Hess's law deals with:

Explanation

Hess's law of constant heat summation states that the total enthalpy change for a reaction is the same, no matter whether the reaction occurs in one step or multiple steps. This principle allows for the calculation of the heat of reaction by adding the enthalpy changes of known reactions that sum to the overall reaction. Option A is correct because it directly relates to Hess's law.Option B is incorrect because it concerns the rate of reaction, which is a topic in kinetics, not thermochemistry. Option C is incorrect because the equilibrium constant pertains to the balance of reactants and products at equilibrium, not to enthalpy changes. Option D is incorrect because the effect of pressure on the volume of a gas is explained by Boyle's Law, not Hess's Law.