Chemical equations
A chemical equation shows what happens in a reaction. The reactants (the substances that react) are on the left and the products (the substances made) are on the right of an arrow. A word equation uses names: magnesium + oxygen → magnesium oxide. A symbol equation uses formulae and state symbols: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
Balancing equations
Atoms are not created or destroyed, so there must be the same number of each kind of atom on both sides. Balance by putting numbers (coefficients) in front of formulae. Never change the small numbers in a formula.
Types of chemical reactions
| Type | What happens | Example |
|---|---|---|
| Combination (synthesis) | Two or more substances join to make one | |
| Decomposition | One substance breaks into two or more | |
| Displacement | A more reactive element takes the place of a less reactive one | |
| Neutralisation | Acid + base → salt + water | |
| Precipitation | Two solutions react to make an insoluble solid | |
| Combustion | A substance burns in oxygen giving out heat and light |
Energy changes
Breaking bonds takes in energy and making bonds gives it out. In an exothermic reaction more energy is given out than taken in (the surroundings get hotter): burning, neutralisation, respiration. In an endothermic reaction more energy is taken in (the surroundings get colder): photosynthesis, dissolving ammonium nitrate.
Reactions in daily life
| Reaction | Where we meet it |
|---|---|
| Rusting of iron | Iron + oxygen + water make hydrated iron oxide; protected by paint, oil or galvanising |
| Burning of fuels | Cooking with charcoal, gas or firewood; vehicle engines |
| Respiration | Glucose + oxygen → carbon dioxide + water + energy in our cells |
| Photosynthesis | Plants make glucose from carbon dioxide and water using light |
| Fermentation | Yeast turns sugar into ethanol and carbon dioxide: bread and beer |
| Digestion and cooking | Enzymes and heat change food chemically |
Magnesium burns: 2Mg + O₂ → 2MgO. Choose the mass of magnesium. Find the oxygen it takes in and the oxide formed. (Mg = 24, O = 16)
Key terms
- Reactant
- A substance that takes part in a reaction.
- Product
- A substance made in a reaction.
- Exothermic
- A reaction that gives out heat to the surroundings.
- Endothermic
- A reaction that takes in heat from the surroundings.
Summary
- A balanced equation has the same atoms on both sides because atoms are conserved.
- Types: combination, decomposition, displacement, neutralisation, precipitation and combustion.
- Exothermic reactions give out energy; endothermic reactions take it in.
- Rusting, burning, respiration, photosynthesis and fermentation are reactions in daily life.