A Level Chemistry OCR Practice Exam

What does the (standard) enthalpy change of formation (∆Hfө) indicate?

1. The enthalpy change when one mole of a compound is formed

2. The enthalpy change during a decomposition reaction

3. The total energy released when a substance is burned

4. The enthalpy change when a gas is formed from a liquid

The correct answer is: The enthalpy change when one mole of a compound is formed

The standard enthalpy change of formation (∆Hfө) specifically indicates the heat change that occurs when one mole of a compound is formed from its elements in their standard states. This definition is crucial in thermochemistry, as it provides a basis for comparing the stability of different compounds and understanding energy changes in chemical reactions. When considering the definition of the standard enthalpy of formation, it is essential to recognize that it reflects the formation of a compound from its elements under standard conditions (typically 1 atmosphere pressure and a specified temperature, usually 298 K). This enthalpy change is used to assess how much energy is either absorbed or released during the formation process, allowing chemists to calculate the overall energy changes in a reaction using Hess's law. The other options present different scenarios that do not accurately describe the standard enthalpy of formation. For example, the decomposition of a compound would involve breaking it down into its constituent elements rather than forming it, which is a different thermodynamic process. Similarly, the total energy released when a substance is burned pertains to the enthalpy of combustion rather than formation. Lastly, the enthalpy change associated with a gas forming from a liquid is related to phase changes and does not correspond to the