in the original exercise.
• For ions, the oxidation state is equal to their charge, such as \( ext{Ce}^{3+} \) having a +3 oxidation state.
• In compounds like \( ext{ClO}_3^- \), the oxidation state of Oxygen is typically -2, which, alongside other rules, is used to determine Chlorine's oxidation state.

Understanding these states helps predict electron transfer processes in redox reactions, which are essential for identifying whether a substance will act as an oxidant or a reductant.

Oxidants

An oxidant, or oxidizing agent, is a substance that gains electrons in a redox chemical reaction, facilitating the oxidation of another species. This means that oxidants themselves are reduced during the process.

To identify an oxidant, look for species that:

• Have high oxidation states, as they are likely to accept electrons. For instance, \( ext{Ce}^{3+} \) wants to acquire another electron to reach \( ext{Ce}^{4+} \)
• Are able to gain electrons and thus undergo a decrease in oxidation state, like \( ext{ClO}_3^- \)

Both Ce\( ^{3+} \) and \( ext{ClO}_3^- \) demonstrate the characteristic behavior of oxidants in reactions, playing essential roles in chemical processes. Recognizing oxidants is particularly important in applications ranging from industrial synthesis to biological metabolic pathways.

Reductants

Reductants, or reducing agents, are substances that lose electrons in a redox reaction, thereby causing another substance to be reduced. This means reductants are oxidized in the process.

Characteristics of reductants include:

• The ability to easily lose electrons, as seen in metals like \( ext{Ca}(s) \), which oxidizes by losing electrons to form \( ext{Ca}^{2+} \)
• Compounds that can release electrons due to their high electron count, such as \( ext{N}_2 ext{O}_5(g) \)

In the original exercise, both \( ext{Ca}(s) \) and \( ext{N}_2 ext{O}_5(g) \) act as reductants. Understanding reductants is vital for both theoretical comprehension and practical applications involving energy generation, battery technology, and combating oxidative stress in biological systems.