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Hess’s Law
The total enthalpy change for a reaction is the sum of the enthalpy changes for each step of the reaction pathway.
Standard enthalpy of combustion
The standard enthalpy of combustion ($ΔH_c^\ominus$) is the energy change when one mole of a substance is completely burned in oxygen under standard conditions (298 K and 1 atm).
Standard enthalpy of formation
The standard enthalpy of formation ($ΔH_f^\ominus$) is the energy change when one mole of a compound is formed from its elements in their standard states under standard conditions.