Ch 6 · Alcohols · IGCSE Chemistry 0620
Topic 6 of 8 in Organic Chemistry — IGCSE Chemistry 0620 — 56 lessons.
Introduction
Ethanol is the alcohol in drinks, the solvent in perfume and the biofuel blended into petrol. Industry makes it in two completely different ways: an ancient biological process, fermentation, and a modern chemical one, the hydration of ethene. You need the exact conditions of both, the combustion equation, the two uses, and (Extended) a balanced comparison of the methods plus the oxidation of ethanol to ethanoic acid.
Key ideas — Ethanol and the alcohols
Alcohols contain the -OH functional group and have the general formula mathrmC_nH_2n+1OH. Ethanol, CH_3CH_2OH (often written C_2H_5OH), is a colourless, flammable liquid that boils at 78 °C and mixes with water in all proportions.
🧑🏫 Teacher's explanation
Think of alcohols as a family of molecules where every member has a specific 'sticky' part called the hydroxyl group (-OH). This group is like a magnet that allows alcohol molecules to cling to each other and to water, which is why they are such good solvents.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass (M_r) of ethanol, C_2H_5OH. (Given: C=12, H=1)
🔍 Worked example — Standard
A student uses 100 g of glucose (C_6H_12O_6) to produce ethanol (C_2H_5OH) via fermentation. Write the balanced equation for this reaction.
🔍 Worked example — Exam-style
Calculate the volume of CO_2 gas produced at RTP from the fermentation of 0.50 mol of glucose, assuming the stoichiometry from the equation C_6H_12O_6 arrow 2C_2H_5OH + 2CO_2. (Molar volume at RTP = 24 dm^3/mol)
⚠️ Common errors
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🎯 Exam question
Key ideas — Manufacture by fermentation
Fermentation uses yeast to convert aqueous glucose (from sugar cane, sugar beet or the starch in grain) into ethanol and carbon dioxide:
🧑🏫 Teacher's explanation
Imagine you are making bread. You add yeast to flour and water, and the yeast 'eats' the sugars, releasing gas that makes the dough rise. Fermentation is essentially a controlled version of this biological process. Instead of making bread rise, we are using the yeast to convert sugar into alcohol.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of glucose required to produce 46 g of ethanol via fermentation, assuming a 100\% yield. Equation: ceC_6H_12O_6 arrow 2ceC_2H_5OH + 2ceCO_2
🔍 Worked example — Standard
A sample of ethanol produced by fermentation has a concentration of 12\%. If the total volume of the solution is 500 cm^3, calculate the volume of pure ethanol present.
🔍 Worked example — Exam-style
Compare the two methods of ethanol production. Which method is better for producing high-purity ethanol for use as a fuel, and why?
⚠️ Common errors
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🎯 Exam question
Key ideas — Manufacture by hydration of ethene
Ethene, obtained from cracking, is reacted with steam by catalytic addition:
🧑🏫 Teacher's explanation
Imagine you are trying to force two people to hold hands. If they are moving too fast (high temperature) or there is too much chaos (low pressure), they will keep bouncing apart. In chemistry, we use high pressure to squeeze the molecules together and a specific temperature to ensure they have enough energy to react but not so much that they fly apart immediately.
📖 Definitions
🔍 Worked example — Easy
Calculate the molar mass of ethanol (C_2 extH_5 extOH) given A_r values: C=12, H=1.
🔍 Worked example — Standard
A reaction uses 0.5 moles of ethene and 0.5 moles of steam to produce ethanol. If the yield is 80\%, how many moles of ethanol are produced?
⚠️ Common errors
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🎯 Exam question
Key ideas — Combustion and uses
Ethanol burns in air with a clean blue flame to give carbon dioxide and water, releasing a lot of energy:
🧑🏫 Teacher's explanation
Think of ethanol as a versatile 'chemical tool' that can play two very different roles depending on how you use it. When you burn it, you are breaking its chemical bonds to release stored energy, much like burning wood in a fireplace. Because ethanol has a high carbon-to-hydrogen ratio compared to some other fuels, it burns very cleanly if there is enough oxygen, producing a blue flame rather than a smoky orange one.
📖 Definitions
🔍 Worked example — Easy
Write a balanced chemical equation for the complete combustion of ethanol, ceC2H5OH
🔍 Worked example — Standard
Calculate the mass of ethanol (ceC2H5OH) required to produce 88 g of ceCO2 upon complete combustion.
🔍 Worked example — Exam-style
A student uses the hydration of ethene to produce ethanol. The reaction is ceC2H4 + H2O arrow C2H5OH. If 12 g of ethene is reacted with excess steam, calculate the theoretical yield of ethanol.
⚠️ Common errors
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🎯 Exam question
Key ideas — Comparing the methods, and oxidation to ethanoic acid
🧑🏫 Teacher's explanation
Think of making ethanol like making bread versus making soda. Fermentation is like baking bread: you use yeast and sugar, it takes a long time (days), and you get a messy, diluted mixture that needs cleaning up. Hydration of ethene is like a high-tech factory line: you use high pressure and heat to force ethene and steam together. It is much faster and cleaner, but it uses oil (petroleum), which won't last forever.
📖 Definitions
🔍 Worked example — Easy
Calculate the mass of ethanol (C_2H_5OH) produced from 180 g of glucose (C_6\ extH_12O_6) via fermentation, assuming 100\% yield. Equation: C_6H_12O_6 arrow 2C_2H_5OH + 2CO_2. (M_r of glucose = 180, M_r of ethanol = 46)
🔍 Worked example — Standard
Identify the observation when acidified potassium manganate(VII) is added to ethanol and heated.
🔍 Worked example — Exam-style
A student uses the hydration of ethene to produce ethanol. The reaction is C_2H_4 + H_2O arrow C_2H_5OH. If the student starts with 5.6 g of ethene (C_2H_4), calculate the theoretical yield of ethanol. (M_r of C_2H_4 = 28, M_r of C_2H_5OH = 46)
⚠️ Common errors
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🎯 Exam question
Worked example
Question. Glucose is fermented to make ethanol. (a) State the conditions needed for fermentation. [3] (b) Calculate the maximum mass of ethanol and the volume of carbon dioxide at room temperature and pressure that can be obtained from 90 g of glucose. (M_r: glucose = 180, ethanol = 46; one mole of gas occupies 24 dm³ at r.t.p.) [4]
🔍 Worked example — Standard
A student uses 180 g of glucose to produce ethanol. Calculate the volume of CO_2 gas produced at r.t.p. (M_r: glucose = 180; 1 mole of gas = 24 dm³). Equation: C_6H_12O_6 arrow 2C_2H_5OH + 2CO_2
🔍 Worked example — Exam-style
In industrial production, ethene (C_2H_4) reacts with steam (H_2O) to form ethanol (C_2H_5OH). If 28 g of ethene is reacted completely, calculate the mass of ethanol produced. (M_r: ethene = 28, ethanol = 46). Equation: C_2H_4 + H_2O arrow C_2H_5OH
⚠️ Common errors
Common mistakes
Practice
Answers Ethene and steam [B1]; 300 °C and 6000 kPa [B1]; acid (phosphoric acid) catalyst [B1]; C_2H_4 + H_2O arrow C_2H_5OH [B1]. C_2H_5OH + 3O_2 arrow 2CO_2 + 3H_2O: products [B1], balancing [B1]. Solvent [B1]; fuel [B1]; e.g. as a solvent it dissolves substances insoluble in water and evaporates easily, or as a fuel it burns cleanly releasing a lot of energy [B1]. Advantages, any two: renewable raw material; low temperature so little energy needed; simple, cheap equipment [B1 each]. Disadvantage, any one: slow; batch process; dilute, impure product needing distillation; uses farmland [B1]. Purple solution turns colourless [B1]; ethanoic acid [B1].