Thermochemistry

Thermochemistry studies heat changes accompanying chemical reactions and physical transformations, using energy measurements to characterize processes.

Enthalpy: A thermodynamic state function equal to internal energy plus pressure–volume work. Reaction heat at constant pressure is measured as a change in this quantity.
First law of thermodynamics: The law that energy is conserved and transferred as heat or work. It constrains the heat and work accompanying every thermochemical process.
Combustion: A rapid chemical reaction with an oxidizer that releases energy, often as heat and light. Fuel heating values quantify the heat released when fuels burn.
Entropy: A thermodynamic state function measuring energy dispersal and the number of accessible microscopic arrangements. Enthalpy alone cannot determine whether a process is thermodynamically spontaneous.
Internal energy: The total microscopic kinetic and potential energy of a system. Its change connects heat and work through the first law of thermodynamics.
Bond energy: The energy required to break a chemical bond in the gas phase. Breaking and forming bonds accounts for much of a reaction's energy change.
Hess cycle: A thermochemical calculation that combines reaction enthalpies around a closed reaction pathway. It estimates otherwise difficult-to-measure heats of formation and reaction.
Gibbs free energy: A thermodynamic potential that predicts spontaneity at constant temperature and pressure. It combines enthalpy and entropy rather than describing heat change alone.
Heat capacity: The heat required to raise a system's temperature by one degree. It lets calorimetry translate measured temperature changes into heat transfer.
Exothermic reaction: A chemical reaction that releases heat to its surroundings. Its negative enthalpy change means products have lower enthalpy than reactants.

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