Calculation of enthalpy changes of reaction using bond energies
2 marks · NormalState two reasons why calculated enthalpy changes using bond energies are only estimates and not always exactly the same as values from calorimetry experiments.
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Bond energies are average values as actual bond strength can vary; real molecules may have slightly different environments; calorimetry also measures specific compounds and conditions.
Also accept: Bond strengths vary with molecular environment; bond energies are averages; experiments use real compounds; measurement conditions differ
Common mistakes: Only giving 'experimental error' without reason
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Marking points
B1 · 1 mark · States bond energies are average values/bond strength varies with surroundings.
B2 · 1 mark · Mentions calorimetry is specific to compounds used or experimental conditions may differ from calculated mean values.
Worked solution
Calculated values use average bond energies, but actual bond strengths differ in various molecules. Also, calorimetry measures enthalpy for specific compounds and conditions, so experimental ΔH may differ from the calculated estimate.