Carbon Dioxide and Methane Worked ExamplesIGCSE Chemistry 0620

Statement 1 is wrong. Methane is natural gas; its complete combustion forms carbon dioxide and water: $\text{CH}_4 + 2\text{O}_2 \rightarrow \text{CO}_2 + 2\text{H}_2\text{O}$.

Statement 2 is correct: methane comes from the decomposition of vegetation and waste gases from digestion in animals.

Statement 3 is correct: clean, dry air contains a small amount of carbon dioxide, along with the noble gases.

Statements 2 and 3 only: answer C. A misses statement 3; B and D include the false statement 1.

(a) Coal, natural gas and petroleum.

(b) Methane, $\text{CH}_4$.

(c) A compound that contains hydrogen and carbon only.

(d) The complete combustion of carbon-containing fuels.

(e) The decomposition of vegetation, and waste gases from digestion in animals (such as cattle).

(a) B: some of the Sun's energy is reflected by the Earth's surface back to space.

C: the warmed Earth's surface emits thermal energy (infrared), which is absorbed by carbon dioxide and methane molecules in the atmosphere.

D: the greenhouse gas molecules re-emit thermal energy in all directions, and some goes back towards the Earth's surface, so less thermal energy is lost to space (arrow E is smaller than C).

(b) More carbon dioxide and methane absorb and re-emit more thermal energy, so thermal energy loss to space is reduced. The Earth's average temperature rises (increased global warming), which leads to climate change.

Greenhouse gases act on the thermal energy emitted by the Earth's surface, not on incoming sunlight. They absorb it and re-emit it in all directions, so less is lost to space. That is B.

A and D are about sunlight — and D would actually cool the surface. C is wrong: methane does not burn in the atmosphere to warm it.

Answer: B.

(a) Check: C 1 = 1; H 4 = 4; O 4 = 2 + 2.

(b) Moles of methane ($M_r = 12 + 4 = 16$):

(i) $\text{CH}_4 : \text{CO}_2 = 1 : 1$ and $M_r(\text{CO}_2) = 44$.

(ii)

(iii) $\text{CH}_4 : \text{O}_2 = 1 : 2$.

(a) The ice cools the gases so that water vapour condenses to a liquid in the U-tube.

(b) Add the liquid to anhydrous copper(II) sulfate: it turns from white to blue. (Or: anhydrous cobalt(II) chloride turns from blue to pink.)

(c) The limewater turns milky (cloudy), showing carbon dioxide.

(d) The pump draws the gases from the flame through the apparatus.

(e) The fuel contains hydrogen (it forms water) and carbon (it forms carbon dioxide) — as expected for a hydrocarbon.

(a) Photosynthesis is the reaction between carbon dioxide and water to produce glucose and oxygen, in the presence of chlorophyll and using energy from light.

(b) Check: C 6 = 6; H 12 = 12; O 12 + 6 = 18 = 6 + 12.

(c) Moles of carbon dioxide ($M_r = 44$):

$\text{CO}_2 : \text{C}_6\text{H}_{12}\text{O}_6 = 6 : 1$, and $M_r(\text{C}_6\text{H}_{12}\text{O}_6) = 6(12) + 12(1) + 6(16) = 180$.

$\text{CO}_2 : \text{O}_2 = 6 : 6 = 1 : 1$.

(a) 1960: $317$ ppm; 2020: $414$ ppm.

(b)

(c) The rate increases — the line gets steeper. From 1960 to 1970 the rise is $326 - 317 = 9$ ppm, but from 2010 to 2020 it is $414 - 389 = 25$ ppm.

(d) Burning fossil fuels (complete combustion of carbon-containing fuels). Higher carbon dioxide levels lead to increased global warming, which leads to climate change.

(a) Methane: moles needed for $1000$ kJ; each mole gives $1$ mol of $\text{CO}_2$ ($M_r = 44$).

Octane: each mole gives $8$ mol of $\text{CO}_2$.

Convert to mass.

(b) Methane — it releases less carbon dioxide (a greenhouse gas) for the same amount of energy.

(c) Hydrogen contains no carbon, so its only combustion product is water ($2\text{H}_2 + \text{O}_2 \rightarrow 2\text{H}_2\text{O}$) — no carbon dioxide is released.

(a) Climate change — trees remove carbon dioxide from the air by photosynthesis.

(b) Climate change — fewer animals produce less methane from digestion.

(c) Acid rain — the converter removes oxides of nitrogen (reducing them to nitrogen).

(d) Acid rain — calcium oxide removes sulfur dioxide from the waste gases of power stations.

(e) Both — less fossil fuel is burned, so less carbon dioxide (climate change) and less sulfur dioxide and oxides of nitrogen (acid rain) are released.

(a) X is the activation energy, $E_a$ — the minimum energy that colliding particles need to react. Y is the enthalpy change, $\Delta H$ (negative, as the products are lower in energy).

(b) Bonds broken in $\text{CH}_4 + 2\text{O}_2$ (endothermic):

Bonds made in $\text{CO}_2 + 2\text{H}_2\text{O}$ (exothermic): two C=O and four O–H.

$\Delta H$ = energy to break bonds − energy released making bonds.

(c) More energy is released making the new bonds ($3462$ kJ) than is taken in breaking the old bonds ($2648$ kJ).

(d)

(a) Volume ratio = mole ratio:

$20 \text{ cm}^3$ of methane uses $40 \text{ cm}^3$ of oxygen, leaving $50 - 40 = 10 \text{ cm}^3$ of oxygen, and forms $20 \text{ cm}^3$ of carbon dioxide.

At room temperature the water is a liquid with negligible volume. So syringe 2 reads $20 + 10 = 30 \text{ cm}^3$.

(b) The $20 \text{ cm}^3$ of carbon dioxide is absorbed; the remaining $10 \text{ cm}^3$ is oxygen (the excess).

(c) Check: C 2 = 2; H 8 = 8; O 6 = 2 + 4. The ratio $\text{O}_2 : \text{CH}_4 = 3 : 2$ matches $30 : 20$ exactly.

CO : $\text{CH}_4$ = 2 : 2, so $20 \text{ cm}^3$ of carbon monoxide forms.

(d) Carbon monoxide is a toxic gas (and it is colourless and odourless, so it cannot be detected by people).