Problem 39 - Chemistry The Molecular Nature of Matter

Question

For the following reactions, identify the substance oxidized, the substance reduced, the oxidizing agent, and the reducing agent. $$ \begin{array}{l} \text { (a) } 2 \mathrm{HNO}_{3}+3 \mathrm{H}_{3} \mathrm{AsO}_{3} \longrightarrow \\ 2 \mathrm{NO}+3 \mathrm{H}_{3} \mathrm{AsO}_{4}+\mathrm{H}_{2} \mathrm{O} \\ \text { (b) } \mathrm{NaI}+3 \mathrm{HOCl} \longrightarrow \mathrm{NaIO}_{3}+3 \mathrm{HCl} \end{array} $$ (c) \(2 \mathrm{KMnO}_{4}+5 \mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}+3 \mathrm{H}_{2} \mathrm{SO}_{4} \longrightarrow\) $$ 10 \mathrm{CO}_{2}+\mathrm{K}_{2} \mathrm{SO}_{4}+2 \mathrm{MnSO}_{4}+8 \mathrm{H}_{2} \mathrm{O} $$ (d) \(6 \mathrm{H}_{2} \mathrm{SO}_{4}+2 \mathrm{Al} \longrightarrow \mathrm{Al}_{2}\left(\mathrm{SO}_{4}\right)_{3}+3 \mathrm{SO}_{2}+6 \mathrm{H}_{2} \mathrm{O}\)

Step-by-Step Solution

Answer

(a) Oxidized: H3AsO3, Reduced: HNO3, Oxidizing agent: HNO3, Reducing agent: H3AsO3. (b) Oxidized: NaI, Reduced: HOCl, Oxidizing agent: HOCl, Reducing agent: NaI. (c) Oxidized: H2C2O4, Reduced: KMnO4, Oxidizing agent: KMnO4, Reducing agent: H2C2O4. (d) Oxidized: Al, Reduced: H2SO4 (to SO2), Oxidizing agent: H2SO4 (producing SO2), Reducing agent: Al.

Step 1: Analyze Reaction (a)

Observe changes in oxidation states: In HNO3, N has an oxidation state of +5 and gets reduced to +2 in NO. In H3AsO3, As has an oxidation state of +3 and gets oxidized to +5 in H3AsO4.

Step 2: Identify in Reaction (a)

Substance oxidized: H3AsO3 (As goes from +3 to +5). Substance reduced: HNO3 (N goes from +5 to +2). Oxidizing agent: HNO3 (provides oxygen and gets reduced). Reducing agent: H3AsO3 (loses oxygen and gets oxidized).

Step 3: Analyze Reaction (b)

In NaI, I has an oxidation state of -1 and gets oxidized to +5 in NaIO3. In HOCl, Cl has an oxidation state of +1 and gets reduced to -1 in HCl.

Step 4: Identify in Reaction (b)

Substance oxidized: NaI (I goes from -1 to +5). Substance reduced: HOCl (Cl goes from +1 to -1). Oxidizing agent: HOCl (Cl gets reduced). Reducing agent: NaI (I gets oxidized).

Step 5: Analyze Reaction (c)

In KMnO4, Mn has an oxidation state of +7 and gets reduced to +2 in MnSO4. In H2C2O4, C has an oxidation state of +3 which gets oxidized to +4 in CO2.

Step 6: Identify in Reaction (c)

Substance oxidized: H2C2O4 (C goes from +3 to +4). Substance reduced: KMnO4 (Mn goes from +7 to +2). Oxidizing agent: KMnO4 (Mn gets reduced). Reducing agent: H2C2O4 (C gets oxidized).

Step 7: Analyze Reaction (d)

In H2SO4, S has an oxidation state of +6; it is reduced to +4 in SO2 and remains at +6 in Al2(SO4)3. Al starts at an oxidation state of 0 and gets oxidized to +3 in Al2(SO4)3.

Step 8: Identify in Reaction (d)

Substance oxidized: Al (Al goes from 0 to +3). Substance reduced: H2SO4 (S goes from +6 to +4 in SO2). Oxidizing agent: H2SO4 (S gets reduced in SO2). Reducing agent: Al (Al gets oxidized).

Key Concepts

Oxidation and Reduction

Understanding oxidation and reduction is fundamental for mastering redox reactions in chemistry. These processes are essentially two sides of the same coin, occurring simultaneously during a redox reaction.

Oxidation refers to the loss of electrons by an atom, molecule, or ion. As it loses electrons, its oxidation state increases. In the classroom, it's often memorized using the phrase 'LEO says GER,' where 'LEO' means 'Loss of Electrons is Oxidation.' For example, in the provided exercise, arsenic in H₃AsO₃ is oxidized from an oxidation state of +3 to +5 in H₃AsO₄, meaning it has lost electrons.

Reduction is the gain of electrons, leading to a decrease in the oxidation state. 'GER' in the mnemonic corresponds to 'Gain of Electrons is Reduction.' Nitrogen in HNO₃ is reduced as it gains electrons, transitioning from an oxidation state of +5 in HNO₃ to +2 in NO.

The concept of electron transfer is at the heart of understanding these reactions, and keeping track of electron movement can help identify what is being oxidized and what is being reduced in a given chemical equation.

Oxidizing and Reducing Agents

Identifying the oxidizing and reducing agents in a redox reaction is key to understanding the direction of electron flow. The oxidizing agent, or oxidant, is the substance that accepts electrons and gets reduced in the process. It 'promotes' oxidation of the other substance. In contrast, the reducing agent, or reductant, donates electrons and is oxidized.

In the exercise, for instance, nitric acid (HNO₃) acts as an oxidizing agent because it gains electrons (is reduced) while inducing the oxidation of H₃AsO₃. Conversely, H₃AsO₃ serves as the reducing agent since it loses electrons (is oxidized) and causes the reduction of HNO₃.

An easy way to remember this is that a reducing agent 'reduces' the other substance and itself gets 'oxidized,' whilst an oxidizing agent 'oxidizes' the other substance and gets 'reduced' itself. These agent roles are essential for driving redox reactions forward.

Oxidation States

The oxidation state, also known as oxidation number, is a useful concept that helps track the transfer of electrons in redox reactions. It is an indicator of the degree of oxidation or reduction an atom undergoes in a compound or when it forms an ion.

For example, in reaction (c) of the exercise, manganese (Mn) in KMnO₄ has an oxidation state of +7 and is reduced to a +2 state in MnSO₄. This signifies that Mn has gained electrons. Oxidation states can be assigned using a set of rules, such as the fact that the sum of oxidation states in a neutral compound is zero, and in a polyatomic ion, it is equal to the ion's charge.

When sketching out a redox reaction, careful accounting of the changes in oxidation states can reveal the subtleties of the electron transfer occurring. This provides a clear framework for identifying the substances undergoing oxidation and reduction, and the agents driving these processes.