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Question
Answer in brief.
How will you calculate reaction enthalpy from data on bond enthalpies?
Solution
Reaction and bond enthalpies:
- In a chemical reaction, bonds are broken and formed.
- The enthalpies of reactions involving substances having covalent bonds are calculated by knowing the bond enthalpies of reactants and those in products.
- The calculations assume all the bonds of a given type are identical.
- Enthalpy change of a reaction can be calculated using the following expression:
ΔrH° = ∑ ΔH° (reactant bonds) - ∑ ΔH° (product bonds)
e.g. Consider the reaction, H2(g) + I2(g) → 2HI(g)
The enthalpy is given by
ΔrH° = [ΔH° (H - H) + ΔH° (I - I)] - [2ΔH° (H - I)]
Note:
i. If reactants and products are diatomic molecules, the above equation gives accurate results. The bond enthalpies are known accurately.
ii. For reactions involving polyatomic molecules, the reaction enthalpies calculated using above equation would be approximate and refer to average bond enthalpies.
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