Hess law states that :

Heat evolved or absorbed, at constant pressure or at constant volume, in a given chemical reaction is same whether it occurs in a single step or in several steps.

Total enthalpy change of a thermochemical reaction depends on the number of steps and nature of intermediates involved in the given chemical reaction under similar conditions.

Total enthalpy change of a thermochemical reaction at constant pressure or at constant volume does not depend on the number and nature of intermediates formed but depends only on initial and final states.

enthalpy of formation of a compound is same as the enthalpy of decomposition of the
compound into constituent elements, but with opposite sign.