How does enthalpy differ from heat?
During a exothermic reaction, the chemical energy (enthalpy) inside a substance decreases as this form of energy is transformed into heat energy, resulting in the substances and their surroundings getting hotter. Conservation of energy is satisfied. During an endothermic reaction, the enthalpy increases: the products have more chemical energy than the reactants. This energy has to come from somewhere, so it is taken from the surroundings, which lose heat energy as a result.