5.1.1—Chemical reactions are accompanied by enthalpy

An enthalpy change, ΔH, is the energy transferred during a chemical reaction under stated conditions and for a stated amount of reaction. It is a property of the reaction as written, not an undefined amount of heat detached from the equation.

In an exothermic reaction, energy is transferred from the reacting system to the surroundings, so ΔH is negative and the surroundings warm. The products are at lower enthalpy than the reactants for the stated reaction direction.

In an endothermic reaction, energy is transferred from the surroundings to the reacting system, so ΔH is positive and the surroundings cool. The products are at higher enthalpy than the reactants for the stated reaction direction.

Always attach the sign to the reaction direction and amount shown. Do not treat a negative ΔH as energy created, a positive ΔH as a faster reaction, or the sign convention as independent of whether the reaction is written forwards or reversed.