For an exothermic reaction at constant pressure, the system's change in enthalpy, Δ H, is negative due to the products of the reaction having a smaller enthalpy than the reactants, and equals the heat released in the reaction if no electrical or shaft work is done. In other words, the overall decrease in enthalpy is achieved by the generation of heat. Conversely, for a constant-pressure endothermic reaction, Δ H is positive and equal to the heat absorbed in the reaction.