What a learner can do afterwards
- Assigns oxidation numbers in a compound or a polyatomic ion using the standard rules
- Identifies what is oxidised and what is reduced from the changes in oxidation number
- Balances a half equation in acid solution using water and hydrogen ions
- Combines two half equations so that the electrons cancel exactly
1 · Read
A breathalyser catches a drunk driver with this same bookkeeping: it watches an orange chromium ion turn green as its oxidation number falls. An oxidation number is that bookkeeping charge: the charge an atom would carry if every bond were fully ionic. The rules are small and strict. Oxygen sits at -2, hydrogen at +1, Group 1 at +1, Group 2 at +2, fluorine at -1, and uncombined elements sit at 0. Everything must add to the overall charge, which is how the unknown falls out.
Try sulfur in sulfuric acid, H2SO4. The two hydrogen atoms give +2 and the four oxygen atoms give -8, and the molecule is neutral, so sulfur must be +6. In the manganate ion MnO4-, the four oxygens give -8 against an overall charge of -1, so manganese is +7. Run the sum every time and the numbers assign themselves.
Once numbers are assigned, redox reads off directly. The species whose number rises has lost electrons, so it is oxidised. The one whose number falls has gained electrons, so it is reduced. You saw this when chlorine displaced bromine, and in zinc meeting copper(II) ions zinc goes from 0 to +2 while copper goes from +2 to 0, so zinc is oxidised and copper is reduced. If electrons move between species, reach for the half equation method.
In acid, balance other atoms first, then water for oxygen, hydrogen ions for hydrogen, electrons for charge. Manganate(VII) to Mn2+ needs 5 electrons per ion. To combine, match electron counts, add and cancel them.
Oxidation numbers track the electrons, and balanced halves combine by cancelling electrons.
2 · Watch
Take it off screen
Where it sits
8 questions wait behind this lesson, each with its answer explained. Every answer feeds the sky: stars light as they are learned, and dim when it is time to come back.