What is a chemical reaction?
In a chemical reaction, the atoms in substances rearrange to form new substances. Bonds between atoms break, and new bonds form. No atoms are lost or created; they just change partners.
reactants → products
Writing chemical equations
A chemical equation uses symbols and formulas to show a reaction.
Word equation: methane + oxygen → carbon dioxide + water
Chemical equation: CH₄ + 2O₂ → CO₂ + 2H₂O
- The arrow means "yields" or "produces."
- The plus sign separates substances.
- State symbols are sometimes added: (s) solid, (l) liquid, (g) gas, (aq) dissolved in water.
The law of conservation of mass
In 1789, French chemist Antoine Lavoisier carefully weighed substances before and after reactions in sealed containers. He found the total mass never changed. This became the law of conservation of mass: matter is neither created nor destroyed in a chemical reaction.
Why does a burning log seem to lose mass? Its gases (CO₂ and water vapour) escape into the air. In a closed system, where nothing can enter or leave, mass stays the same.
| System | Example | Mass appears to… |
|---|---|---|
| Open | Burning wood outdoors | Decrease (gases escape) |
| Open | Iron rusting in air | Increase (oxygen from air joins in) |
| Closed | Baking soda and vinegar in a sealed bag | Stay the same |
Balancing equations
Because atoms aren’t created or destroyed, a chemical equation must have the same number of each kind of atom on both sides. To balance it, put a coefficient (a whole number) in front of formulas. Never change the subscripts, because that would change the substance.
Example: hydrogen and oxygen make water
Unbalanced: H₂ + O₂ → H₂O
| Atom | Left | Right |
|---|---|---|
| H | 2 | 2 |
| O | 2 | 1 |
Add a 2 in front of H₂O: H₂ + O₂ → 2H₂O. Now there are 4 H on the right, so put 2 in front of H₂:
2H₂ + O₂ → 2H₂O
| Atom | Left | Right |
|---|---|---|
| H | 4 | 4 |
| O | 2 | 2 |
Balanced!
Tips for balancing
- Count atoms of each element on both sides.
- Balance elements that appear in only one substance on each side first.
- Balance hydrogen and oxygen last.
- Check your final count.
Energy in reactions
| Exothermic | Endothermic | |
|---|---|---|
| Energy | Released | Absorbed |
| Surroundings feel | Warmer | Colder |
| Examples | Burning, hand warmers, respiration | Photosynthesis, instant cold packs, baking soda and vinegar (slightly) |
Rates of reaction
Reactions happen faster when:
- the temperature is higher
- reactants are more concentrated
- particles are smaller (more surface area)
- a catalyst is present (a substance that speeds a reaction without being used up)
Enzymes in your body are biological catalysts.
Common mistakes
- Changing subscripts to balance an equation. Change only coefficients.
- Thinking mass disappears when things burn. It becomes gases.
- Counting atoms wrong. In 3H₂O there are 6 H atoms and 3 O atoms.
Try it yourself
Balance these equations:
- Na + Cl₂ → NaCl
- Mg + O₂ → MgO
- CH₄ + O₂ → CO₂ + H₂O
- N₂ + H₂ → NH₃
(Answers: 2Na + Cl₂ → 2NaCl; 2Mg + O₂ → 2MgO; CH₄ + 2O₂ → CO₂ + 2H₂O; N₂ + 3H₂ → 2NH₃)
Quick recap
- In a chemical reaction, atoms rearrange into new substances.
- Chemical equations show reactants and products using formulas.
- Mass is conserved; in a closed system the total mass doesn’t change.
- Balance equations with coefficients, never by changing subscripts.
- Reactions can be exothermic (release energy) or endothermic (absorb energy).