Alcohols

1. Overview

Alcohols are a homologous series of organic compounds containing the hydroxyl (-OH) functional group. This topic focuses primarily on ethanol ($C_2H_5OH$), exploring how it is manufactured industrially, its chemical reactions, and its vital role as both a solvent and a sustainable fuel source.

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Key Definitions

• Functional Group: An atom or group of atoms that determine the chemical properties of a homologous series. For alcohols, this is the –OH group.
• Fermentation: A biological process where enzymes in yeast convert sugars into ethanol and carbon dioxide in the absence of oxygen.
• Hydration (Catalytic Addition): A chemical reaction where water (in the form of steam) reacts with an unsaturated molecule like ethene to produce an alcohol.
• Anaerobic: Conditions that occur in the absence of oxygen.

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Core Content

A. Manufacture of Ethanol by Fermentation

Ethanol can be produced from renewable plant sugars (glucose).

• Process: Yeast is added to an aqueous solution of glucose.
• Conditions:
  • Temperature: 25–35 °C (Optimum temperature for yeast enzymes).
  • Atmosphere: Absence of oxygen (anaerobic). If oxygen is present, the yeast will produce $CO_2$ and $H_2O$ instead, or the ethanol will oxidize into ethanoic acid (vinegar).
  • Catalyst: Yeast (contains enzymes).

Equations:

• Word: Glucose → Ethanol + Carbon Dioxide
• Symbol: $C_6H_{12}O_6(aq) \rightarrow 2C_2H_5OH(aq) + 2CO_2(g)$

B. Manufacture of Ethanol by Catalytic Addition (Hydration)

Ethanol is produced from ethene, which is obtained from the cracking of crude oil.

• Process: Steam is reacted with ethene.
• Conditions:
  • Temperature: 300 °C.
  • Pressure: 6000 kPa / 60 atm.
  • Catalyst: Phosphoric acid ($H_3PO_4$).

Equations:

• Word: Ethene + Steam → Ethanol
• Symbol: $C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(l)$

C. Combustion of Ethanol

Ethanol burns cleanly in the presence of oxygen, making it an effective fuel.

• Word: Ethanol + Oxygen → Carbon Dioxide + Water
• Symbol: $C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)$

D. Uses of Ethanol

1. As a Solvent: Used in perfumes, cosmetics, glues, and inks because it dissolves many substances that are insoluble in water.
2. As a Fuel: Used in spirit burners and mixed with petrol (bioethanol) to power car engines, reducing reliance on fossil fuels.

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Extended Content (Extended Curriculum Only)

Comparison of Manufacture Methods

In the exam, you may be asked to compare these two methods.

Feature	Fermentation	Catalytic Addition (Hydration)
Raw Material	Renewable (Glucose from crops)	Non-renewable (Ethene from crude oil)
Type of Process	Batch process (Slow)	Continuous process (Fast)
Purity	Low (produces a dilute solution)	High (produces pure ethanol)
Energy Requirement	Low (Low temperatures)	High (High temp/pressure required)
Reaction Rate	Very Slow	Very Fast

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Key Equations

1. Fermentation:

   • $C_6H_{12}O_6(aq) \rightarrow 2C_2H_5OH(aq) + 2CO_2(g)$
   • Note: Glucose is aqueous; $CO_2$ is a gas.

2. Hydration of Ethene:

   • $C_2H_4(g) + H_2O(g) \rightarrow C_2H_5OH(l)$
   • Note: Water must be in gas state (steam).

3. Complete Combustion:

   • $C_2H_5OH(l) + 3O_2(g) \rightarrow 2CO_2(g) + 3H_2O(l)$
   • Note: Ensure the oxygen ($O_2$) is balanced (3 moles).

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Common Mistakes to Avoid

• ❌ Wrong: Forgetting the specific conditions for fermentation.
  • ✅ Right: Always state 25–35 °C and absence of oxygen. If the temperature is too high (>40 °C), yeast enzymes denature.
• ❌ Wrong: Writing $H_2O(l)$ for the hydration of ethene.
  • ✅ Right: Use $H_2O(g)$ because the reaction uses steam at 300 °C.
• ❌ Wrong: Using the wrong catalyst for ethene hydration.
  • ✅ Right: The catalyst is Phosphoric acid ($H_3PO_4$), not yeast or nickel.

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Exam Tips

• Command Words:
  • "State": Give a brief answer (e.g., "State the conditions for fermentation" → 30 °C, yeast, no oxygen).
  • "Name": Provide the chemical name only (e.g., "Name the catalyst" → Phosphoric acid).
• Question Types: Expect questions asking you to compare the "Greenness" of the two manufacturing methods. Fermentation is often considered "greener" as it uses renewable resources, but hydration is more efficient.
• Real-world contexts: Be prepared for questions about "Biofuel." Remember that ethanol from fermentation is a "carbon-neutral" concept because the $CO_2$ released during combustion was originally absorbed by the plants during photosynthesis.
• Typical Values: Remember 300 °C and 60 atm for hydration. These numbers appear frequently in multiple-choice and structured questions.

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Exam-Style Questions

Practice these original exam-style questions to test your understanding. Each question mirrors the style, structure, and mark allocation of real Cambridge 0620 Theory papers.

Exam-Style Question 1 — Short Answer [6 marks]

Question:

Ethanol is an important industrial chemical that can be manufactured by two different methods.

(a) State the two raw materials required for the production of ethanol by fermentation. [2]

(b) Describe two advantages of producing ethanol by fermentation compared to the catalytic addition of steam to ethene. [4]

Worked Solution:

(a)

1. Aqueous glucose This is the sugar source that will be fermented.
2. Yeast Yeast contains enzymes that catalyse the fermentation process.

How to earn full marks:

• State "aqueous glucose" or a named sugar (e.g., sucrose, starch) for 1 mark.
• State "yeast" for 1 mark.

(b)

1. Fermentation uses renewable resources. Glucose can be obtained from crops, which are renewable.
2. Fermentation requires lower temperatures and pressures. This reduces energy consumption and costs.

How to earn full marks:

• State one advantage of fermentation (renewable resources, lower temperatures) for 2 marks.
• State a second advantage of fermentation for 2 marks.

Common Pitfall: Many students only state "glucose" without specifying "aqueous glucose." Remember that the yeast needs to be in a solution to effectively ferment the sugar. Also, be sure to clearly articulate the advantages of fermentation, focusing on the environmental and economic aspects.

Exam-Style Question 2 — Short Answer [5 marks]

Question:

Ethanol is a member of the homologous series of alcohols.

(a) Define the term homologous series. [2]

(b) State two general characteristics of a homologous series. [2]

(c) Name the alcohol with three carbon atoms. [1]

Worked Solution:

(a)

1. A homologous series is a series of organic compounds This indicates the type of compounds being referred to.
2. with the same functional group and similar chemical properties This highlights the key similarities between members of the series.
3. which differ by $CH_2$ in their molecular formula. This points out the consistent difference in structure between successive members.

How to earn full marks:

• Mention "series of organic compounds" for 1 mark.
• Mention "same functional group" and "similar chemical properties" for 1 mark.

(b)

1. They have the same general formula. This refers to the algebraic representation of the series.
2. They show a gradual change in physical properties (e.g., boiling point) This points to the trend in physical characteristics.

How to earn full marks:

• State one general characteristic for 1 mark.
• State a second general characteristic for 1 mark.

(c)

1. Propanol

How to earn full marks:

• Correct name of the alcohol: 1 mark.

Common Pitfall: When defining a homologous series, students often forget to mention both the same functional group and similar chemical properties. Also, remember that the "gradual change in physical properties" is a key feature. Finally, double-check your alcohol names to ensure you've used the correct prefix for the number of carbon atoms.

Exam-Style Question 3 — Extended Response [8 marks]

Question:

Ethanol ($C_2H_5OH$) undergoes complete combustion in the presence of oxygen.

(a) Write a balanced chemical equation for the complete combustion of ethanol. [2]

(b) Calculate the mass of carbon dioxide produced when 9.2 g of ethanol is completely combusted. $[A_r: C = 12, H = 1, O = 16]$ [6]

Worked Solution:

(a)

1. $C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O$ This shows the reactants and products of the reaction.

How to earn full marks:

• Correct reactants and products (unbalanced equation) for 1 mark.
• Correct balancing of the entire equation for 1 mark.

(b)

1. $M_r$ of $C_2H_5OH = (2 \times 12) + (6 \times 1) + (1 \times 16) = 46$ This calculates the relative molecular mass of ethanol.
2. Moles of $C_2H_5OH = \frac{9.2}{46} = 0.2 , mol$ This calculates the number of moles of ethanol.
3. From the equation, 1 mol of $C_2H_5OH$ produces 2 mol of $CO_2$. This identifies the mole ratio between ethanol and carbon dioxide.
4. Moles of $CO_2 = 2 \times 0.2 = 0.4 , mol$ This calculates the number of moles of carbon dioxide produced.
5. $M_r$ of $CO_2 = (1 \times 12) + (2 \times 16) = 44$ This calculates the relative molecular mass of carbon dioxide.
6. Mass of $CO_2 = 0.4 \times 44 = \boxed{17.6 , g}$ This calculates the mass of carbon dioxide produced.

How to earn full marks:

• Calculate $M_r$ of $C_2H_5OH$ correctly (46) for 1 mark.
• Calculate moles of $C_2H_5OH$ correctly (0.2 mol) for 1 mark.
• Correct mole ratio between ethanol and carbon dioxide for 1 mark.
• Calculate moles of $CO_2$ correctly (0.4 mol) for 1 mark.
• Calculate $M_r$ of $CO_2$ correctly (44) for 1 mark.
• Calculate the mass of $CO_2$ correctly (17.6 g) for 1 mark.

Common Pitfall: Balancing combustion equations can be tricky. Double-check that you've accounted for all atoms on both sides. Also, remember to use the correct mole ratio from the balanced equation when calculating the moles of carbon dioxide produced. Don't forget the units in your final answer!

Exam-Style Question 4 — Extended Response [9 marks]

Question:

A student investigates the rate of fermentation of glucose using yeast. They set up the apparatus shown below, collecting carbon dioxide gas produced in a measuring cylinder.

The student measures the volume of carbon dioxide gas collected every minute for 10 minutes. The results are shown in the table:

Time (minutes)	Volume of $CO_2$ (cm$^3$)
0	0
1	5
2	11
3	16
4	21
5	25
6	29
7	32
8	35
9	37
10	38

(a) Plot a graph of volume of $CO_2$ (y-axis) against time (x-axis). [4]

(b) Describe the shape of the graph and explain what it shows about the rate of fermentation. [3]

(c) Suggest two ways to increase the rate of fermentation in this experiment. [2]

Worked Solution:

(a)

1. 📊A graph with time (0-10 min) on the x-axis and volume of CO2 (0-40 cm^3) on the y-axis. The points from the table are plotted accurately. A smooth curve of best fit is drawn through the points.
   *The graph shows the volume of carbon dioxide produced over time.*
2. Correctly labeling both axes with units. Ensures the graph is properly defined.
3. Correctly plotting all points. Ensures the data is accurately represented.
4. Drawing a smooth curve of best fit. Shows the trend in the data.

How to earn full marks:

• Correctly labeled axes with units for 1 mark.
• Correctly plotting all points for 2 marks.
• Drawing a smooth curve of best fit for 1 mark.

(b)

1. The graph shows that the rate of $CO_2$ production decreases with time. This identifies the overall trend in the data.
2. Initially, the rate of reaction is high, as there are plenty of glucose molecules for the yeast to act on. This explains why the reaction starts quickly.
3. As time increases, the glucose concentration decreases, so the rate of reaction slows down. This explains why the rate decreases over time.

How to earn full marks:

• State that the rate of reaction decreases with time for 1 mark.
• Explain that the initial rate is high due to high glucose concentration for 1 mark.
• Explain that the rate slows down as the glucose concentration decreases for 1 mark.

(c)

1. Increase the temperature (but not too high, as this will denature the enzymes in the yeast). Temperature affects the rate of enzyme activity.
2. Increase the concentration of glucose. More substrate available for the yeast.

How to earn full marks:

• Suggest one way to increase the rate of fermentation for 1 mark.
• Suggest a second way to increase the rate of fermentation for 1 mark.

Common Pitfall: When plotting graphs, always double-check that your axes are labeled with both the quantity and the correct units. When describing the graph's shape, be specific about the rate of reaction changing, not just the volume of gas. Also, remember that enzymes have an optimal temperature range; too high, and they'll denature.