enthalpy of reaction
The enthalpy of reaction is the heat a chemical reaction gives off or soaks up when it runs at constant pressure — the warmth you feel from a hand-warmer or the chill of an instant cold pack, made into a number. It is written ΔH and asks a simple question: as reactants turn into products, does energy spill out into the surroundings, or does the reaction quietly draw energy in?
Concretely, it is the difference between the total enthalpy of the products and that of the reactants. When the products hold less enthalpy than the reactants, the surplus escapes as heat and ΔH is negative — the reaction is exothermic. When the products hold more, the reaction must pull heat in from outside and ΔH is positive — it is endothermic. The size of ΔH tells you how much heat, usually quoted per mole and in kilojoules.
Because enthalpy is a state function, the reaction enthalpy depends only on what you start and end with, not on the mechanism in between. That is what lets chemists compute it by combining tabulated formation enthalpies, by applying Hess's law, or by estimating from bond enthalpies — all roads lead to the same ΔH.
Mix baking soda with vinegar and the cup turns noticeably cold: that reaction has a positive enthalpy of reaction — it draws heat in from your hand and the air. It is endothermic.
A positive reaction enthalpy means heat flows in and the mixture cools.
Reaction enthalpy depends on how the equation is written. Doubling the coefficients doubles ΔH, and reversing the reaction flips its sign. It is also usually quoted under standard conditions and for a specific physical state of each substance, since melting or vaporizing changes the numbers.