Q.The enthalpies of formation of Al$_2$O$_3$ and Cr$_2$O$_3$ are – 1596 kJ and – 1134 kJ, respectively. $\Delta H$ for the reaction $2Al + Cr_2O_3 \rightarrow 2Cr + Al_2O_3$ is

Hess's Law says the overall enthalpy change of a reaction is the same whether it happens directly or via intermediate steps, because enthalpy is a state function.

Hess's Law of Constant Heat Summation (1840): the total enthalpy change (Delta H) accompanying a chemical reaction is the same whether the reaction is carried out in a single step, or in several steps (via one or more intermediate reactions/compounds), PROVIDED the initial reactants and final products are identical in both cases (same physical states, same conditions).

This follows directly from the fact that enthalpy (H) is a STATE FUNCTION — its value depends only on the current state of the system (defined by variables like temperature, pressure, composition), not on how that state was reached. Since Delta H for any process equals H(final) - H(initial), and these depend only on the endpoints, any path connecting the same two endpoints must give the same overall Delta H, regardless of how many intermediate steps are involved or what those intermediate species are.

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