Chemical Reactions and Equations
In one line
In a chemical reaction, substances change into new substances, and a balanced equation shows this with the same number of each atom on both sides.
Key points
- Signs that a reaction has happened: change in colour, gas given off, change in temperature, a solid (precipitate) forms, or a change in state.
- Reactants are on the left of the arrow, products on the right.
- Mass is never created or destroyed (law of conservation of mass), so equations must be balanced.
- States are written in brackets: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
- Exothermic reactions give out heat (burning, respiration). Endothermic reactions take in heat (decomposing limestone).
How to balance an equation
Fe + H₂O → Fe₃O₄ + H₂ (unbalanced)
- Count each atom on both sides.
- Balance the element in the biggest formula first (here, oxygen in Fe₃O₄: put 4 in front of H₂O).
- Balance the rest: 4 H₂ on the right, 3 Fe on the left.
- Never change the small numbers inside a formula, only the numbers in front.
3Fe(s) + 4H₂O(g) → Fe₃O₄(s) + 4H₂(g)
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Types of reactions
| Type | What happens | Example |
|---|---|---|
| Combination | Two or more substances join to form one | CaO + H₂O → Ca(OH)₂ (slaked lime, gives out heat) |
| Decomposition | One substance breaks into simpler ones, using heat, light or electricity | CaCO₃ → CaO + CO₂ (on heating) |
| Displacement | A more reactive element pushes out a less reactive one | Fe + CuSO₄ → FeSO₄ + Cu (blue solution turns green) |
| Double displacement | Two compounds swap ions, often forming a precipitate | Na₂SO₄ + BaCl₂ → BaSO₄↓ + 2NaCl (white precipitate) |
| Redox | One substance is oxidised while another is reduced | CuO + H₂ → Cu + H₂O |
Oxidation and reduction
- Oxidation
- Gain of oxygen or loss of hydrogen. In CuO + H₂ → Cu + H₂O, hydrogen is oxidised.
- Reduction
- Loss of oxygen or gain of hydrogen. In the same reaction, copper oxide is reduced.
- Corrosion
- A metal slowly eaten away by air and moisture. Rusting of iron is the common example. Prevent it by painting, oiling or galvanising.
- Rancidity
- Fats and oils in food get oxidised and smell and taste bad. Prevent it with airtight containers, nitrogen-filled chip packets or antioxidants.
Common mistakes
- Changing subscripts to balance. Writing H₂O₂ instead of 2H₂O changes the substance itself.
- Mixing up displacement and double displacement. Displacement has a single element on the left; double displacement has two compounds.
- Forgetting that oxidation and reduction happen together. If something is oxidised, something else is reduced.
Quick check
Revise in 30 seconds
| Balance | Same atoms both sides; change only the numbers in front |
|---|---|
| Combination | A + B → AB |
| Decomposition | AB → A + B |
| Displacement | A + BC → AC + B |
| Double displacement | AB + CD → AD + CB |
| Redox | Oxidation = gain O / lose H · Reduction = lose O / gain H |