Question

Identify the oxidizing and reducing agents in the following reactions: $$ \begin{array}{l} \text { (a) } 5 \mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}(a q)+2 \mathrm{MnO}_{4}^{-}(a q)+6 \mathrm{H}^{*}(a q) \longrightarrow \\ 2 \mathrm{Mn}^{2+}(a q)+10 \mathrm{CO}_{2}(g)+8 \mathrm{H}_{2} \mathrm{O}(l) \end{array} $$ (b) \(3 \mathrm{Cu}(s)+8 \mathrm{H}^{+}(a q)+2 \mathrm{NO}_{3}^{-}(a q) \longrightarrow\) $$ 3 \mathrm{Cu}^{2+}(a q)+2 \mathrm{NO}(g)+4 \mathrm{H}_{2} \mathrm{O}(l) $$

Short answer

a) Oxidizing agent: \(\mathrm{MnO}_{4}^{-}\), reducing agent: \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\). b) Oxidizing agent: \(\mathrm{NO}_{3}^{-}\), reducing agent: \(\mathrm{Cu}\).

Step-by-step solution

Identify Oxidation States - Part (a)

Determine the oxidation states of each element in the reaction (a). For example, in \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\), hydrogen has an oxidation state of +1, oxygen has -2, and carbon balanced to +3 (since two carboxyl groups are present). For \(\mathrm{MnO}_{4}^{-}\), manganese has an oxidation state of +7 and oxygen is -2.

Identify Changes in Oxidation States - Part (a)

In the product side of the reaction (a), \(\mathrm{Mn}^{2+}\) indicates manganese has an oxidation state of +2. Compare this change: Manganese goes from +7 to +2 (reduction), and carbon goes from +3 in \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) to +4 in \(\mathrm{CO}_{2}\) (oxidation).

Identify Agents - Part (a)

The substance that gets reduced is the oxidizing agent. Hence, \(\mathrm{MnO}_{4}^{-}\) is the oxidizing agent, and \(\mathrm{H}_{2} \mathrm{C}_{2} \mathrm{O}_{4}\) is the reducing agent because it is oxidized.

Identify Oxidation States - Part (b)

Determine the oxidation states of each element in the reaction (b). For \(\mathrm{Cu}\), copper in elemental form is 0, and in \(\mathrm{Cu}^{2+}\), it is +2. For \(\mathrm{NO}_{3}^{-}\), nitrogen is +5.

Identify Changes in Oxidation States - Part (b)

In the product side of reaction (b), nitrogen in \(\mathrm{NO}\) is +2. Compare this change: Copper goes from 0 in \(\mathrm{Cu}\) to +2 in \(\mathrm{Cu}^{2+}\) (oxidation), and nitrogen goes from +5 in \(\mathrm{NO}_{3}^{-}\) to +2 in \tt{NO}\ (reduction).

Identify Agents - Part (b)

The substance that gets reduced is the oxidizing agent. Hence, \(\mathrm{NO}_{3}^{-}\) is the oxidizing agent, and \(\mathrm{Cu}\) is the reducing agent because it is oxidized.

Key concepts

Oxidation States

Understanding oxidation states is crucial for identifying oxidizing and reducing agents in redox reactions.
An oxidation state, also called the oxidation number, is an indicator of the degree of oxidation of an atom in a chemical compound.
It shows the hypothetical charge an atom would have if all bonds were 100% ionic.
Oxidation numbers are assigned according to a set of rules, such as:

• Elements in their elemental form have an oxidation state of 0. For example, O₂ and H₂ have oxidation states of 0.
• The oxidation state of a monoatomic ion equals the charge of the ion. For instance, Na⁺ has an oxidation state of +1.
• In compounds, hydrogen usually has an oxidation state of +1, while oxygen usually has -2.
• The sum of oxidation states in a neutral molecule must be zero, and in an ion, it must be equal to the ion's charge.

For example, in H₂C₂O₄, hydrogen has an oxidation state of +1, oxygen has -2, and carbon's state is balanced to +3.

Redox Reactions

Redox reactions, also known as oxidation-reduction reactions, are processes where there is a transfer of electrons between two species.
These reactions are characterized by the change in oxidation states of the involved chemical elements.

In a redox reaction, one substance undergoes oxidation (loses electrons), and another undergoes reduction (gains electrons).

• Oxidation: An increase in oxidation state.
• Reduction: A decrease in oxidation state.

For instance, in the reaction 5 H₂C₂O₄ + 2 MnO₄^- + 6 H⁺ → 2 Mn²⁺ + 10 CO₂ + 8 H₂O, oxalic acid (H₂C₂O₄) gets oxidized, whereas permanganate (MnO₄^-) gets reduced.

Oxidizing Agent

An oxidizing agent, or oxidant, is a substance that gains electrons in a redox reaction and gets reduced.
It effectively causes the oxidation of another substance.
Identifying the oxidizing agent involves finding which substance undergoes a decrease in oxidation state.

For example, in the reaction 5 H₂C₂O₄ + 2 MnO₄^- + 6 H⁺ → 2 Mn²⁺ + 10 CO₂ + 8 H₂O, manganese in MnO₄^- goes from +7 to +2.
Here, MnO₄^- acts as the oxidizing agent.

Reducing Agent

A reducing agent, or reductant, is a substance that loses electrons in a redox reaction and gets oxidized.
It causes the reduction of another substance.
Identifying the reducing agent involves finding which substance undergoes an increase in oxidation state.

For example, in the reaction 5 H₂C₂O₄ + 2 MnO₄^- + 6 H⁺ → 2 Mn²⁺ + 10 CO₂ + 8 H₂O, carbon in H₂C₂O₄ goes from +3 to +4.
Here, H₂C₂O₄ acts as the reducing agent.

Chemical Reactions

Chemical reactions involve the transformation of reactants into products through the breaking and forming of bonds.
Redox reactions are a specific type of chemical reaction where oxidation and reduction occur simultaneously.
The understanding of redox reactions is essential because they are involved in various processes such as cellular respiration, photosynthesis, combustion, and industrial applications.

Redox reactions can often be identified by observing changes in oxidation states of the involved substances, signaling the transfer of electrons.

• Important real-world applications include energy production in batteries, metal corrosion and prevention, and biochemical pathways like respiration.

Hence, learning to balance redox reactions and identifying oxidation and reduction processes is vital for chemistry students.