Heat (Q) is included in the equation of an endothermic reaction as a product. This means that the heat enters the system from the surroundings during the reaction. The equation for an endothermic reaction can be represented as:

Reactants + Heat → Products

For example, the equation for the endothermic reaction of dissolving ammonium chloride in water is:

NH₄Cl(s) + H₂O(l) + Heat → NH₄Cl(aq)

In this reaction, ammonium chloride (NH₄Cl) dissolves in water (H₂O), and heat is absorbed from the surroundings to break the bonds between the ammonium and chloride ions in the solid ammonium chloride. This results in the formation of ammonium and chloride ions in the aqueous solution.