A transport cooperative is comparing diesel vans with vans powered by liquefied petroleum gas, LPG. LPG is stored as a liquid under pressure in a steel cyli...
Question 1 Report
A transport cooperative is comparing diesel vans with vans powered by liquefied petroleum gas, LPG. LPG is stored as a liquid under pressure in a steel cylinder, but it becomes a gas when released. Fig. 1 shows a displayed formula for propane, one hydrocarbon present in LPG. The company will use the fuel in an engine where oxygen from air reacts with the gas.
Fig. 1
Table 1 gives average emissions for vans travelling 1 km.
| fuel | carbon dioxide released / g per km | carbon monoxide released / g per km |
|---|---|---|
| diesel | 180 | 0.05 |
| LPG | 145 | 0.03 |
The equation for complete combustion of propane is
C3H8 + 5O2 → 3CO2 + 4H2O
Relative formula masses: C3H8 = 44, CO2 = 44.
(a) State the molecular formula of propane. State the homologous series to which propane belongs. [2]
(b) Give two reasons why the LPG cylinder must be kept away from flames. [2]
(c) Calculate the mass of carbon dioxide made when 17.6 g of propane undergoes complete combustion. [3]
(d) Use Table 1 to explain why changing to LPG could reduce environmental harm, but would not make the vans pollution-free. [5]
Answer Details
(a) Propane has formula \(\mathrm{C_3H_8}\) and belongs to the alkanes. [2]
(b) LPG is flammable. If it leaks, it can mix with air to form a flammable mixture that a flame could ignite, causing a fire or explosion. [2]
(c) \[n(\mathrm{C_3H_8})=\frac{17.6}{44}=0.40\text{ mol}\] \[n(\mathrm{CO_2})=0.40\times3=1.20\text{ mol}\] \[m(\mathrm{CO_2})=1.20\times44=\mathbf{52.8\text{ g}}\] [3]
(d) LPG releases 145 g CO2 per km compared with 180 g per km for diesel, which is 35 g less per km. As carbon dioxide is a greenhouse gas, this reduces the contribution to climate change. LPG also produces less carbon monoxide, 0.03 rather than 0.05 g per km. However, it still produces carbon dioxide and is a fossil fuel, so the vans are not pollution-free. [5]
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