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Alcohols

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This topic is examined in Paper 1, Paper 2, Paper 3, Paper 4, Paper 5, and Paper 6.

Manufacture of Ethanol: Fermentation
Ethanol can be produced biologically through fermentation. This is the process where yeast acts as a biological catalyst to break down sugars. For Cambridge exams, you must specify the raw material as aqueous glucose.

The reaction requires three specific conditions to occur effectively:

  1. Temperature: The mixture must be kept between 25–35°C. If the temperature is too low, the reaction is too slow; if it is above 40°C, the yeast enzymes denature (stop working).
  2. Anaerobic Conditions: The process must occur in the absence of oxygen (air). If oxygen is present, the yeast will respire aerobically to produce carbon dioxide and water instead of ethanol.
  3. Yeast: Yeast must be present to provide the enzymes required for fermentation.
The word equation for this process is:
\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}

Building on previous concepts: This is an example of anaerobic respiration. The carbon dioxide produced can be detected by bubbling the gas through limewater, which turns milky.

Manufacture of Ethanol: Catalytic Addition of Steam to Ethene
Ethanol can also be produced industrially from ethene (an alkene) via catalytic addition. This is a chemical synthesis method rather than a biological one.

The reaction involves adding steam (water vapour) to ethene. The specific conditions required are:

  1. Temperature: Approximately 300°C.
  2. Pressure: High pressure of approximately 6000 kPa (or 60 atm).
  3. Catalyst: An acid catalyst (typically phosphoric acid, H_3PO_4).
The word equation is:
\text{ethene} + \text{steam} \rightarrow \text{ethanol}

Note on Notation: In chemical equations, steam is represented as H_2O(g) or simply 'steam'. The reaction is an addition reaction because the double bond in ethene breaks to allow the water molecule to add across it.

Advantages and Disadvantages of Production Methods
Disadvantages
Slow reaction rate (takes several days).
Produces a dilute solution of ethanol (approx. 15%), requiring expensive fractional distillation to purify.
Uses agricultural land that could be used for food.
Uses non-renewable resources (ethene is derived from crude oil/petroleum).
Requires high temperature and pressure, consuming significant energy.
Combustion is the reaction of a substance with oxygen that releases energy in the form of heat and light. For alcohols like ethanol, complete combustion occurs when there is sufficient oxygen.

When describing the combustion of ethanol in an exam, you must include both the physical observation and the chemical products.

  1. Observation: Ethanol burns with a clean blue flame. This indicates complete combustion (no soot/carbon particles are produced).
  2. Products: The reaction produces carbon dioxide and water.
The symbol equation for the complete combustion of ethanol is:
C_2H_5OH + 3O_2 \rightarrow 2CO_2 + 3H_2O

Why this matters: This reaction is exothermic (releases heat), which is why ethanol is used as a fuel. The 'clean blue flame' is a key identifier for alcohols compared to hydrocarbons like methane, which may burn with a slightly yellowish or sooty flame if oxygen is limited.

Ethanol has two primary uses that are frequently tested:

  1. As a Fuel: Ethanol is used as a fuel for vehicles (often mixed with petrol/gasoline). It burns cleanly, producing less pollution than fossil fuels.
  2. As a Solvent: Ethanol is an excellent solvent because it can dissolve both polar substances (like water) and non-polar substances (like oils and fats). This makes it useful in perfumes, paints, and cleaning agents.
⚠︎ Fermentation Conditions and Products
Mistake 1: Incorrect Temperature Range
Students often write '37°C' or 'room temperature'. While 37°C is body temperature, the Cambridge syllabus specifically requires 25–35°C. Writing a single value outside this range may lose marks. Always state the range.
Mistake 2: Missing 'Absence of Oxygen'
Students frequently forget to mention that fermentation must be anaerobic (without oxygen). If oxygen is present, yeast undergoes aerobic respiration, producing CO_2 and H_2O, not ethanol. This is a critical condition.
Mistake 3: Confusing Fermentation Products
Students sometimes list 'ethene' as a product of fermentation. Ethene is the reactant for the industrial method, not a product of fermentation. The products are ethanol and carbon dioxide.
⚠︎ Industrial Conditions and Combustion Description
Mistake 1: Vague Industrial Conditions
When asked for conditions for catalytic addition, students often write 'high temperature' or 'catalyst'. You must be specific: 300°C, 6000 kPa (or 60 atm), and an acid catalyst. The markscheme requires these specific values.
Mistake 2: Incomplete Combustion Description
When asked to 'describe' combustion, writing only the equation is insufficient. You must describe the observation: 'burns with a clean blue flame'. Without this physical description, you will lose marks for not answering the command word 'describe'.
Mistake 3: Vague Uses
Writing 'medicine' or 'cleaning' is too vague. The specific uses are solvent and fuel. If you write 'disinfectant', it is acceptable only if linked to its solvent properties, but 'solvent' and 'fuel' are the core syllabus answers.
Specifying Conditions for Industrial Manufacture
When to use: When answering questions about the catalytic addition of steam to ethene (Paper 3/4).

Why examiners accept this: The markscheme is strict on numerical values. 'High pressure' is not accepted; you must write 6000 kPa or 60 atm. Similarly, 'high temperature' is rejected; you must write 300°C.

Example:
Question: State the conditions for making ethanol from ethene.
Correct Answer: Temperature of 300°C, pressure of 6000 kPa, and an acid catalyst.

Describing Fermentation Completeness
When to use: When interpreting practical data or describing the end of fermentation (Paper 5/6).

Why examiners accept this: Fermentation produces carbon dioxide gas. As long as yeast is active and glucose remains, bubbles are produced. The reaction stops when glucose is used up or ethanol concentration becomes too high for the yeast.

Example:
Question: How can you tell fermentation is complete?
Correct Answer: There are no more bubbles (of gas) observed.

Fermentation Conditions and Products
Q:
State two conditions required for the fermentation of glucose.
A:
  1. Temperature between 25–35°C.
  2. Absence of oxygen (anaerobic conditions).
    (Or: Presence of yeast)
Q:
Name the raw material used to make ethanol by fermentation.
A:
Glucose
Industrial Manufacture Conditions
Q:
State the temperature and pressure used in the catalytic addition of steam to ethene.
A:
Temperature: 300°C
Pressure: 6000 kPa (or 60 atm)
Q:
Name the catalyst used in the manufacture of ethanol from ethene.
A:
Acid catalyst (or Phosphoric acid)
Combustion and Uses
Q:
Describe the observation when ethanol burns in air.
A:
It burns with a clean blue flame.
Q:
State two products formed during the complete combustion of ethanol.
A:
  1. Carbon dioxide
  2. Water
Q:
State two uses of ethanol.
A:
  1. As a solvent.
  2. As a fuel.
Comparing Production Methods
Q:
State one advantage of manufacturing ethanol by fermentation.
A:
Uses renewable resources (glucose from plants).
Q:
State one disadvantage of manufacturing ethanol by fermentation.
A:
Produces a dilute solution requiring distillation.
(Or: Slow reaction rate.)
Q:
State one advantage of manufacturing ethanol from ethene.
A:
Fast reaction rate (or produces pure ethanol).
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