Chemistry · Rung 11 of 24

Oxidation and reduction

To oxidise is to lose electrons and to reduce is to gain them, and since there are no loose electrons lying around, the two always happen together. To keep track we use oxidation numbers, which are bookkeeping: oxygen counts −2 except in peroxides, hydrogen +1 except with metals, a free element counts 0, and everything must add up to the compound's charge. Whatever gets oxidised is the reducing agent and whatever gets reduced is the oxidising agent, which is the opposite of what it sounds like and is why it is always confused. This explains the battery: connect two metals with different appetites for giving up electrons and the electrons travel down the wire, and that is a current. It also explains corrosion, which is a battery nobody asked for, and electrolysis, which is the same reaction pushed backwards with a current from outside.

Where this sits

Comes after
Acids and bases
Opens up
Batteries and electrolysis

Exercises from this rung

Generated by the app, from easiest to hardest

  1. What oxidation number does oxygen take in an ordinary compound?

    • −2
    • −1
    • +2
    • 0

    Why Almost always −2, and that settles nearly every exercise. There are two exceptions worth knowing: in peroxides such as H₂O₂ it is −1, because there is an O—O bond; and in OF₂ it is +2, because fluorine is the only thing more electronegative. A free element, like the O₂ in air, is always 0.

    easier

  2. In Fe₂O₃, what oxidation number does iron take?

    • +3
    • −3
    • +2
    • +6

    Why Three oxygens at −2 add to −6, and the compound is neutral, so the two irons must add to +6: each is +3. The sum is always the same: fix the ones you know, add up and solve for the missing one. That is why red rust is Fe₂O₃, iron(III) oxide, while the black Fe₃O₄ holds a mixture of iron(II) and iron(III).

    in between

  3. What does electrolysis do?

    • It forces a reaction with an outside current
    • Heats a solution until it decomposes
    • Generates current from a reaction
    • Separates mixtures by size

    Why It is a battery in reverse. Instead of using a spontaneous reaction to draw current, current is put in to force one that is not: splitting water into hydrogen and oxygen, winning aluminium from its oxide — which consumes 3 % of the world's electricity — or laying down a chrome plating. The same chemistry, the same equation, pushed the other way.

    harder