What is the difference between standard enthalpy of formation and standard enthalpy of combustion?

• A. ΔH°f is the energy change for breaking the compound into its elements.
• B. ΔH°f is the energy change to burn the compound in oxygen.
• C. ΔH°f is the enthalpy change to form one mole of the compound from its elements.
• D. ΔH°comb is the enthalpy change for burning one mole of a substance in oxygen.

Answer: (C)

The key idea here is distinguishing two different heat changes: how a compound is formed versus how it is burned. Standard enthalpy of formation is the heat change when one mole of a compound is formed from its elements in their most stable forms at standard conditions. In other words, it’s the energy associated with making the compound from its elements—a synthesis from the elements. Standard enthalpy of combustion, by contrast, is the heat released when one mole of a substance is burned completely in excess oxygen to form oxides and water under standard conditions. So the difference lies in the process: formation from elements versus burning in oxygen. The statement that correctly defines the formation enthalpy is that it is the enthalpy change to form one mole of the compound from its elements. The other descriptions describe burning or the reverse of formation, not the formation process itself.