Under what conditions is the change in internal energy, δe rxn, equal to the heat evolved in a chemical reaction?

the first law says that the change in internal energy of a system is given by:

δ E = δq + δw

where δ E is the i change in internal energy,

δq is the amount of thermal energy added to the system from the surroundings

δw is the l work done * on * the system * by * the surroundings.

For a system only undergoing expansion work,

δw = - p δ V, so:

δE = δq - p δ V

when δV = 0, then δe=δq