Redox and Oxidation Numbers Exam Questions

(a) Oxidation = gain of oxygen; Reduction = loss of oxygen [1]

(b) Oxidation = loss of hydrogen; Reduction = gain of hydrogen [1]

(c) Oxidation = loss of electrons; Reduction = gain of electrons [1]

Examiner note: OILRIG: Oxidation Is Loss (of electrons), Reduction Is Gain (of electrons). All three definitions are equivalent ways of describing the same process. The electron definition is the most fundamental and applies to all redox reactions.

(a) Hydrogen (H2) is oxidised [1]

(b) Copper oxide (CuO) is reduced [1]

(c) H2 gains oxygen (becomes H2O) — oxidation; CuO loses oxygen (becomes Cu) — reduction [1]

Examiner note: In this reaction, hydrogen acts as the reducing agent (it causes the CuO to be reduced) and CuO acts as the oxidising agent (it causes the H2 to be oxidised). The reducing agent is itself oxidised.

(a) S = +6 [1] (Na = +1 each, O = -2 each; 2(+1) + S + 4(-2) = 0, so S = +6)

(b) Mn = +7 [1] (K = +1, O = -2 each; +1 + Mn + 4(-2) = 0, so Mn = +7)

(c) Cr = +3 [1] (O = -2 each; 2Cr + 3(-2) = 0, so 2Cr = +6, Cr = +3)

(d) Fe = +2 [1] (the charge on the ion equals its oxidation number)

Examiner note: Rules: uncombined elements = 0; oxygen = -2 (except in peroxides); hydrogen = +1 (except in metal hydrides); group I = +1; group II = +2; the sum of oxidation numbers in a compound = 0; in an ion, the sum = the charge.

(a) Zinc is oxidised [1] (Zn goes from 0 to +2) Copper is reduced [1] (Cu goes from +2 to 0)

(b) Oxidation: Zn → Zn2+ + 2e- [1] Reduction: Cu2+ + 2e- → Cu [1]

Examiner note: Zinc loses electrons (oxidised) and copper ions gain electrons (reduced). The sulfate ion is a spectator — its oxidation numbers do not change. This is a displacement reaction, which is always a redox reaction.

(a) Oxidation [1] — iron has lost an electron (oxidation number increases from +2 to +3)

(b) Reduction [1] — each chlorine atom has gained an electron (oxidation number decreases from 0 to -1)

(c) Neither [1] — no change in oxidation number

Examiner note: Oxidation = increase in oxidation number / loss of electrons. Reduction = decrease in oxidation number / gain of electrons. If the oxidation number stays the same, no redox has occurred.

• An oxidising agent causes another substance to be oxidised / is itself reduced [1]
• A reducing agent causes another substance to be reduced / is itself oxidised [1]
• Fe2O3 is the oxidising agent (it is reduced — Fe goes from +3 to 0); carbon is the reducing agent (it is oxidised — C goes from 0 to +4) [1]

Examiner note: This is the extraction of iron from its ore. Carbon (coke) reduces the iron oxide. Remember: the reducing agent is oxidised, and the oxidising agent is reduced. They do the opposite of what their name suggests to themselves.

(a) Bromide (Br-) is oxidised; chlorine (Cl2) is reduced [1] [1]

(b) Br goes from -1 in KBr to 0 in Br2 — the oxidation number increases, which is oxidation [1] Cl goes from 0 in Cl2 to -1 in KCl — the oxidation number decreases, which is reduction [1]

Examiner note: This is a halogen displacement reaction. Chlorine is more reactive than bromine, so it displaces bromide ions from solution. Chlorine is the oxidising agent (it oxidises Br- by taking its electrons).