Unit 8
Thermochemistry
Why does a hand warmer get hot and a cold pack cold? This unit treats every reaction as an energy ledger: separate system from surroundings and heat from work, measure reaction enthalpies yourself in a calorimeter, use Hess’s law and enthalpies of formation to calculate reactions nobody can measure directly, and finally weigh bond-enthalpy estimates and the energy stored in fuels.
- 1 Energy, heat and enthalpy System and surroundings exchange energy in only two ways: heat and work. Enthalpy turns heat at constant pressure into a state function, so exothermic and endothermic changes fit on one energy-level diagram. 25 min
- 2 Calorimetry: measuring heats of reaction A thermometer reads temperature, not heat. With q = mcΔT, the calorimeter constant and a heat-leak extrapolation, a coffee cup and a steel bomb yield heats of neutralisation, solution and combustion. 30 min
- 3 Hess’s law and enthalpies of formation Enthalpy only cares where you start and finish. Snap measurable reactions together like building blocks to get ones nobody can measure; one table of standard enthalpies of formation covers thousands of reactions. 25 min
- 4 Bond enthalpies and fuels Breaking bonds costs, forming bonds pays: average bond enthalpies estimate gas-phase reaction enthalpies — until a molecule like benzene breaks the estimate. Why do fuels release heat? The answer hides in the weak O=O bond. 25 min