What is a chemical change?
In a chemical change a new substance forms. You cannot easily get the old one back. Signs: a new colour, gas bubbles, a solid forming, or heat and light.
Inside, atoms do not vanish. They only swap partners. The same atoms are there before and after. That is why we balance equations.
Acids and bases
An acid gives hydrogen ions (H⁺) in water. A base gives hydroxide ions (OH⁻). The pH scale tells how acidic: below 7 is acid, 7 is neutral, above 7 is base.
Neutralisation: H⁺ + OH⁻ → H₂O. Acid + base gives salt + water. This is how an antacid tablet calms stomach acid, and how factories make many salts.
Oxidation and reduction (redox)
Oxidation is losing electrons. Reduction is gaining electrons. They always happen together, so we say redox. Remember OIL RIG: Oxidation Is Loss, Reduction Is Gain.
Rusting, burning and a battery are redox. In a battery the electrons are made to travel through a wire, and that flow is electricity.
Chemical reactions and heat
Breaking bonds takes energy. Making bonds gives energy back. The difference is the heat of the reaction.
- Exothermic: more energy given out than taken in. It feels hot. Burning fuel, neutralisation.
- Endothermic: more energy taken in. It feels cold. A cold pack, photosynthesis.
The heat change is written as ΔH. It is negative for exothermic and positive for endothermic. Units: kJ per mole.
Reaction rate and chemical equilibrium
Rate is how fast reactants turn into products. It goes up with higher temperature, higher concentration, smaller pieces (more surface) and a catalyst.
In a closed container many reactions are reversible (⇄). At equilibrium the forward and backward changes go at the same speed, so the amounts stop changing, but the reaction has not stopped. Factories use this: they choose conditions that push the balance towards the product (Le Chatelier's idea).
Nuclear energy
In chemical change only the outer electrons rearrange. In nuclear change the nucleus itself changes.
Fission: a big nucleus (like uranium-235) splits and releases a lot of energy and more neutrons, which can split more nuclei (a chain reaction). Fusion: small nuclei join, as in the Sun. Per kilogram, nuclear change gives about a million times the energy of burning. Safe handling and waste storage matter a lot.
Key formulas and definitions
- Acid + base → salt + water; H⁺ + OH⁻ → H₂O
- Oxidation = loss of electrons; reduction = gain of electrons (OIL RIG)
- ΔH < 0 exothermic; ΔH > 0 endothermic
- Rate = change in amount ÷ time
- At equilibrium: rate forward = rate backward
- Nuclear (Einstein): E = m c²
Worked examples
1. 20 mL of acid is exactly neutralised by 20 mL of base of the same strength. What is left in the beaker?
Only salt and water. Every H⁺ met an OH⁻ and became water, so the mixture is neutral (pH about 7).
2. Zinc gives two electrons to copper ions: Zn → Zn²⁺ + 2e⁻ and Cu²⁺ + 2e⁻ → Cu. Which is oxidised and which is reduced?
Zinc loses electrons, so zinc is oxidised. Copper ions gain electrons, so they are reduced. Together this is a redox reaction.
3. A reaction heats the test tube from 25 °C to 40 °C. Is it exothermic or endothermic? What is the sign of ΔH?
The surroundings got warmer, so heat went out. It is exothermic and ΔH is negative.
4. In a reaction at 20 °C a gas takes 60 s to fill a bag. At 40 °C it takes 20 s. By what factor did the rate change?
Rate = 1 ÷ time. The time fell from 60 s to 20 s, so the rate became 60 ÷ 20 = 3 times faster.
Common mistakes
- Thinking oxidation always needs oxygen. It means losing electrons; oxygen is only one case.
- Thinking equilibrium means the reaction has stopped. Both directions still go on, at equal speed.
- Mixing up the sign of ΔH: exothermic is negative because the chemicals lose energy.
- Thinking nuclear change is just a bigger chemical reaction. Here the nucleus itself changes.