GCSE Chemistry Energy changes revision

Exothermic reactions transfer energy to the surroundings, usually as heat: the reaction mixture warms up. Combustion, oxidation and neutralisation are the headline exothermic families — burning fuel, hand warmers (iron rusting in a pouch), and acid meeting alkali all pour heat out. Endothermic reactions take energy from the surroundings, cooling them: thermal decomposition needs continuous heating, and the citric-acid-plus-sodium-hydrogencarbonate reaction chills so effectively it powers novelty cold packs and self-chilling cans.

A reaction profile plots energy against the progress of the reaction. Reactants sit on the left, products on the right, and a hump separates them — the activation energy, the minimum energy colliding particles need to react. In an exothermic profile the products finish lower than the reactants: the drop is the energy released, ΔH negative. In an endothermic profile the products finish higher: energy was absorbed, ΔH positive. The hump’s height is what catalysts attack — a catalyst offers a different route with a lower activation energy, speeding the reaction without changing ΔH.

Every bond holds energy. Breaking a bond takes in energy; making one releases it. The energy change of a gas-phase reaction is therefore ΔH = total energy to break bonds − total energy made by forming bonds. If the making releases more than the breaking cost, ΔH is negative — exothermic. For H₂ + Cl₂ → 2HCl with H–H = 436, Cl–Cl = 242, H–Cl = 431 kJ/mol: broken = 436 + 242 = 678; made = 2 × 431 = 862; ΔH = 678 − 862 = −184 kJ/mol. Strongly exothermic, as every chemist who has mixed hydrogen and chlorine in sunlight can attest.

1. 1Method

   Place 30 cm³ of dilute hydrochloric acid in a polystyrene cup (an insulator). Record its temperature. Add 5 cm³ of sodium hydroxide, stir, record the peak temperature.

2. 2Vary

   Repeat, adding 5 cm³ more alkali each time, always starting with fresh acid — the total volume grows by 5 cm³ each run.

3. 3Plot

   Plot maximum temperature change against volume of alkali. Two straight lines appear: temperature rising, then falling.

4. 4Read

   The intersection marks the volume of alkali that exactly neutralises the acid — the maximum temperature change, where neither reactant is in excess.

A chemical cell turns the electron-handover of a redox reaction into a usable current: two different metals (or a metal and carbon) dip into an electrolyte, and the more reactive metal pushes electrons around the circuit. The voltage depends on how different the two metals are — magnesium against copper gives a punchier cell than zinc against copper — and on the electrolyte. When the reactive metal or electrolyte runs out, the cell goes flat; rechargeable cells simply reverse the reaction with external electricity to restore the reactants.

A hydrogen–oxygen fuel cell feeds hydrogen to the anode and oxygen to the cathode; inside, the overall reaction 2H₂ + O₂ → 2H₂O releases electricity directly. In acidic electrolyte the half equations are: anode, H₂ → 2H⁺ + 2e⁻; cathode, O₂ + 4H⁺ + 4e⁻ → 2H₂O. The only exhaust is water — no carbon dioxide at the point of use. The trade-offs: hydrogen must be manufactured (often using fossil energy), stored under pressure, and supplied continuously; the cell is expensive. Fuel cells shine where weight and refuelling beats charging — spacecraft have used them since the 1960s, and buses and cars increasingly do.