The net enthalpy change of a reaction
Question:

The net enthalpy change of a reaction is the amount of energy required to break all the bonds in reactant molecules minus the amount of energy required to form all the bonds in the product molecules. What will be the enthalpy change for the following reaction?

H2(g) + Br2(g) → 2HBr(g)

Given that Bond energy of H2, Br2 and HBr is 435 kJ mol–1, 192 kJ mol–1 and 368 kJ mol–1 respectively.

Solution:

For the reaction

H2(g) + Br2(g) →2HBr(g)

Enthalpy change

= (Bond energy of H-H bond + Br-Br bond) – (2 × bond energy of H-Br)

= (435 + 192) kJ mol–1 – (2 × 368) kJ mol–1

= -109 kJ mol–1

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