Methane gas, CH₄, can be captured from the breakdown of waste in landfills - VCE - SSCE Chemistry - Question 6 - 2020 - Paper 1

First, we need to calculate the number of moles of CH₄:

$n(CH₄) = \frac{20.0 \text{ g}}{16.0 \text{ g/mol}} = 1.25 \text{ mol}$

Next, we convert the temperature to Kelvin:

$T = 25 °C + 273.15 = 298.15 \, K$

Using the ideal gas law, we find the pressure:

$P = \frac{nRT}{V}$

Substituting the values:

$P = \frac{(1.25 \, mol)(8.31 \, kPa \, L/(mol \, K))(298.15 \, K)}{5.0 \, L} = 6.02 \times 10^2 \, kPa$

To find the energy absorbed by water, we use:

$Q = mcΔT$

where:

• $m = 350.0 \, g$ (mass of water)
• $c = 4.18 \, J/g°C$ (specific heat of water)
• $ΔT = 32.3 °C - 20 °C = 12.3 °C$

Thus:

$Q = (350.0 \, g)(4.18 \, J/g°C)(12.3 \, °C) = 18,264.3 \, J$

Next, we calculate the energy released from the combustion of 0.485 g of CH₄:

1. The molar mass of CH₄ is 16.0 g/mol, so: $n(CH₄) = \frac{0.485 \, g}{16.0 \, g/mol} = 0.0303 \, mol$
2. The heat of combustion for CH₄ is approximately 890 kJ/mol, thus: $Q_{combustion} = n(CH₄) \times 890 \text{ kJ/mol} = 0.0303 \, mol \times 890 \, kJ/mol = 26.92 \, kJ = 26,920 \, J$

Percentage of energy lost is given by:

$\text{Percentage Lost} = \left(\frac{Q_{combustion} - Q}{Q_{combustion}}\right) \times 100$

Substituting the values:

$\text{Percentage Lost} = \left(\frac{26,920 \, J - 18,264.3 \, J}{26,920 \, J}\right) \times 100 = 32.42\%$

The environmental impact of methane, CH₄, derived from different sources can be compared as follows:

Similarity:

• Both sources produce atmospheric carbon dioxide (CO₂) as a byproduct of combustion.
• Both types of methane can contain trace amounts of nitrogen and sulfur, and thus can lead to the formation of oxides such as SOₓ and NOₓ.

Difference:

• Landfill Methane: This methane is renewable as it is produced through natural processes. However, its extraction can be more carbon-neutral. It also may involve less environmental damage as it is captured from waste.
• Natural Gas Methane: This is non-renewable and its extraction can cause significant environmental damage through processes like fracking, which can result in the release of greenhouse gases and other pollutants.
• Overall, methane from landfill tends to have a lesser impact on the environment compared to natural gas due to its renewable nature and lesser contribution to atmospheric CO₂.