Oxidation Numbers and Balancing Redox Equations · seed 1 · A4, ink-friendly. The answer key prints on its own page for grown-ups.

Bookkeeping electrons with oxidation numbers

Science · Chemistry · ages 16-18
Name ______________________   Date ____________

  1. In sulfuric acid H2SO4, what is the oxidation number of sulfur? Type the number with its sign as a plain number.

    Answer: ______________

  2. In most compounds, what oxidation number does oxygen take?

    • -2
    • -1
    • +1

  3. A species changes from +2 to 0 during a reaction. Has it been oxidised or reduced?

    • Oxidised, because it gained oxygen
    • Reduced, since electrons were gained
    • Nothing changed, the number is just bookkeeping

  4. Which order balances a half equation in acid solution?

    • Electrons first, then water, then hydrogen ions
    • Hydrogen ions first, then electrons, then water
    • Other atoms, water, hydrogen ions, electrons

  5. Manganate(VII) ions turn into Mn2+ ions in acid. How many electrons does each manganate ion take?

    Answer: ______________

  6. How do you combine two finished half equations?

    • Match electrons, add, cancel them
    • Add them at once and ignore the electrons
    • Subtract the smaller from the larger

  7. A student balances manganate to Mn2+ using 3 electrons and says the charge works out. What is wrong?

    • The atom balance is fine and 3 electrons is correct
    • Electrons are not needed at all in acid
    • Add water and H+ first, then 5 electrons

  8. Zinc metal meets copper(II) ions. Which species is oxidised?

    • Copper ions, going from +2 to 0
    • Zinc, 0 to +2 by losing electrons
    • Neither, because no oxygen is involved
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Answer key

For grown-ups. Fold this page away before handing over the rest.

Bookkeeping electrons with oxidation numbers W1-mt_4xuYKYgUex-s1

  1. 6 · Two hydrogens give +2 and four oxygens give -8, so sulfur is +6.
  2. -2 · Oxygen sits at -2 in almost every compound you meet.
  3. Reduced, since electrons were gained · A falling number means electrons gained, which is reduction.
  4. Other atoms, water, hydrogen ions, electrons · The fixed order is other atoms, water, hydrogen ions, electrons.
  5. 5 · Balancing the half equation needs 5 electrons per ion.
  6. Match electrons, add, cancel them · Matching electron counts lets the electrons cancel exactly.
  7. Add water and H+ first, then 5 electrons · Skipping water and hydrogen ions gives the wrong electron count.
  8. Zinc, 0 to +2 by losing electrons · Zinc loses electrons as its number rises, so zinc is oxidised.
Worksheet · LightMySky