能量变化

• Exothermic 放热 reactions give energy out — the surroundings warm; endothermic 吸热 reactions take it in — they cool.
• Reaction profile 反应能量图s show the activation energy 活化能 and the overall change; (HT) bond energies add up the same story.
• Chemical cells and fuel cell 燃料电池s (chemistry only) turn reaction energy into electricity.

Energy is conserved: energy transferred to the surroundings comes off the products' store. An exothermic reaction transfers energy out — the surroundings' temperature rises (combustion, many oxidations, neutralisation; uses: hand warmers, self-heating cans). An endothermic reaction takes energy in — the temperature falls (thermal decomposition, citric acid + sodium hydrogencarbonate; uses: sports injury packs). Judge each from the temperature change of the surroundings; ΔH calculations are not required.

Reaction profiles: reactions need colliding particles with at least the activation energy. Draw and read energy-level diagrams — relative energies of reactants and products, the activation energy arrow from the reactants' level, the overall energy change, curved line between. Exothermic: products below reactants; endothermic: products above.

(HT) Bond energies 键能: energy is supplied to break reactant bonds and released when product bonds form:

• exothermic — forming releases more than breaking costs;
• endothermic — breaking costs more than forming releases.

Overall change = Σ(bond energies broken) − Σ(bond energies formed). A catalyst lowers the activation energy — it does not change the overall energy change.

RP4: investigate the variables affecting temperature changes in reacting solutions (e.g. volume/concentration) — plan variables, measure ΔT, plot, conclude.

Cells contain chemicals that react to produce electricity. The voltage depends on the type of electrode and electrolyte; a simple cell is two different metals in an electrolyte — the more reactive metal produces the higher voltage (it releases electrons more readily). Batteries = cells in series for greater voltage.

Non-rechargeable 可充电 (alkaline) cells stop when a reactant runs out; rechargeable cells reverse their reactions when an external current is supplied.

Fuel cells: fuel (e.g. hydrogen) + oxygen supplied continuously; the fuel is oxidised electrochemically to give a potential difference. Overall: hydrogen → water. Evaluate against rechargeable batteries: no pollutants at point of use, continuous refuel; but hydrogen storage, production and cost weigh against. (HT) half-equations for the hydrogen fuel cell:

• negative electrode: 2 H₂ → 4 H⁺ + 4e⁻ (or 2 H₂ + 4 OH⁻ → 4 H₂O + 4e⁻)
• positive electrode: O₂ + 4 H⁺ + 4e⁻ → 2 H₂O (or O₂ + 2 H₂O + 4e⁻ → 4 OH⁻)