Calculate the specific energy of methane, in MJ kg⁻¹, using sections 1, 6 and 13 of the data booklet.

Calculate the maximum electric energy output, in MJ, which can be obtained from burning 1.00 kg of methane by using your answer from (a).

Hydroelectric power plants produced 16% of the world’s energy in 2015, down from 21% in 1971.

Suggest why hydroelectric power production has a higher efficiency than the other sources given in (b) and why its relative use has decreased despite the high efficiency.

Reason for higher efficiency:

Reason for decreased use:

Methane can also be obtained by fractional distillation of crude oil.

[Source: Image used with kind permission of science-resources.co.uk]

Draw a circle on the diagram to show where the methane fraction is withdrawn.

List the following products, which are also obtained by fractional distillation, according to decreasing volatility: asphalt, diesel, gasoline, lubricating motor oil.

Explain how methane absorbs infrared (IR) radiation by referring to its molecular geometry and dipole moment.

Compare methane’s atmospheric abundance and greenhouse effect to that of carbon dioxide.

« $\frac{891\text{kJ mo}{\text{l}}^{-1}}{16.05\text{g mo}{\text{l}}^{-1}}$ =» 55.5 kJ g^–1 =» 55.5 «MJ kg^–1» ✔

«55.5 MJ × 58 % =» 32.2 «MJ»   [✔]

Reason for higher efficiency:
no heat/energy loss in producing steam
OR
no need to convert chemical energy of the fuel into heat and then heat into mechanical energy
OR
direct conversion of «gravitational» potential energy to mechanical energy    [✔]

 Note: Accept “less energy lost as heat” but do not accept “no energy lost”.

Reason for decreased use:
limited supply of available hydroelectric sites
OR
rapid growth of electrical supply in countries with little hydroelectric potential
OR
not building «new hydroelectric» dams because of environmental concerns    [✔]

Note: Accept “new/alternative/solar/wind power sources «have taken over some of the demand»”.

Accept “lower output from existing stations due to limited water supplies”.

  [✔]

gasoline > diesel > lubricating motor oil > asphalt   [✔]

Note: Accept products written in this order whether separated by >, comma, or nothing.

methane is tetrahedral
OR
methane has zero dipole moment/is non-polar/bond polarities cancel    [✔]

Any two of:
IR absorption can result in increased vibrations/bending/stretching    [✔]

only modes that cause change in dipole absorb IR    [✔]

for methane this is asymmetric bending/stretching    [✔]

methane is less abundant AND has a greater effect «per mol»   [✔]

Calculations of specific energy of methane and the maximum electric energy output in parts (a) and (b)(i) were done well.

Calculations of specific energy of methane and the maximum electric energy output in parts (a) and (b)(i) were done well.

Suggesting reasons for hydroelectric power having higher efficiency but lower relative use than other energy sources in was not answered well by most candidates. Often the reasons for higher efficiency were given in vague terms that did not meet the detail required.

Required candidates to circle a fractionating tower to show where the methane fraction could be withdrawn. Despite the expectation that candidates know methane is a gas at room temperature, there were many varied answers to this question.

Required products of fractional distillation of crude oil to be ranked according to decreasing volatility. This should have been able to be worked out from first principles and did not have to be memorized as one G2 respondent suggested.

Many candidates scored the first mark for stating that methane is tetrahedral. Further details to explain how methane absorbs IR radiation were generally insufficient. Many candidates referred to “dipole movements” despite dipole moment being in the stem of the question.

Most candidates correctly answered comparing methane’s atmospheric abundance and greenhouse effect to that of carbon dioxide.