Ethanol is used as a fuel because it burns readily in air. Table 6.1 gives the energy released when 1 g of some fuels is completely burned. Table 6.1 Fuel F...
Question 1 Report
Ethanol is used as a fuel because it burns readily in air. Table 6.1 gives the energy released when 1 g of some fuels is completely burned.
| Fuel | Formula | Energy released (kJ/g) |
|---|---|---|
| methanol | CH4O | 23 |
| ethanol | C2H6O | 30 |
| petrol (octane) | C8H18 | 48 |
| hydrogen | H2 | 143 |
(a) Write the balanced symbol equation for the complete combustion of ethanol. [2]
(b) Name the two products formed when ethanol burns completely, and give one chemical test, with its result, that would confirm one of them. [2]
(c) Use Table 6.1 to calculate the energy released when 9.2 g of ethanol is completely burned. [2]
(d) When the supply of air is limited, ethanol undergoes incomplete combustion. Explain how incomplete combustion happens and name the poisonous gas that is produced. [3]
(e) Use Table 6.1 to suggest, with a reason, whether ethanol or methanol is the better fuel per gram. [2]
Answer Details
This question tests combustion of ethanol, the energy calculation from a data table, and the danger of incomplete combustion.
(a) Complete combustion in plenty of oxygen gives carbon dioxide and water:
\[\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}\]
correct formulae [1]; balanced [1].
(b) The two products are carbon dioxide and water [1]. A confirming test: bubble the gas through limewater, which turns milky (carbon dioxide), or the liquid turns anhydrous copper(II) sulfate from white to blue (water) [1].
(c) From the table ethanol releases 30 kJ per gram, so for 9.2 g:
\[E = 9.2 \times 30\ [1] = 276\ \text{kJ}\ [1]\]
(d) Incomplete combustion happens because there is insufficient (limited) oxygen for complete combustion [1]; instead of carbon dioxide, carbon monoxide (and carbon / soot) is formed [1]; the poisonous gas is carbon monoxide, which is toxic because it binds to haemoglobin and reduces oxygen transport in the blood [1].
(e) Per gram, ethanol is the better fuel [1]; because it releases more energy per gram than methanol (30 kJ/g compared with 23 kJ/g) [1].
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