En anglais

Bookkeeping electrons with oxidation numbers

Science · Chemistry · ages 16-18

Name ______________________ Date ____________

In sulfuric acid H2SO4, what is the oxidation number of sulfur? Type the number without the plus sign, for example 4 for +4.
Answer: ______________
In most compounds, what oxidation number does oxygen take?
- -2
- -1
- +1
A species changes from +2 to 0 during a reaction. Has it been oxidised or reduced?
- Oxidised, because the number rose
- Reduced, because the number fell
- No redox, because the charge stayed
Which order balances a half equation in acid solution?
- Balance charge with electrons before water
- Other atoms, water, H+, electrons
- Add H+ before balancing oxygen
Manganate(VII) ions turn into Mn2+ ions in acid. How many electrons does each manganate ion take?
Answer: ______________
How do you combine two finished half equations?
- Match electrons, add, cancel
- Add halves without checking charge
- Subtract the smaller electron count
A student balances sulfate becoming SO2 in acid and writes SO4 2- + 4H+ + 4e- -> SO2 + 2H2O. What went wrong?
- Sulfate's 2- charge was missed, so only 2e- are needed
- The oxygen is unbalanced, so another water is needed
- Nothing is wrong; four H+ ions always need four electrons
Zinc metal meets copper(II) ions. Which species is oxidised?
- Copper ions, +2 to 0
- Zinc, 0 to +2 by losing electrons
- Neither, because no oxygen is involved

Answer key

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

6 · The two hydrogen atoms contribute +2 and the four oxygen atoms contribute -8. Neutral H2SO4 needs sulfur to contribute +6.
-2 · The usual rule assigns oxygen -2. Peroxides are a named exception.
Reduced, because the number fell · A fall in oxidation number represents gain of electrons, so the species is reduced.
Other atoms, water, H+, electrons · Balance atoms other than oxygen and hydrogen, then oxygen with water, hydrogen with H+, and charge with electrons.
5 · The balanced reduction is MnO4- + 8H+ + 5e- -> Mn2+ + 4H2O, so each manganate ion takes five electrons.
Match electrons, add, cancel · Multiply the half equations until the electrons lost and gained match. Add the halves and cancel those electrons.
Sulfate's 2- charge was missed, so only 2e- are needed · Count every charge. On the left, sulfate gives -2 and 4H+ give +4, a total of +2. The right side is neutral. So 2e- are needed: SO4 2- + 4H+ + 2e- -> SO2 + 2H2O. Dropping the 2- made the left look like +4, which led to 4e-.
Zinc, 0 to +2 by losing electrons · Zinc changes from 0 to +2 and loses electrons, so zinc is oxidised. Copper ions gain those electrons.