10.0     DEFINITIONS OF OXIDATION AND REDUCTION

• Oxidation is defined as the addition of oxygen and the removal of hydrogen.
• Reduction is defined as the removal of oxygen and addition of hydrogen.

In terms of electron transfer,

• Oxidation is a process involving the loss of electron(s).
• Reduction is a process involving the gain of electron(s).

In terms of oxidation number,

• Oxidation is defined as an increase in oxidation number.
• Reduction is defined as a decrease in oxidation number.

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         10.1     OXIDIZING AND REDUCING AGENTS

AN OXIDIZING AGENT is a substance which accepts electron(s) i.e. undergoes reduction. Examples include MnO₂, KMnO₄, NO₂, K₂Cr₂O₇, conc. HNO₃, hot conc. H₂SO₄ etc.

TEST FOR OXIDIZING AGENT

• It oxidizes sulphide ions, S^2– in hydrogen sulphide, to a yellow precipitate of sulphur.
• It oxidizes green iron(II) chloride to brown iron(III) chloride.

A REDUCING AGENT is a substance which donates electron(s) i.e. undergoes oxidation. Examples include H₂S, SO₄, KI, NO₂, CO, H₂ etc.

TEST FOR REDUCING AGENT

• It reduces purple acidified potassium tetraoxomanganate(VII) ion MnO₄ ^– , to manganese (II) ion, Mn²⁺.
• It reduces orange acidified potassium heptaoxochromate(VI) ion, Cr₂O₇ ^2– , to green chromium (III) ion, Cr³⁺ .

         10.2     BALANCING REDOX REACTION

A redox reaction (short for reduction-oxidation reaction) is a chemical reaction in which electrons are transferred between two substances. In every redox reaction, oxidation and reduction happen simultaneously, meaning one substance is oxidized while another is reduced. For example, in the reaction between zinc and copper sulfate:

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In balancing a redox reaction, the following steps are involved:

• Write the Unbalanced Redox Reaction – Separate the overall reaction into two half-reactions (oxidation and reduction).
• Balance Atoms Other Than Oxygen and Hydrogen – In each half-reaction, first balance all atoms except oxygen (O) and hydrogen (H).
• Balance Oxygen Atoms – Add water (H₂O) molecules to balance the oxygen atoms in each half-reaction.
• Balance Hydrogen Atoms – Add hydrogen ions (H⁺) to balance the hydrogen atoms in each half-reaction.
• Balance the Charge – Add electrons (e⁻) to one side of the half-reaction to balance the charge. Electrons are added to the reduction half-reaction on the left side and to the oxidation half-reaction on the right side.
• Equalize the Number of Electrons – Multiply the half-reactions by appropriate factors so that the number of electrons in both half-reactions is the same.
• Combine the Half-Reactions – Add the two half-reactions together, canceling out the electrons and any species that appear on both sides of the equation.
• Final Check – Ensure that both mass (atoms) and charge are balanced in the final equation.

If the reaction is in basic solution, follow these additional steps:

• Neutralize Excess H⁺ – Add OH⁻ ions to both sides to neutralize any H⁺ ions, forming water.
• Simplify the Equation – Cancel any water molecules that appear on both sides to finalize the balanced redox reaction.

EXAMPLE –Balance the redox reaction between MnO₄⁻ and Fe²⁺ in an acidic solution.