A Level Chemistry OCR Practice Exam

What is average bond enthalpy?

1. The energy required to form a bond

2. The change associated with homolytic fission of a mole of bonds

3. The total energy of all bonds in a molecule

4. The average energy stored in a chemical bond

The correct answer is: The change associated with homolytic fission of a mole of bonds

Average bond enthalpy refers to the energy change that occurs during the homolytic fission of a specific type of bond in a molecule, measured per mole of that bond. In simpler terms, it’s the average energy needed to break a particular type of bond in a gaseous substance, leading to the formation of two radicals. This is calculated from various molecules containing that bond type, providing a general estimate that accounts for variations in bond strengths across different environments. The first choice pertains to the energy required to form a bond, which is more accurately the opposite of bonding energy rather than the average bond enthalpy. The third option mentions the total energy of all bonds in a molecule, which is not representative of average bond enthalpy since this notion focuses on individual bonds rather than cumulative energy. The last choice, regarding the average energy stored in a chemical bond, is misleading because bond enthalpy specifically deals with the energy required to break bonds rather than the energy inherent in the bonds themselves.