What is the oxidation number of an uncombined element?
Always 0 (e.g. O₂, Na, Cl₂).
What is the oxidation number of a simple, monatomic ion?
Equal to its charge (e.g. Na⁺ = +1, O²⁻ = –2).
What is the oxidation number of oxygen in compounds?
Usually –2, except in peroxides (–1) and in OF₂ (+2).
What is the oxidation number of hydrogen in compounds?
Usually +1, except in metal hydrides (–1, e.g. NaH).
What is the oxidation number of fluorine?
Always –1 in its compounds (most electronegative element).
What is the sum of oxidation numbers in a neutral compound?
0
What is the sum of oxidation numbers in a polyatomic ion?
Equal to the overall ionic charge
Define oxidation in terms of electrons
Loss of electrons (OIL – Oxidation Is Loss).
Define reduction in terms of electrons
Gain of electrons (RIG – Reduction Is Gain).
Define oxidising agent.
A species that causes oxidation by accepting electrons and is reduced itself.
Define reducing agent.
A species that causes reduction by donating electrons and is oxidised itself.
How can redox reactions be identified using oxidation numbers?
A change in oxidation number shows redox. Increase = oxidation, decrease = reduction.
What are the steps to write a redox half-equation in acidic conditions?
1️⃣ Balance atoms other than O and H 2️⃣ Add H₂O to balance O 3️⃣ Add H⁺ to balance H 4️⃣ Add e⁻ to balance charge.
How are half-equations combined into a full redox equation?
Multiply half-equations so electrons cancel, then add together.
Why must electrons be balanced in a redox equation?
To ensure overall charge conservation.
Write the half-equation for Fe²⁺ → Fe³⁺.
Fe²⁺ → Fe³⁺ + e⁻ (oxidation)
Write the half-equation for MnO₄⁻ → Mn²⁺ in acidic conditions.
MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O (reduction)
Write the overall equation for Fe²⁺ reducing MnO₄⁻ in acid.
5Fe²⁺ + MnO₄⁻ + 8H⁺ → 5Fe³⁺ + Mn²⁺ + 4H₂O
What is the purpose of a redox titration?
To determine the concentration of a species involved in a redox reaction.
What colour change is seen in a MnO₄⁻ / Fe²⁺ titration?
Colourless → permanent pale pink at the end point (first excess MnO₄⁻).
