For a reaction carried out at constant pressure, the heat exchanged is equal to the change in

Explanation

Enthalpy was defined precisely so that its change measures the heat of a process at constant pressure, which is the condition of almost every reaction done in an open vessel. At constant volume, as in a bomb calorimeter, the heat measured is the change in internal energy instead, since no expansion work is done. This is why the two quantities differ slightly for reactions involving gases.