Determine the enthalpy change for the following reaction shown below using the bond energies listed in the box on the right.

Question

1. - 544 kJ
2. - 1130 kJ
3. + 1130 kJ
4. + 544 kJ
5. + 21 kJ

Answer 1

It takes energy to break bonds (reactants).

Energy is released when bonds are formed (products)

Reactants: H₂ and F₂

H₂ which is H-H .. 432 kJ/mol x 1 mol = 432 kJ

F₂ which is F-F .. 154 kJ/mol x 1 mol = 154 kJ

Total needed to break the bonds in the reactants = 432 kJ + 154 kJ = +586 kJ

Products: 2HF

HF which is H-F .. 565 kJ/mol x 2 mol = -1130 kJ (note minus sign since energy is liberated)

Total energy released when bonds are formed in the products = -1130 kJ

Enthalpy change for the reaction = 586 kJ - 1130 kJ = -544 kJ (ANSWER 1)