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Q1.
Methane burns in oxygen. The equation below represents this reaction.
CH4(g) + 2O2(g) → CO2(g) + 2H2O(g)
A molecule of methane has the following structure.
The following table gives the average bond energies for some bonds in kJ/mol of bonds.
(a) (i) Use the information to calculate the bond energy in 1 mol of carbon dioxide.
……………………………………………………………………………………………….
…………………………………………………………………………………………… kJ
1 mark
(ii) Calculate the total bond energy in 2 mol of water.
……………………………………………………………………………………………….
………………………………………………………………………………………….. kJ
1 mark
(iii) Calculate the energy change when 1 mol of gaseous carbon dioxide and 2 mol of gaseous water are formed from their constituent atoms in their gaseous states. Show whether energy is absorbed or evolved.
……………………………………………………………………………………………….
……………………………………………………………………………………………kJ
1 mark
The energy level diagram for the reaction of methane with oxygen is:
(b) Using the energy diagram, calculate the energy required to break all the bonds in 1 mol of methane gas and 2 mol of oxygen gas.
……………………………………………………………………………………………………….
……………………………………………………………………………………………………….
…………………………………………………………………………………………………….kJ
1 mark
(c) (i) Use your answer to part (b) to find a value for the total bond energy of
1 mol of methane gas.
………………………………………………………………………………………………..
……………………………………………………………………………………………..kJ
1 mark
(ii) Calculate the average bond energy of C–H.
……………………………………………………………………………………………….
………………………………………………………………………………………kJ/mol
1 mark
Maximum 6 marks
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(a)
(i) Carbon dioxide (CO₂) has 2 C=O bonds, and the energy for each is 800 kJ/mol.
So:
2 × 800 = 1600 kJAnswer: 1600 kJ
(ii) Each water molecule (H₂O) has 2 O–H bonds, and 2 moles of water, so:
4 × 460 = 1840 kJAnswer: 1840 kJ
(iii) Total energy to form CO₂ and H₂O =
1600 + 1840 = 3440 kJSince this is energy released when making bonds,
–3440 kJ or “energy is evolved”Answer: –3440 kJ
(b)
From the diagram, the energy needed to break bonds in CH₄ and 2O₂ is:2590 = 3440 – 850
Answer: 2590 kJ
(c)
(i) In CH₄ there are 4 C–H bonds.
Total = 2590 kJ (from b)
2O₂ = 2 × 500 = 1000 kJSo:
2590 – 1000 = 1590 kJAnswer: 1590 kJ
(ii) There are 4 C–H bonds in CH₄.
So the average bond energy =
1590 ÷ 4 = 397.5 kJ/molAnswer: 397.5 kJ/mol
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