Enthalpy is the thermodynamic quantity used to describe heat transfer in a chemical reaction, and because no absolute quantity of heat can be defined it is always expressed as a change, ΔH, taken strictly from the perspective of the system (the reaction) rather than the surroundings. ΔH equals the enthalpy of products minus that of reactants and carries units of energy per mole of reactant, so that the extent of reaction is accounted for; a positive ΔH defines an endothermic reaction, which absorbs heat from the surroundings, and a negative ΔH defines an exothermic reaction, which releases heat to them. This concept belongs to thermodynamics and physical chemistry, connects to biology through the energetics of physiological processes, and matters beyond heat accounting because enthalpy is one component of Gibbs free energy, the criterion chemists use to judge whether a reaction proceeds spontaneously.
Enthalpy is the thermodynamic quantity used to describe heat transfer in a chemical reaction, and because no absolute quantity of heat can be defined it is always expressed as a change, ΔH, taken str…