Alcohols Revision Notes

Fermentation is a biological process where yeast converts sugars into ethanol and carbon dioxide in the absence of oxygen. The sugar solution, commonly glucose (C6H12O6), is maintained at a temperature between 25 °C and 35 °C to optimise yeast activity. Temperatures below 25 °C slow the reaction, while temperatures above 35 °C can denature the yeast enzymes, stopping fermentation.

Yeast cells metabolise glucose anaerobically, producing ethanol (C2H5OH) and carbon dioxide (CO2). The process is anaerobic because oxygen inhibits ethanol production and causes respiration instead.

The balanced chemical equation for fermentation is: C6H12O6 (aq) → 2 C2H5OH (aq) + 2 CO2 (g)

• Fermentation requires sugar solution (e.g., glucose).
• Temperature range: 25–35 °C for optimal enzyme activity.
• Anaerobic conditions (absence of oxygen) are essential.
• Yeast acts as a biological catalyst.
• Products: ethanol and carbon dioxide.

During fermentation, carbon dioxide gas bubbles out of the solution, which can be observed as fizzing or frothing. The sugar solution gradually becomes less sweet as glucose is consumed, and ethanol concentration increases.

The reaction mixture is often kept in a sealed container with a delivery tube to allow CO2 to escape but prevent oxygen entry, maintaining anaerobic conditions.

Fermentation is relatively slow, taking several hours to days depending on conditions and sugar concentration. The ethanol concentration typically reaches about 15% by volume because higher concentrations inhibit yeast activity and kill the cells.

• CO2 gas evolution observed as bubbling.
• Sugar concentration decreases; ethanol concentration increases.
• Anaerobic conditions maintained by sealing the container.
• Fermentation time varies from hours to days.
• Maximum ethanol concentration limited to ~15% due to yeast tolerance.

The catalytic hydration method produces ethanol by reacting ethene (C2H4) with steam (water vapour) in the presence of an acid catalyst, typically phosphoric acid (H3PO4) supported on silica.

This reaction occurs at a high temperature of about 300 °C and a high pressure of approximately 6000 kPa (60 atm) to increase the rate and yield of ethanol.

The reaction is an example of an electrophilic addition, where the double bond in ethene opens to add a hydroxyl group (–OH) and a hydrogen atom, forming ethanol.

• Reactants: ethene gas and steam.
• Conditions: 300 °C temperature, 6000 kPa pressure.
• Catalyst: acid catalyst such as phosphoric acid on silica.
• Reaction type: electrophilic addition (hydration).
• Product: ethanol (C2H5OH).

The balanced chemical equation for the catalytic hydration of ethene is: C2H4 (g) + H2O (g) → C2H5OH (g)

The acid catalyst protonates the ethene double bond, forming a carbocation intermediate. Water then attacks the carbocation, followed by deprotonation to yield ethanol.

The catalyst is not consumed in the reaction and provides an alternative pathway with lower activation energy, increasing the reaction rate without affecting the equilibrium position.

• Balanced equation: C2H4 (g) + H2O (g) → C2H5OH (g)
• Acid catalyst protonates ethene double bond.
• Carbocation intermediate formed.
• Water attacks carbocation, then loses a proton.
• Catalyst remains unchanged after reaction.

Fermentation is a biological process operating at mild temperatures and atmospheric pressure, producing ethanol with some impurities and limited concentration. It uses renewable sugar sources and is suitable for small-scale or bioethanol production.

Catalytic hydration is a chemical process requiring high temperature and pressure, producing high-purity ethanol from ethene derived from crude oil. It is faster and suitable for large-scale industrial production but depends on non-renewable feedstocks.

• Fermentation: mild conditions, renewable sugars, slower, lower ethanol concentration.
• Catalytic hydration: high temperature and pressure, non-renewable ethene, faster, high purity ethanol.
• Fermentation produces CO2 as a by-product; catalytic hydration does not.
• Catalytic hydration requires acid catalyst; fermentation uses yeast enzymes.
• Choice depends on feedstock availability and production scale.

Ethanol (C2H5OH) is an organic compound classified as an alcohol. When ethanol burns in the presence of oxygen, it undergoes a combustion reaction, releasing energy in the form of heat and light. This reaction is important in both laboratory and industrial contexts, especially as ethanol is used as a fuel.

• Ethanol is a flammable liquid.
• Combustion requires oxygen from the air.
• Energy is released during combustion as heat and light.

Complete combustion occurs when ethanol burns with a plentiful supply of oxygen. In this reaction, ethanol reacts fully with oxygen to produce carbon dioxide and water only. This process releases the maximum amount of energy.

The balanced chemical equation for complete combustion of ethanol is:

C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) (balanced)

• Complete combustion requires sufficient oxygen.
• Products are carbon dioxide and water vapour.
• The reaction is exothermic, releasing heat and light.
• Flame is typically blue and clean-burning.

If oxygen supply is limited, ethanol undergoes incomplete combustion. This produces carbon monoxide (CO), a toxic gas, and carbon (soot) instead of carbon dioxide. Less energy is released compared to complete combustion.

Incomplete combustion can be represented by the simplified equation:

2C2H5OH + 5O2 → 4CO + 6H2O (incomplete combustion example)

• Insufficient oxygen causes incomplete combustion.
• Products include carbon monoxide and soot (carbon).
• Flame is yellow and smoky due to soot particles.
• Incomplete combustion is less efficient and hazardous due to CO.

Combustion of ethanol is an exothermic reaction, meaning it releases energy. This energy can be observed as heat and light from the flame.

During complete combustion, the flame is blue and steady, indicating efficient burning.

During incomplete combustion, the flame is yellow and smoky, indicating soot formation and less energy released.

• Energy released can be used for heating or powering engines.
• Flame colour indicates combustion completeness.
• Soot deposits can accumulate on surfaces during incomplete combustion.

Balance the equation for the complete combustion of ethanol: C2H5OH + O2 → CO2 + H2O.

Step 1: Write unbalanced equation: C2H5OH + O2 → CO2 + H2O

Step 2: Balance carbon atoms: 2 carbons in ethanol → 2 CO2 molecules on right: C2H5OH + O2 → 2CO2 + H2O

• Step 3: Balance hydrogen atoms: 6 hydrogens in ethanol → 3 H2O molecules: C2H5OH + O2 → 2CO2 + 3H2O
• Step 4: Balance oxygen atoms: Left side oxygen atoms = from ethanol (1) + from O2 (x), Right side oxygen atoms = 2×2 (from CO2) + 3×1 (from H2O) = 4 + 3 = 7
• Step 5: Oxygen atoms on left: 1 (ethanol) + 2x (O2) = 7 → 2x = 6 → x = 3
• Final balanced equation: C2H5OH + 3O2 → 2CO2 + 3H2O

Ethanol (C2H5OH) is an alcohol with a polar hydroxyl (-OH) group and a non-polar ethyl (C2H5-) group, giving it the ability to dissolve a wide range of substances. This dual polarity allows ethanol to dissolve both polar compounds (such as salts and sugars) and non-polar compounds (such as oils and fats).

In laboratories and industries, ethanol is commonly used as a solvent for pharmaceuticals, perfumes, and paints due to its effectiveness and relatively low toxicity compared to other organic solvents.

Its miscibility with water and many organic solvents makes it ideal for preparing solutions and extracting compounds.

• Ethanol contains a polar -OH group and a non-polar hydrocarbon chain.
• It dissolves polar substances by hydrogen bonding and non-polar substances by van der Waals forces.
• Used in pharmaceuticals, cosmetics, and chemical synthesis as a solvent.

The hydroxyl group in ethanol forms hydrogen bonds with polar molecules, increasing solubility of substances like sugars and salts.

The ethyl group interacts with non-polar molecules through London dispersion forces, allowing ethanol to dissolve oils and fats.

This amphipathic nature makes ethanol a versatile solvent in chemical reactions and extractions.

• Hydrogen bonding with polar solutes enhances solubility.
• Non-polar interactions allow dissolution of hydrophobic substances.
• Ethanol's solvent properties depend on molecular structure.

Ethanol is used as a renewable fuel either in pure form (anhydrous ethanol) or blended with petrol (gasoline) to produce mixtures such as E10 (10% ethanol, 90% petrol).

It burns cleanly in combustion reactions, producing carbon dioxide and water, releasing energy that powers engines.

The combustion reaction of ethanol is: C2H5OH(l) + 3O2(g) → 2CO2(g) + 3H2O(g) (balanced equation)

• Ethanol is a biofuel derived from biomass.
• Used in vehicles to reduce fossil fuel consumption.
• Combustion produces CO2 and H2O, releasing energy.

Ethanol is renewable, as it can be produced from fermentation of sugars in crops like sugarcane and maize.

It burns more cleanly than petrol, producing fewer particulates and carbon monoxide emissions.

Blending ethanol with petrol reduces overall greenhouse gas emissions and dependence on fossil fuels.

• Renewable and sustainable energy source.
• Reduces harmful emissions compared to pure petrol.
• Can be blended with petrol to improve fuel octane rating.

Ethanol is flammable and must be handled with care, stored away from ignition sources.

Its use as a solvent requires consideration of purity and concentration depending on the application.

Fuel ethanol must be anhydrous (water-free) to prevent engine corrosion and maintain combustion efficiency.

• Store ethanol in labelled, sealed containers away from flames.
• Use appropriate concentrations for solvent applications.
• Fuel-grade ethanol requires removal of water to avoid engine damage.

Fermentation is a biological process where glucose (a sugar) is converted into ethanol and carbon dioxide by yeast enzymes under anaerobic conditions.

The general equation for fermentation is: C6H12O6 (aq) → 2 C2H5OH (aq) + 2 CO2 (g)

This reaction occurs at moderate temperatures, typically between 25 °C and 35 °C, and requires the absence of oxygen to prevent oxidation of ethanol to ethanoic acid (vinegar). Yeast acts as a biological catalyst, producing enzymes that break down glucose into ethanol and carbon dioxide.

• Uses renewable biomass sources such as sugarcane, corn, or other carbohydrate-rich crops.
• Operates at relatively low temperatures and atmospheric pressure, reducing energy consumption.
• Simple equipment and technology allow small-scale production.
• Produces ethanol with a concentration of about 10–15% by volume; higher concentrations require distillation.

Fermentation uses renewable raw materials, making it sustainable if crops are grown responsibly.

The process requires less energy input compared to chemical methods, as it occurs at mild temperatures and pressures.

It is a relatively simple and low-cost process, suitable for rural or small-scale production where petrochemical infrastructure is unavailable.

• Renewable feedstock reduces dependence on fossil fuels.
• Lower energy costs due to mild reaction conditions.
• Simplicity allows decentralised production and supports local economies.

Fermentation produces ethanol at relatively low concentrations (10–15%), necessitating energy-intensive distillation to obtain pure ethanol.

The process is slow, often taking several days to complete, limiting production rates.

Large-scale fermentation requires significant agricultural land, which may compete with food production and contribute to deforestation or habitat loss.

• Low ethanol yield per batch increases processing costs.
• Time-consuming process limits industrial throughput.
• Land use for biomass cultivation can impact food supply and biodiversity.

This chemical method involves the reaction of ethene (C2H4), derived from crude oil or natural gas, with steam in the presence of an acid catalyst, typically phosphoric acid supported on silica.

The reaction is carried out at high temperature (~300 °C) and high pressure (~6000 kPa) to increase yield and rate.

The balanced equation is: C2H4 (g) + H2O (g) ⇌ C2H5OH (g) (catalyst: H3PO4, conditions: 300 °C, 6000 kPa)

• Produces ethanol with high purity, often close to 100%.
• Fast reaction rate allows continuous industrial production.
• Does not require agricultural land, reducing competition with food crops.

High yield and purity reduce the need for further purification steps, lowering overall production costs.

The process is rapid and continuous, enabling large-scale industrial ethanol production.

No reliance on biomass feedstock means less vulnerability to agricultural fluctuations.

• Efficient conversion with high ethanol concentration.
• Continuous process suitable for large-scale manufacture.
• Reduces pressure on agricultural resources.

Relies on ethene derived from fossil fuels, which are non-renewable and contribute to greenhouse gas emissions.

Requires high temperatures and pressures, increasing energy consumption and operational costs.

Extraction and processing of fossil fuels have significant environmental impacts, including pollution and habitat disruption.

• Dependence on finite petrochemical resources.
• High energy input increases carbon footprint.
• Environmental concerns from fossil fuel extraction and use.