Oxidation vs Reduction
Adding oxygen or removing hydrogen.
What this lesson covers
Why it matters
Pass hydrogen over black copper oxide and it turns to shiny reddish-brown copper, while the hydrogen leaves as water. In one reaction, one substance loses oxygen and the other gains it.
Predict first
In CuO + H₂ → Cu + H₂O, which substance is oxidised?
Oxidation = gain of oxygen / loss of hydrogen / loss of electrons. Reduction = loss of oxygen / gain of hydrogen / gain of electrons. The two always happen together (redox).
- Hydrogen — it gains oxygen to become water — correct
- Copper oxide — it gains oxygen
- Neither — it is only a colour change
What you do
Sort each description: is it oxidation, or reduction?
Check yourself
In CuO + H₂ → Cu + H₂O, the reducing agent is…
Hydrogen reduces CuO by taking its oxygen, so hydrogen is the reducing agent (and CuO the oxidising agent).
Oxidation and reduction always occur…
Electrons (or oxygen/hydrogen) transferred from one species are taken by another — they're two halves of one redox reaction.
- hydrogen — it removes oxygen from copper oxide — correct
- copper oxide — it supplies the oxygen
- water — it is the product
- together — one substance is oxidised as another is reduced — correct
- separately, in different reactions
- only when oxygen gas is present