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Chemistry 30

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Oxidation and reduction

Ten questions of mixed difficulty, covering Redox reactions. Print it, or work through it on screen — the answer key starts on its own page.

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Oxidation and reduction

Chemistry 30 · maddyhelps.com

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Circle the best answer for each question. Show your work in the space provided.

  1. In balancing a redox equation, electrons are balanced by

    1. a) adding water molecules until the charges match
    2. b) ignoring them, since they cancel automatically
    3. c) multiplying each half-reaction so the electrons lost equal the electrons gained
    4. d) adding electrons to both sides of the final equation

  2. What is the oxidation number of sulfur in SO₄²⁻?

    1. a) +6
    2. b) +2
    3. c) −2
    4. d) +4

  3. A cell is built from Zn²⁺/Zn (E° = −0.76 V) and Cu²⁺/Cu (E° = +0.34 V). The standard cell potential is

    1. a) −1.10 V
    2. b) +0.42 V
    3. c) −0.42 V
    4. d) +1.10 V

  4. In the reaction 2Na(s) + Cl₂(g) → 2NaCl(s), the reducing agent is

    1. a) NaCl(s), because it contains both elements
    2. b) Na(s), because it loses electrons
    3. c) Cl₂(g), because it gains electrons
    4. d) Na⁺, because it is positively charged

  5. In the half-reaction Cu²⁺(aq) + 2e⁻ → Cu(s), copper is

    1. a) reduced, because it loses electrons
    2. b) neither oxidized nor reduced
    3. c) oxidized, because it gains electrons
    4. d) reduced, because it gains electrons

  6. In a redox reaction, the oxidizing agent

    1. a) is oxidized, and loses electrons
    2. b) is reduced, and loses electrons
    3. c) is reduced, and gains electrons
    4. d) does not change oxidation number

  7. The oxidation number of oxygen in most compounds is

    1. a) −2
    2. b) +2
    3. c) −1
    4. d) 0

  8. A redox reaction is spontaneous when the standard cell potential, E°cell, is

    1. a) greater than 1.00 V
    2. b) negative
    3. c) positive
    4. d) zero

  9. When the half-reaction MnO₄⁻(aq) → Mn²⁺(aq) is balanced in acidic solution, the number of electrons transferred is

    1. a) 8
    2. b) 5
    3. c) 3
    4. d) 2

  10. Which of the following is a redox reaction?

    1. a) HCl(aq) + NaOH(aq) → NaCl(aq) + H₂O(l)
    2. b) CO₂(g) + H₂O(l) → H₂CO₃(aq)
    3. c) 2Mg(s) + O₂(g) → 2MgO(s)
    4. d) AgNO₃(aq) + NaCl(aq) → AgCl(s) + NaNO₃(aq)

Answer key · Oxidation and reduction

Chemistry 30 · maddyhelps.com

  1. c) multiplying each half-reaction so the electrons lost equal the electrons gained — Every electron lost by one species is gained by another, so the half-reactions are scaled until those numbers match and the electrons cancel. Electrons should never appear in the final balanced equation.
  2. a) +6 — Four oxygens at −2 give −8. The whole ion is −2, so sulfur must be +6: (+6) + (−8) = −2.
  3. d) +1.10 V — Copper has the higher reduction potential, so it is reduced at the cathode and zinc is oxidized: E°cell = E°cathode − E°anode = 0.34 − (−0.76) = +1.10 V. The positive value confirms the cell runs on its own.
  4. b) Na(s), because it loses electrons — Sodium goes from 0 to +1, losing electrons, so it is oxidized — which makes it the reducing agent. Chlorine goes from 0 to −1, so it is the oxidizing agent.
  5. d) reduced, because it gains electrons — Electrons appear on the reactant side, so they are being gained: reduction. The oxidation number falls from +2 to 0.
  6. c) is reduced, and gains electrons — An oxidizing agent oxidizes something else, which it can only do by taking electrons — so it is itself reduced. The names describe what each substance does to its partner, not what happens to it.
  7. a) −2 — Oxygen is −2 in nearly every compound. The exceptions worth knowing are peroxides such as H₂O₂, where it is −1, and elemental O₂, where it is 0.
  8. c) positive — A positive E°cell means the reaction can drive electrons through a circuit on its own. A negative value means the reverse reaction is the spontaneous one, and zero means the system is at equilibrium.
  9. b) 5 — Manganese goes from +7 in MnO₄⁻ to +2, a drop of 5, so 5 electrons are gained. The balanced half-reaction is MnO₄⁻ + 8H⁺ + 5e⁻ → Mn²⁺ + 4H₂O.
  10. c) 2Mg(s) + O₂(g) → 2MgO(s) — Magnesium goes from 0 to +2 and oxygen from 0 to −2, so electrons move. In the other three, every element keeps the same oxidation number — they are neutralization, precipitation and a combination reaction.