5.4 Bond energy calculations
5 exam questions worth 12 marks. Write your answer, open the mark scheme, then give yourself the marks you earned. Your answers are saved on this device.
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A reaction has an energy change of +120 kJ/mol. What does this tell you?
Choose one answer.
Correct Not quite: the answer is It is endothermic
Mark scheme
B
Model answer
B: a positive value means more energy is taken in breaking bonds than given out making them, so it is endothermic.
Calculate the energy change for: 2H2 + O2 → 2H2O
Bond energies: H–H 436, O=O 498, O–H 464 kJ/mol
Mark scheme
- bonds broken = (2 × 436) + 498 = 1370 (1)
- bonds made = 4 × 464 = 1856 (1)
- 1370 − 1856 = −486 kJ/mol (1)
Model answer
Broken: 872 + 498 = 1370 kJ/mol. Made: 4 × 464 = 1856 kJ/mol. Energy change = −486 kJ/mol.
Calculate the energy change for: N2 + 3H2 → 2NH3
Bond energies: N≡N 945, H–H 436, N–H 391 kJ/mol
Mark scheme
- bonds broken = 945 + (3 × 436) = 2253 (1)
- bonds made = 6 × 391 = 2346 (1)
- 2253 − 2346 = −93 kJ/mol (1)
Model answer
Broken: 945 + 1308 = 2253 kJ/mol. Made: 6 × 391 = 2346 kJ/mol. Energy change = −93 kJ/mol.
Calculate the energy change for: 2HBr → H2 + Br2 and state whether it is exothermic or endothermic.
Bond energies: H–Br 366, H–H 436, Br–Br 193 kJ/mol
Mark scheme
- bonds broken = 2 × 366 = 732 (1)
- bonds made = 436 + 193 = 629 (1)
- 732 − 629 = +103 kJ/mol (1)
- endothermic (1)
Model answer
Broken: 732 kJ/mol. Made: 629 kJ/mol. Energy change = +103 kJ/mol, so the reaction is endothermic.
Explain why bond breaking is endothermic.
Mark scheme
- energy is needed to overcome the attraction between the atoms / break the bond (1)
Model answer
Energy must be supplied to overcome the attraction holding the atoms together.
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