Chemistry
Thermochemistry
3,784 words, 10 sections. The first two are free.
The opening sections
Definitions first, then the reasoning. Each section is short enough to read in one sitting.
Thermochemistry is the accounting of energy in chemical change, and the paper treats it as the most computational chapter in the subject. The guide holds 183 distinct questions on it, drawn from 82 official papers between 2000 and 2026. Chemistry carries 45 of the 180 marks, and inside this chapter enthalpy with calorimetry accounts for 109 questions, the first law with Hess's law for 37, and the classification of reactions as exothermic or endothermic for a further 37. Mean item difficulty is 1.54 on the guide scale, the lowest in the chemistry guide, and 719 active questions sit in the practice bank. A student who has this chapter under control can attach the right sign to a heat change, run a or a calculation with correct units, and close a Hess cycle without losing a step.
A thermodynamic system is the part of the universe under study; everything outside it is the surroundings, and the two are separated by a boundary. Systems are classified by what can cross that boundary. An open system exchanges both energy and matter with the surroundings, a closed system exchanges energy but not matter, and an isolated system exchanges neither. A reaction in an open beaker on a bench is open, the same beaker with a lid is closed, and a sealed insulated flask approaches isolated. Thermodynamics is the study of these energy transfers.
Thermodynamic quantities divide into state functions, whose value depends only on the current state and not on how the system reached it, and path functions, whose value depends on the route. Internal energy, enthalpy, volume, pressure and temperature are state functions; work and heat are path functions. This is why a reaction carried out in one step and the same reaction carried out in several steps have the same overall enthalpy change, which is the substance of Hess's law. A second division is between extensive properties, which depend on the amount of matter, and intensive properties, which do not. Gibbs free energy is extensive; density, pressure and temperature are intensive.
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