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Breaking the bonds in one mole of hydrogen molecules takes 436 kJ. What happens when one mole of H2 forms from hydrogen atoms?
AThe same 436 kJ is absorbed
BThe same 436 kJ is released
CTwice that energy is released
DHalf that energy is absorbed
Answer & Solution
Correct answer: B. The same 436 kJ is released
1. Breaking a bond needs energy put in, which makes it an endothermic step.
2. Forming the same bond runs that step backwards, so energy comes out.
3. The size of the energy is the same either way, only the sign flips.
4. One mole of H2 forming therefore releases 436 kilojoules.
5. Saying the energy is absorbed would make bond formation endothermic, which reverses the physics.
_Source: OpenStax Chemistry (CC BY 4.0), Chs 7 and 17, section 7.2 Covalent Bonding_
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