Enthalpy (given the symbol H) is a measure of how much heat energy can be released by a chemical. ΔH is defined as the amount of heat energy released or taken in when two chemicals react. If ΔH is negative it means that the reaction is exothermic and will give out heat as it progresses and will feel hot.

If ΔH is positive then the reaction is endothermic and will therefore absorb heat as it progresses, and will feel cold.