Questions: Oxidation-Reduction Reactions

Question 1

In the reaction Zn + CuSO₄ → ZnSO₄ + Cu, zinc is oxidized. Which species is the oxidizing agent?

• A. Zinc (Zn)
• B. Copper ion (Cu²⁺)
• C. Sulfate ion (SO₄²⁻)
• D. Zinc sulfate (ZnSO₄)

Answer: (B)

The oxidizing agent is the species that causes oxidation by accepting electrons — it is the species that gets reduced itself. Cu²⁺ gains two electrons to become Cu(0), so it is reduced. Therefore Cu²⁺ is the oxidizing agent. Zinc loses electrons (is oxidized) making it the reducing agent. The naming is counterintuitive: the oxidizing agent is itself reduced.

Question 2

An atom's oxidation state in a compound is equal to its actual electric charge.

• T. True
• F. False

Answer: False

Oxidation states are a bookkeeping formalism, not measurements of real charge. They are assigned by applying a set of rules (e.g., shared electrons go to the more electronegative atom). In covalent compounds like CO₂, carbon has an oxidation state of +4, but it carries nothing close to a +4 charge — the electrons are shared, not transferred. Only in purely ionic compounds do oxidation states approximate actual charges.

Question 3

What does 'OIL RIG' stand for, and why is each half of a redox reaction called a 'half-reaction'?

Model answer: OIL RIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). Each half-reaction is called that because it captures only one side of the electron transfer: the oxidation half shows the species losing electrons, and the reduction half shows the species gaining them. Together they sum to the complete redox reaction.

Separating a redox reaction into two half-reactions is useful because it lets you balance electrons, atoms, and charge independently in each half before combining. The number of electrons lost in oxidation must equal the number gained in reduction, which is enforced by multiplying the half-reactions by appropriate coefficients before adding.