Why the change of reaction enthalpy is equal to reaction heat at constant pressure? - QS Study
QS Study

The change of reaction enthalpy is equal to reaction heat at constant pressure

At constant temperature, when heat applied to the gas, that heat does not destroy, it divided into two part. One pan increasing the motion of gas molecules, As a result, change the internal energy. Another part used for increasing volume of gas.

Applying heat = Internal energy increase + Increasing volume

So, the reaction heat is equal to enthalpy at constant pressure.