What is the enthalpy change of formation?

• A. The enthalpy change when 1 mole of a compound is formed from its elements in their standard states under standard conditions (measured in kJ/mol)
• B. The energy change when a compound dissolves in water
• C. The energy change when a gas is ionized
• D. The entropy change of the system at standard conditions

Answer: (A)

Enthalpy change of formation is the enthalpy change when one mole of a compound is formed from its elements in their standard states under standard conditions (298 K, 1 atm). In this context, the standard enthalpy of formation for elements in their standard states is zero, so the value for a compound tells you how much energy is absorbed or released to assemble it from the elements. The unit is kilojoules per mole. When the formation releases energy, the enthalpy change is negative; when it requires energy, it is positive. This is different from the energy change when a compound dissolves in water (enthalpy of solution), the energy change when a gas is ionized (ionization energy), or the entropy change of the system (which relates to disorder, not enthalpy). Therefore, the enthalpy change of formation specifically describes forming a compound from its elements in their standard states under standard conditions.