iGCSE Chemistry (0620) Core:11.6 Alcohols: Exam Style Questions Paper 1- New Syllabus
▶️ Answer/Explanation
Detailed solution:
Ethanol can be manufactured industrially by the direct hydration of ethene: $\mathrm{C_2H_4 + H_2O \rightarrow C_2H_5OH}$. This reaction requires specific conditions to achieve a reasonable yield at an acceptable rate. The temperature used is $300^{\circ}C$, which is high enough to provide the activation energy and give a decent rate of reaction. The pressure is $6000\mathrm{kPa}$ (equivalent to about 60 atmospheres), which favours the forward reaction because there are fewer moles of gas on the product side. A phosphoric acid catalyst is also used. The low temperature of $35^{\circ}C$ in options A and B corresponds to fermentation (a completely different biological method using yeast), not the ethene/steam route. Option C has the right temperature but a pressure that is far too low. Only option D has the correct combination of $300^{\circ}C$ and $6000\mathrm{kPa}$.
▶️ Answer/Explanation
Detailed solution:
Let’s check each statement. A is wrong: alcohols have the functional group $-\text{OH}$ (hydroxyl), while carboxylic acids have $-\text{COOH}$ (carboxyl). B is incorrect: fermentation occurs at 25–35°C, not 300°C (that’s the temperature for the catalytic addition of steam to ethene). C is incorrect: ethanol is produced by the addition of steam (water) to ethene, not hydrogen—adding hydrogen to ethene would produce ethane. D is correct: when alcohols combust, they react with oxygen to produce carbon dioxide and water: $\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}$. So water is indeed produced, making option D the right choice.
▶️ Answer/Explanation
Detailed solution:
Ethene is an alkene with a carbon-carbon double bond. When it reacts with steam ($H_2O$) in the presence of a phosphoric acid catalyst, a water molecule adds across the double bond. This is an addition reaction that produces ethanol ($C_2H_5OH$). It is one of the two main industrial methods for manufacturing ethanol, the other being fermentation. The reaction specifically gives ethanol, not ethane, ethanoic acid, or a polymer.