How is standard enthalpy change defined in relation to exothermic and endothermic reactions?

Standard enthalpy change is defined as the heat change that occurs when a reaction takes place under standard conditions (1 atmosphere and a specified temperature, usually 25 °C). This value can differ between exothermic and endothermic reactions.

In exothermic reactions, energy is released to the surroundings as heat, resulting in a negative standard enthalpy change. Conversely, in endothermic reactions, energy is absorbed from the surroundings, leading to a positive standard enthalpy change. Thus, the enthalpy change depends on whether the reaction is releasing or absorbing energy.

Consequently, standard enthalpy change is not fixed but varies depending on the type of reaction being considered. This variability highlights the importance of reaction specifics—different reactions will yield different enthalpy changes, reinforcing the notion that this property is reaction-dependent.