Chemical Reactions
Types of Reaction
Chemical reactions form new substances. Signs include gas, colour change, temperature change and precipitate. In a synthesis reaction elements combine (e.g. metal + oxygen → metal oxide). Decomposition splits a compound. Combustion of hydrocarbons produces carbon dioxide and water when oxygen is plentiful. Displacement occurs when a more reactive metal takes the place of a less reactive one in a compound. Neutralisation is acid + base → salt + water.
Equations and Conservation
Word equations name reactants and products; symbol equations must be balanced because atoms are conserved. State symbols (s, l, g, aq) show physical state. Exothermic reactions release energy to the surroundings (temperature rises); endothermic reactions absorb energy (temperature falls).
Rates of Reaction
Rate increases with higher temperature, higher concentration, greater surface area and a catalyst. Collision theory explains this: more frequent and more energetic collisions between particles. A catalyst speeds the reaction without being used up.
Traps to Watch
- Writing an unbalanced symbol equation and calling it correct.
- Confusing exothermic with "gives out light" only.
- Thinking a catalyst increases the yield rather than the rate.
Worked Example
Balance the combustion of methane: CH₄ + 2O₂ → CO₂ + 2H₂O. Count atoms: C 1=1, H 4=4, O 4=4. Balanced. Without the coefficient 2 before O₂, oxygen atoms would not tally.