Question detail
Calculate the approximate enthalpy change for the reaction: C2H4(g) + H2(g) → C2H6(g). The bond enthalpies are: C=C = 612 kJ/mol, C-H = 412 kJ/mol.
Try the question, check the answer, then read the explanation to understand the curriculum point.
At a glance
MCQ
Type
practice
Style
Topic
Energetics
Question
- A. -130 kJ
- B. -200 kJ
- C. -120 kJ
- D. -100 kJ
Answer
-130 kJ
Explanation
The correct option is -130 kJ. -130 kJ is the best answer because it directly supports the AQA A-Level Chemistry objective to calculate approximate enthalpy changes from mean bond enthalpies. This reasoning is anchored to Bond enthalpies in Energetics, and it separates changes from similar A-Level ideas rather than relying on a vague recall statement. Other options are weaker if they use the wrong evidence, calculation, mechanism, observation, unit, or conclusion for this subtopic.
Common mistake
Incorrect Bond Energy Calculation
Students often forget to distinguish between bonds broken and bonds formed when calculating enthalpy changes using mean bond enthalpies.
To fix this, remember to clearly identify which bonds are broken and which are formed. Use the formula: energy transferred = sum(bonds broken) - sum(bonds formed). For example, if 2 bonds of 400 kJ/mol are broken and 3 bonds of 200 kJ/mol are formed, substitute: energy transferred = (2 * 400) - (3 * 200). This results in energy transferred = 800 - 600 = 200 kJ. Therefore, the enthalpy change is +200 kJ, indicating an endothermic reaction.
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