In a chemical equation, an endothermic reaction is identified by the presence of a positive value for the enthalpy change (ΔH).

ΔH > 0 indicates that the reaction absorbs heat from the surroundings, meaning it requires an input of energy to proceed.

For example:

2H2(g) + O2(g) → 2H2O(g) ΔH = +572 kJ

In this reaction, the enthalpy change is positive, indicating that the reaction is endothermic.