When hydrochloric acid is reacted with zinc heat is released. There is no external energy source such as a flame used, so where does the heat energy come from?

The heat energy comes from the chemical reaction. It is the difference of the enthaply of the products and the reactants.

Explanation:

When a chemical reaction happens, the net change of enthalpy of the products and the reactants is the heat of reaction:

Heat of reaction = ΔH rxn = ∑ ΔH products - ∑ΔH reactants

Then, when ∑ ΔHproducts > ∑ ΔA reactants, ΔH rxn > 0 and heat is released. This is what is called an exothermic reaction.

In an exothermic reaction, heat is released, ΔH > 0, and the surroundings will get hotter.

In and endothermic reaction heat is absorved, ΔH < 0, and the surroundings will get cooler.