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Chemical reactions and energy
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In a chemical reaction, atoms swap partners to make new substances. No atoms are made or destroyed.
Atoms rearrange
In a chemical change, new substances are made. Signs: a gas fizzes off, the colour changes, the temperature changes, or a solid appears. Most are hard to reverse.
The atoms are rearranged, not created or destroyed, so the total mass stays the same (conservation of mass).
We write a word equation: reactants → products.
magnesium + oxygen → magnesium oxide
Or a symbol equation: 2Mg + O2 → 2MgO
The atoms are rearranged, not created or destroyed, so the total mass stays the same (conservation of mass).
We write a word equation: reactants → products.
magnesium + oxygen → magnesium oxide
Or a symbol equation: 2Mg + O2 → 2MgO
Types of reaction
Combustion: burning a fuel in oxygen. methane + oxygen → carbon dioxide + water.
Oxidation: a substance gains oxygen. iron + oxygen → iron oxide (rusting needs water too).
Thermal decomposition: heat breaks one substance into two or more. copper carbonate → copper oxide + carbon dioxide.
Displacement: a more reactive metal pushes a less reactive one out of its compound (more in Year 9).
Oxidation: a substance gains oxygen. iron + oxygen → iron oxide (rusting needs water too).
Thermal decomposition: heat breaks one substance into two or more. copper carbonate → copper oxide + carbon dioxide.
Displacement: a more reactive metal pushes a less reactive one out of its compound (more in Year 9).
Energy and catalysts
Exothermic reactions give out heat, so the temperature rises (burning, neutralisation).
Endothermic reactions take in heat, so the temperature falls (thermal decomposition, dissolving some salts).
During a change of state, the temperature stays the same until all of the substance has melted or boiled.
A catalyst speeds up a reaction without being used up.
Endothermic reactions take in heat, so the temperature falls (thermal decomposition, dissolving some salts).
During a change of state, the temperature stays the same until all of the substance has melted or boiled.
A catalyst speeds up a reaction without being used up.
2.4 g of magnesium burns to make 4.0 g of magnesium oxide. What mass of oxygen reacted?
- Mass of reactants = mass of products.
- 2.4 + oxygen = 4.0
- Oxygen = 4.0 − 2.4
Answer: 1.6 g
Reactions rearrange atoms, so mass is conserved. Combustion, oxidation, thermal decomposition and displacement are types of reaction. Exothermic reactions heat the surroundings; endothermic reactions cool them. Catalysts speed up reactions and are not used up.
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