O-LevelChemistryChemistry of the Environment5070/21 · Oct/Nov 2014 · Q710 Marks

Three important processes in the carbon cycle are combustion, respiration and photosynthesis.

(a)Construct the equation for the complete combustion of propane, C3H8.[1]
(b)(i)Describe how the processes in the carbon cycle regulate the amount of carbon dioxide in the atmosphere.[2]
(b)(ii)Carbon dioxide is a greenhouse gas. What do you understand by the term greenhouse gas?[1]
(b)(iii)Methane is also a greenhouse gas. Give one source of methane in the atmosphere.[1]
(b)(iv)The percentage of methane by volume in the air is 0.00014%. Calculate the mass of methane in 1000dm³ of air. mass =[2]
(c)Plants use water in photosynthesis. Water containing the radioactive isotope 18O is fed to a plant. The resulting radioactivity in the products of photosynthesis is shown in the equation below. 6CO2 + 6H218O → C6H12O6 + 618O2 glucose
(c)(i)What does this tell you about the origin of the oxygen in each of the products?[1]
(c)(ii)Deduce the number of protons, neutrons and electrons in an atom of 18O. protons neutrons electrons[2]

Mark scheme

Marking points from the official mark scheme (Oct/Nov 2014). The mark or mark type is shown in brackets.

[1] (a) C3H8 + 5O2 → 3CO2 + 4H2O (1)
[2] (b)(i) respiration releases CO2 AND photosynthesis absorbs CO2 (1) The (rate of) CO2 released into the atmosphere is (roughly) the same as the amount absorbed from the atmosphere (1)
[1] (b)(ii) gas which absorbs infra-red radiation/gas which absorbs energy/gas which absorbs heat (1)
[1] (b)(iii) waste gas from animals/rice paddy fields/bacterial action/landfill sites etc. (1)
[2] (b)(iv) (0.0014 dm³ in 1000 dm³) and 0.0014/24 = 5.833 × 10⁻⁵ mol CH4 (1) 5.833 × 10⁻⁵ × 16 = 9.33 × 10⁻⁴g (1)
[1] (c)(i) the oxygen in O2 comes from the water/the oxygen in the oxygen molecule comes from the water (1)
[2] (c)(ii) protons = 8 AND electrons = 8 (1) neutrons = 10 (1)

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