What is the standard enthalpy of formation?

• A. The enthalpy change when 1 mole of a compound forms from its elements in their standard states, with all substances at standard states.
• B. The energy required to break bonds in a molecule.
• C. The total energy change for any reaction at standard conditions.
• D. The enthalpy change when a compound dissolves in water.

Answer: (A)

Standard enthalpy of formation is the enthalpy change when one mole of a compound forms from its elements in their standard states, with all substances at standard states (1 atm, 25°C). This quantity, written as ΔH°f, serves as a reference point to determine the heat of other reactions by Hess’s law—you combine the formation enthalpies of products and reactants in proportion to the reaction’s formula. A key point is that elements in their standard states have a formation enthalpy of zero. For example, the formation enthalpy of liquid water is about −285.8 kJ/mol, illustrating how forming a compound from its elements releases energy. The other descriptions correspond to different concepts: bond dissociation energy is the energy to break bonds in a molecule, the standard enthalpy change for a reaction is the overall heat change for a specific reaction, and dissolution enthalpy is the heat change when a substance dissolves in water.