Sec 4 Chemistry Oxidation & Reduction topical questions

Define oxidation in two ways: in terms of oxygen, and in terms of electrons.

In each reaction, state which element is oxidised and which is reduced.

1. (a)2Mg + O₂ → 2MgO[1]
2. (b)Zn + CuSO₄ → ZnSO₄ + Cu[1]
3. (c)Cl₂ + 2KI → I₂ + 2KCl[1]
4. (d)H₂ + CuO → Cu + H₂O[1]

State the oxidation state of the underlined element.

1. (a)Carbon in CO₂[1]
2. (b)Nitrogen in HNO₃[1]
3. (c)Sulfur in SO₄²⁻[1]
4. (d)Chlorine in Cl₂[1]

Name one common oxidising agent and one common reducing agent, each with the colour change that shows it has worked.

Iron(III) oxide reacts with carbon monoxide: Fe₂O₃ + 3CO → 2Fe + 3CO₂. State which substance is oxidised, which is reduced, and name the oxidising agent and the reducing agent. Explain how the oxidation-state changes support your answers.

Magnesium reacts with dilute hydrochloric acid: Mg + 2HCl → MgCl₂ + H₂.

1. (a)Write the two half-equations for the reaction.[2]
2. (b)Combine them and show that electrons cancel.[1]
3. (c)State which species is the oxidising agent.[1]

A student adds aqueous potassium iodide to chlorine water and observes the brown colour of iodine appearing.

1. (a)Write the full equation for the reaction.[2]
2. (b)Write the ionic half-equation for the change happening to the iodide ions.[1]
3. (c)Explain which species is the oxidising agent.[2]

Explain why the reaction between hydrochloric acid and sodium hydroxide is not a redox reaction, but the reaction between chlorine and potassium iodide is.

When dilute sulfuric acid is added to potassium manganate(VII) solution and then sulfur dioxide is bubbled through, the purple colour disappears.

1. (a)State what this observation tells you about the sulfur dioxide.[1]
2. (b)Name the substance reduced and the substance oxidised.[2]
3. (c)Sulfur in SO₂ ends as sulfate ions (S = +6). State the oxidation-state change of sulfur and explain why it confirms your answer to (a).[2]

A scrapyard separates iron from copper by stirring the mixed metal pieces into aqueous copper(II) sulfate containing excess copper ions.

1. (a)Write the two half-equations for the reaction that dissolves the iron.[2]
2. (b)Combine them into the overall ionic equation.[1]
3. (c)Identify the reducing agent and explain the choice.[2]
4. (d)State one visible change during the reaction.[1]

Two identical iron nails are prepared: nail 1 is painted; nail 2 is connected electrically to a strip of magnesium and left bare. Both are left in open beakers of water for a week.

1. (a)State what is needed for rusting and describe the expected appearance of each nail.[2]
2. (b)Explain, in terms of redox, why the magnesium protects nail 2 even where water reaches it.[2]
3. (c)Explain why the paint fails if scratched, and name this type of protection.[2]

Sulfur dioxide is passed through acidified potassium dichromate(VI), turning the solution from orange to green.

1. (a)State the type of agent SO₂ behaves as here.[1]
2. (b)The chromium ends as Cr³⁺. Calculate the oxidation-state change of chromium and state whether dichromate is oxidised or reduced.[2]
3. (c)Sulfur in SO₂ is oxidised to sulfate (+6). State the sulfur state change and write one sentence linking both changes to electron transfer.[2]
4. (d)Explain why this test also identifies SO₂ in the gas tests of qualitative analysis.[1]