Enthalpy change and Hess's law

Enthalpy change, written delta H, is the heat released or absorbed by a reaction at constant pressure. It is negative for an exothermic reaction, which releases heat to its surroundings, and positive for an endothermic reaction, which absorbs heat. A thermochemical equation lists it with the balanced equation: burning methane, CH4 + 2 O2 -> CO2 + 2 H2O, has a delta H of about -890 kJ for each mole of methane when the water formed is a liquid (about -802 kJ if it forms as water vapor). Enthalpy change scales with the amount that reacts, so burning 2 moles of methane releases about twice as much heat, and reversing a reaction flips the sign of delta H. Hess's law says that the total enthalpy change of a reaction is the same whether it happens in one step or in several, because enthalpy depends only on the starting and ending states, not on the path between them. That lets chemists find delta H for a reaction that is hard to measure directly by adding up the delta H values of other reactions that combine to give it. Energy diagrams show the same idea: the products sit lower than the reactants for an exothermic reaction and higher for an endothermic one.

This explanation is AI-generated and can occasionally be wrong. 🚩 Report