Why do reactions either give out or absorb heat?
4 marks · NormalAmmonia is synthesised by reacting nitrogen and hydrogen:
State which bonds must be broken and which bonds must be formed in the reaction above.
Explain, in terms of energy, why this reaction can be exothermic.
Work this out on paper, then compare with the answer and worked solution below. With a free account you can submit written answers for marking.
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(a) Bonds broken: N≡N in N2 and H–H in H2. Bonds formed: N–H in NH3.
(b) The reaction is exothermic because more energy is released making N–H bonds than is required to break N≡N and H–H bonds.
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Marking points
B1 (a) · 1 mark · Identifies the bonds that must be broken (N≡N and H–H).
B2 (a) · 1 mark · Identifies the bonds that must be formed (N–H bonds).
B3 (b) · 1 mark · States that more energy is released making bonds than is needed to break bonds (or reverse).
B4 (b) · 1 mark · Links this to the reaction being exothermic.
Worked solution
a) Bonds broken: N≡N bonds in nitrogen and H–H bonds in hydrogen. Bonds formed: N–H bonds in ammonia.
b) The reaction is exothermic because the energy released when N–H bonds form in NH3 is greater than the energy required to break N≡N and H–H bonds.