The heat exchange in chemical reactions is due to a change in:

The heat exchange involved in chemical reactions is due to change in the enthalpy.

Explanation:

Enthalpy is a type of energy which is associated with the energy of bond in a molecule. The enthalpy in chemical reactions can be positive, negative or zero which is determined by the property of chemical reaction.

The enthalpy in endothermic is positive and in exothermic reaction is negative but the enthalpy in the reaction is never constant (loss or gain of heat).

Enthalpy

Explanation:

The enthalpy is the internal energy of a system at a constant pressure and volume.

The change of enthalpy that result from a chemical reaction may absorb or release energy in form of heat. In the International System of Units, the enthalpy is measured in Joule