In chemistry, the standard enthalpy of reaction is the enthalpy change when reactants in their standard states ( p = 1 bar ; usually T = 298 K ) change to products in their standard states. This quantity is the standard heat of reaction at constant pressure and temperature, but it can be measured by calorimetric methods even if the temperature does vary during the measurement, provided that the initial and final pressure and temperature correspond to the standard state. The value does not depend on the path from initial to final state because enthalpy is a state function.