O Levels Chemistry (5070)•5070/11/O/N/19
Explanation
Bond Energy Determines Reaction Type
Steps:
- Breaking chemical bonds always requires energy input (endothermic process).
- Forming chemical bonds always releases energy (exothermic process).
- Net energy change = energy to break bonds minus energy from forming bonds.
- Endothermic reactions have positive net energy (more absorbed than released); exothermic have negative (more released than absorbed).
Why B is correct:
- Endothermic reactions absorb net energy because bond-breaking energy exceeds bond-forming energy release, per the definition of enthalpy change (ΔH > 0).
Why the others are wrong:
- A: Bond formation always releases energy, so "no energy released" is false.
- C: Bond breaking absorbs energy, not releases it, in exothermic reactions.
- D: Bond breaking always requires energy input, so "no energy needed" is false.
Final answer: B
Topic: Exothermic and endothermic reactions
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