Electrochemistry: Oxidation-Reduction Reactions and Agents

Chapter 20: Electrochemistry

Introduction to Electrochemistry

Electrochemistry is the branch of chemistry that explores the relationship between electrical energy and chemical reactions. It encompasses both spontaneous and nonspontaneous processes, with a primary focus on redox (oxidation-reduction) reactions, batteries, and corrosion. Understanding these concepts is essential for applications such as electric vehicle batteries and the electrochemical signals in biological systems.

Section 20.1: Oxidation States and Oxidation-Reduction (Redox) Reactions

Oxidation Numbers

Oxidation numbers (or states) are assigned to elements in chemical compounds to track electron transfer during reactions. These numbers help identify which elements are oxidized and which are reduced in a reaction.

• Elemental Form: The oxidation number of an element in its pure form is zero (e.g., Na, Fe).

• Monoatomic Ions: The oxidation number equals the ion's charge (e.g., Na+ is +1).

• Compounds: The sum of oxidation numbers for all atoms in a compound is zero.

• Polyatomic Ions: The sum of oxidation numbers equals the ion's charge.

Oxidation and Reduction

Redox reactions involve the transfer of electrons between species. The species that loses electrons is oxidized, while the one that gains electrons is reduced.

• Oxidation: Loss of electrons; increase in oxidation number.

• Reduction: Gain of electrons; decrease in oxidation number.

• Oxidizing Agent: The substance that causes oxidation by accepting electrons (is itself reduced).

• Reducing Agent: The substance that causes reduction by donating electrons (is itself oxidized).

Mnemonic: "LEO the lion says GER" — Losing Electrons is Oxidation; Gaining Electrons is Reduction.

Example: Oxidation of Zinc by Hydrochloric Acid

When zinc reacts with hydrochloric acid, zinc is oxidized (loses electrons) and hydrogen ions are reduced (gain electrons). This is a classic example of a redox reaction.

Exothermic vs. Endothermic Redox Reactions

Redox reactions can be either exothermic (release energy) or endothermic (absorb energy). The reaction between zinc and hydrochloric acid is exothermic, releasing heat as the reaction proceeds.

Sample Exercise 20.1: Identifying Oxidizing and Reducing Agents

Step-by-Step Approach

1. Assign Oxidation States: Use the rules above to determine the oxidation state of each atom in the reaction.

2. Identify Changes: Determine which elements increase (oxidized) or decrease (reduced) in oxidation state.

3. Label Agents: The substance oxidized is the reducing agent; the substance reduced is the oxidizing agent.

Example: Nickel–Cadmium (NiCd) Battery Reaction

• The oxidation state of Cd increases from 0 to +2 (Cd is oxidized; reducing agent).

• The oxidation state of Ni decreases from +4 to +2 (Ni is reduced; oxidizing agent).

Practice Exercise

Given a redox reaction, identify the oxidizing and reducing agents by assigning oxidation numbers and tracking electron transfer.

Summary Table: Key Terms in Redox Chemistry

Term	Definition
Oxidation	Loss of electrons; increase in oxidation number
Reduction	Gain of electrons; decrease in oxidation number
Oxidizing Agent	Substance that is reduced; causes oxidation
Reducing Agent	Substance that is oxidized; causes reduction

Key Equations

• General redox reaction: $\mathrm{Oxidant} + \mathrm{Reductant} \rightarrow \mathrm{Reduced\ form\ of\ oxidant} + \mathrm{Oxidized\ form\ of\ reductant}$

• Example (Zinc and Hydrochloric Acid): $\mathrm{Zn}(s) + 2\mathrm{HCl}(aq) \rightarrow \mathrm{ZnCl}_2(aq) + \mathrm{H}_2(g)$