maddyhelps

Chemistry 30

Chemistry 30 · Worksheets

Mixed review · Redox and cells

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

All worksheets

Mixed review · Redox and cells

Chemistry 30 · maddyhelps.com

Name
Date
Score
/ 10

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

  1. How many moles of electrons are needed to produce 1 mol of aluminum from Al³⁺?

    1. a) 3 mol
    2. b) 2 mol
    3. c) 27 mol
    4. d) 1 mol

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

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

  3. Which of the following is a redox reaction?

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

  4. In any electrochemical cell, oxidation occurs at the

    1. a) anode
    2. b) salt bridge
    3. c) cathode
    4. d) electrolyte

  5. A cell is built from Ag⁺/Ag (E° = +0.80 V) and Ni²⁺/Ni (E° = −0.26 V). The cell potential and anode are

    1. a) +1.06 V, with nickel as the anode
    2. b) −1.06 V, with silver as the anode
    3. c) +1.06 V, with silver as the anode
    4. d) +0.54 V, with nickel as the anode

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

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

  7. Which statement about an electrolytic cell is correct?

    1. a) It runs spontaneously, and its anode is positive
    2. b) It runs spontaneously, and its cathode is negative
    3. c) It needs an external power supply, and its anode is the positive electrode
    4. d) It needs an external power supply, and its anode is the negative electrode

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

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

  9. On a table of standard reduction potentials, a spontaneous reaction occurs when the strongest oxidizing agent present is

    1. a) higher on the left side of the table than the strongest reducing agent is on the right
    2. b) a metal rather than a non-metal
    3. c) in the same row as the reducing agent
    4. d) lower on the table than the reducing agent

  10. The purpose of the salt bridge in a voltaic cell is to

    1. a) supply the energy that drives the reaction
    2. b) keep the two solutions from touching at all
    3. c) carry electrons from one half-cell to the other
    4. d) let ions move between the half-cells so the solutions stay electrically neutral

Answer key · Mixed review · Redox and cells

Chemistry 30 · maddyhelps.com

  1. a) 3 mol — Al³⁺ + 3e⁻ → Al, so each aluminum atom needs three electrons. That is why producing aluminum by electrolysis uses so much electricity.
  2. a) 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.
  3. b) 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.
  4. a) anode — Oxidation always happens at the anode and reduction at the cathode, in both voltaic and electrolytic cells. An old memory hook: AN OX and RED CAT.
  5. a) +1.06 V, with nickel as the anode — Silver has the higher reduction potential, so it is reduced at the cathode and nickel is oxidized at the anode: E°cell = 0.80 − (−0.26) = +1.06 V. Adding the two potentials instead of subtracting gives 0.54 V.
  6. 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.
  7. c) It needs an external power supply, and its anode is the positive electrode — Electrolytic cells drive non-spontaneous reactions, so a power supply is required, and it makes the anode positive. In a voltaic cell the anode is the negative electrode — the one sign difference worth memorizing.
  8. d) 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.
  9. a) higher on the left side of the table than the strongest reducing agent is on the right — Reading a table arranged with the strongest oxidizing agents at the top left: if the oxidizing agent sits above the reducing agent, the reaction is spontaneous. That is the same statement as E°cell being positive.
  10. d) let ions move between the half-cells so the solutions stay electrically neutral — As the cell runs, one half-cell builds up positive charge and the other negative. Ions migrating through the salt bridge cancel that build-up; without it the charge separation stops the reaction almost immediately.