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

First, convert the mass of CH₄ to moles:

$n(CH_4) = \frac{20.0\, g}{16.0\, g/mol} = 1.25\, mol$

Using the ideal gas law, apply it as follows:

• R = 8.31 J/(mol·K)
• V = 5.0 L = 0.005 m³ (since 1 L = 0.001 m³)
• T = 25 °C = 298 K (convert to Kelvin)

Now apply the ideal gas law:

$P = \frac{nRT}{V} = \frac{(1.25\, mol)(8.31\, J/(mol·K))(298\, K)}{0.005\, m³} = 6.20 \times 10^2 \text{kPa}$

To determine this percentage, first calculate the energy released during the combustion of CH₄. The heat absorbed by the water can be calculated as:

$q = m c \Delta T$

where:

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

Thus: $q = 350.0\, g \times 4.18\, J/g·°C \times 12.3 °C = 18152.7\, J$

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

• Combustion of CH₄ releases about 890 kJ/mol. First, convert the mass to moles:

$n(CH_4) = \frac{0.485\, g}{16.0\, g/mol} = 0.03031\, mol$

Energy produced: $E = 0.03031\, mol \times 890\, kJ/mol = 26.93\, kJ = 26930\, J$

Finally, the percentage of energy lost to the environment is given by: $\text{Percentage lost} = \frac{\text{Energy lost}}{\text{Energy produced}} \times 100 = \frac{18152.7}{26930} \times 100 \approx 67.4\%$

Both sources of methane (landfill and natural gas) produce carbon dioxide when combusted, contributing to greenhouse gas emissions. However, there are significant differences:

Similarity:

• Both sources emit CO₂ during combustion, impacting climate change.
• Both sources contain trace amounts of nitrogen and sulfur, which can lead to the formation of nitrogen oxides (NOₓ).

Difference:

• Methane from landfills can be produced renewably and is often considered a lower carbon source, while that from natural gas extraction tends to be more damaging.
• Extraction of fossil fuels can result in significant environmental damage, including habitat destruction and increased atmospheric CO₂ levels. Methane from landfill is already part of the waste cycle.
• Additionally, landfill methane is often captured and utilized, thus preventing it from contributing to ozone layer depletion. In contrast, natural gas mining may cause more pollution and related health risks.