Summary GCSE chemistry- energy change grade 9 revision notes

Energy Change (ΔH- enthalpy)
Exothermic reaction
- Gives out energy (heat); -ΔH
- (more energy released when bonds
formed than require to be broken)
- Burning wood, petrol, gas,
neutralisation, respiration

Endothermic reaction
- Takes energy from surroundings; +ΔH
- Photosynthesis

Activation energy (Ea)
- Energy needed to start reaction

Calculate energy change= energy used in reactant subtract product
Exo (-ΔH) Endo (+ΔH)

Cells
- Chemical store of energy
- Potential energy; ability of atom to gain/ lose electron
- Total voltage= sum of total voltage of cell → more cells increases voltage

Zn ⇌ Zn2+ + 2e-
Cu ⇌ Cu2+ + 2e-

Electrons move from Zn (form Zn2+) to
Cu 2+ (form Cu)
- Induce a current (A)
Voltage; less reactive minus more
reactive




Potential ∝ reactivity
- Large pd → bigger difference in reactivity (metal)

Batteries
- > 2 cells connected in series
- Often alkaline (contains OH-)
- Mostly non-rechargeable (reactant used up in chemical reaction)
- Rechargeable batteries; reactants reversible ⇌ (when external electrical current
supplied)