Q.Define Hess's law of constant heat summation.

Because $\Delta U$ and $\Delta H$ are both state functions, a reaction's heat change depends only on its initial and final states -- never on the path or number of steps taken between them. Hess's law: the enthalpy change of a reaction, at constant volume or constant pressure, is the same whether it happens in one step or several, provided the initial and final states match. Its main value is computing enthalpies that are hard or impossible to measure directly, by building the target reaction as a sum of measurable ones (reversing a known equation flips the sign of its $\Delta H$; scaling an equation scales its $\Delta H$ by the same factor) -- graphite's combustion to pure CO cannot be measured cleanly, but graphite$\rightarrow$CO$_2$ and CO$\rightarrow$CO$_2$ both can, and combining them via Hess's law recovers graphite$\rightarrow$CO indirectly. The same technique underlies computing a heat of formation from combustion data, and computing bond-energy-based enthalpies (like the enthalpy of hydrogenation) from tabulated average bond dissociation energies -- in both cases, a target reaction is assembled as an algebraic combination of reactions whose enthalpies ARE known or measurable.