ZIMSEC O Level · Combined Science · Form 3, Form 4
The reactivity series
From most to least reactive: potassium, sodium, calcium, magnesium, aluminium, (carbon), zinc, iron, lead, (hydrogen), copper, silver, gold.
- A more reactive metal displaces a less reactive one from its compounds: iron + copper sulfate → iron sulfate + copper.
- Metals below carbon can be extracted by heating their oxides with carbon. Iron is extracted in a blast furnace (Zimbabwe has iron ore and coal deposits).
Rusting
Iron rusts when it reacts with both oxygen and water. Prevent it by painting, oiling, galvanising (coating with zinc) or plating.
Signs of a chemical reaction
A new substance forms; there may be a colour change, gas given off, temperature change or precipitate.
- Exothermic reactions give out heat (burning, respiration).
- Endothermic reactions take in heat (thermal decomposition of limestone).
Rate of reaction
Reactions go faster with higher temperature, higher concentration, smaller particle size (larger surface area) and a catalyst.
Exam tip: Rusting needs BOTH air and water – an exam experiment often uses three test tubes to show this.
Check yourself
- Which is more reactive, zinc or copper?
- State two conditions needed for iron to rust.
- Give two ways of increasing the rate of a reaction.
Show answers
- Zinc.
- Oxygen (air) and water.
- Any two: increase temperature, increase concentration, use smaller pieces/powder, add a catalyst.
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