Fermentation

Definition

Fermentation

The conversion of aqueous glucose into ethanol and carbon dioxide by enzymes in yeast in the absence of oxygen.

Ethanol, $C2H5OH\text{C}_2\text{H}_5\text{OH}$ , is made on a large scale by two different methods: fermentation of glucose and catalytic addition of steam to ethene.
Fermentation starts with aqueous glucose, which is glucose dissolved in water, usually obtained from sugar-rich crops such as sugar cane or sugar beet.
Yeast is added to the glucose solution, because yeast cells contain enzymes that catalyse the breakdown of glucose.
Three conditions are needed for fermentation.
1. Yeast must be present to supply the enzymes.
2. The temperature is kept between $∘C25\ ^\circ\text{C}$ and $∘C35\ ^\circ\text{C}$ .
3. Oxygen is kept out, so the process is anaerobic.
Below $∘C25\ ^\circ\text{C}$ the enzymes work slowly, so fermentation takes much longer.
Above about $∘C35\ ^\circ\text{C}$ the enzymes start to become denatured, so the rate falls and fermentation stops.
Oxygen must be excluded because, with air present, the yeast respires aerobically to make carbon dioxide and water instead of ethanol, and bacteria can oxidise the ethanol that has formed.
The fermentation vessel is fitted with an airlock, which lets carbon dioxide escape but stops air getting in.
Glucose is broken down into ethanol and carbon dioxide.
$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$
The balanced symbol equation shows that one glucose molecule gives two ethanol molecules and two carbon dioxide molecules.
$\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} \rightarrow 2\text{C}_2\text{H}_5\text{OH(aq)} + 2\text{CO}_2\text{(g)}$
Fermentation is slow and takes several days, with bubbles of carbon dioxide given off the whole time.
Fermentation stops when the glucose is used up or when the ethanol concentration reaches about $15%15\%$ , because ethanol at this concentration kills the yeast.
The product is a dilute aqueous solution of ethanol, so the yeast is removed by filtration and the ethanol is separated from the water by fractional distillation.

Example

Balancing the fermentation equation

Glucose, $C6H12O6\text{C}_6\text{H}_{12}\text{O}_6$ , is the only reactant, and the products are ethanol, $C2H5OH\text{C}_2\text{H}_5\text{OH}$ , and carbon dioxide, $CO2\text{CO}_2$ .

\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{CO}_2

Glucose has 12 hydrogen atoms and each ethanol molecule has 6, so 2 ethanol molecules are needed.
Two ethanol molecules hold 4 carbon atoms, so the remaining 2 carbon atoms go into 2 carbon dioxide molecules.

\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2

Checking oxygen, the left side has 6 atoms and the right side has $2 + 4 = 6$ atoms, so the equation is balanced.

Practical

Fermenting a glucose solution

Aim: to make ethanol by fermenting aqueous glucose with yeast and to identify the gas given off.
Apparatus: $cm3250\ \text{cm}^3$ conical flask, $g10\ \text{g}$ of glucose, $cm3100\ \text{cm}^3$ measuring cylinder, warm water, $g1\ \text{g}$ of dried yeast, bung fitted with a delivery tube, boiling tube of limewater, thermostatically controlled water bath set to $∘C30\ ^\circ\text{C}$ , filter funnel and filter paper, and eye protection.
Method:
- Dissolve $g10\ \text{g}$ of glucose in $cm3100\ \text{cm}^3$ of warm water in the conical flask and swirl until no solid remains.
- Stir in $g1\ \text{g}$ of dried yeast.
- Fit the bung and delivery tube so that the end of the tube dips into limewater, which lets gas out but keeps air away from the mixture.
- Stand the flask in the water bath at $∘C30\ ^\circ\text{C}$ and leave it for several days.
- When bubbling stops, filter the mixture to remove the yeast and keep the filtrate for fractional distillation.
Results:
- Within about half an hour, bubbles rise steadily through the mixture and a froth forms on the surface.
- The limewater turns milky, showing that the gas is carbon dioxide.
- The filtrate smells of alcohol, and bubbling slows and stops after a few days.
Conclusion: yeast ferments glucose into ethanol and carbon dioxide when it is kept warm and away from oxygen.
Safety: wear eye protection, keep the ethanol-containing filtrate away from naked flames because ethanol is flammable, and never taste the product.

Steam addition to ethene

Definition

Catalyst

A substance that increases the rate of a reaction and is chemically unchanged at the end of the reaction.

The second method reacts ethene, $C2H4\text{C}_2\text{H}_4$ , with steam, $H2O\text{H}_2\text{O}$ , to make ethanol.
The ethene comes from cracking large alkane molecules obtained from petroleum.
Three conditions are used for this reaction.
1. A temperature of about $∘C300\ ^\circ\text{C}$ .
2. A pressure of about $kPa6000\ \text{kPa}$ , which is the same as $atm60\ \text{atm}$ .
3. An acid catalyst, usually phosphoric acid.
Steam adds across the $C=C\text{C=C}$ double bond, so ethene and steam join together to form ethanol as the only product.
$\text{ethene} + \text{steam} \rightarrow \text{ethanol}$
The balanced symbol equation is shown below.
$\text{C}_2\text{H}_4\text{(g)} + \text{H}_2\text{O(g)} \rightarrow \text{C}_2\text{H}_5\text{OH(g)}$
The reaction is reversible, so only a small fraction of the ethene reacts each time the gases pass over the catalyst.
The ethanol is condensed by cooling, and the unreacted ethene and steam are recycled back through the reactor.
The process runs continuously, with fresh ethene and steam fed in while ethanol is removed.

Combustion of ethanol

Ethanol is a flammable liquid that burns readily in air with a pale blue flame.
Combustion is exothermic, so burning ethanol releases a large amount of thermal energy.
In a plentiful supply of oxygen, ethanol undergoes complete combustion to form carbon dioxide and water only.
$\text{ethanol} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$
Each ethanol molecule needs three oxygen molecules for complete combustion.
$\text{C}_2\text{H}_5\text{OH(l)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{CO}_2\text{(g)} + 3\text{H}_2\text{O(l)}$
Ethanol already contains an oxygen atom in its molecule, so it needs less oxygen from the air than a hydrocarbon with the same number of carbon atoms.
Ethanol burns with a clean flame that produces very little soot.
In a limited supply of oxygen, incomplete combustion can also produce toxic carbon monoxide.

Example

Balancing the combustion of ethanol

Start with the unbalanced equation for complete combustion.

\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}

Ethanol has 2 carbon atoms, so 2 carbon dioxide molecules are formed.
Ethanol has 6 hydrogen atoms, so 3 water molecules are formed.
The products now contain $\times 2) + 3 = 7$ oxygen atoms.
Ethanol supplies 1 of these oxygen atoms, so the other 6 come from 3 oxygen molecules.

\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}

Uses of ethanol

Ethanol as a solvent
1. Ethanol dissolves many substances that are insoluble in water, such as oils, resins and many dyes.
2. Ethanol also mixes with water, so it can carry both kinds of substance in one solution.
3. Ethanol evaporates quickly at room temperature, so it leaves a surface dry soon after it is applied.
4. These properties make ethanol a common solvent in perfumes, aftershaves, medicines, inks, varnishes and hand sanitisers.
Ethanol as a fuel
1. Ethanol burns easily and releases a large amount of thermal energy, so it is a useful fuel.
2. Complete combustion gives only carbon dioxide and water, and the clean flame produces little soot.
3. Ethanol is mixed with petrol to fuel cars, and it is used on its own in some vehicles, in spirit burners and in camping stoves.

Common Mistake

Do not give a fermentation temperature above $∘C35\ ^\circ\text{C}$ , because the enzymes in the yeast are denatured at higher temperatures.
Do not leave carbon dioxide out of the products of fermentation, because two molecules form for every glucose molecule.
Do not mix up the raw materials, because fermentation uses glucose while the catalytic process uses ethene and steam.
Do not write "no air" for steam addition, because the absence of oxygen is a condition for fermentation only.

Exam technique

Stating conditions and equations

When asked for the conditions for fermentation, give all three: yeast, a temperature of $∘C25\text{–}35\ ^\circ\text{C}$ , and absence of oxygen or anaerobic.
When asked for the conditions for steam addition, give $∘C300\ ^\circ\text{C}$ , $kPa6000\ \text{kPa}$ or $atm60\ \text{atm}$ , and an acid catalyst, always with units.
Write "aqueous glucose" rather than "sugar" when describing the starting material for fermentation.
Check that the combustion equation has 3 oxygen molecules, 2 carbon dioxide molecules and 3 water molecules before moving on.
For a use of ethanol, name the use and give its reason, such as "a fuel because it releases a lot of energy when it burns".

Self review

What three conditions are needed to make ethanol by fermentation?
Why does fermentation slow down and stop above about $∘C35\ ^\circ\text{C}$ ?
What temperature, pressure and catalyst are used to add steam to ethene?
Write the balanced equation for the complete combustion of ethanol.
Give two uses of ethanol.

Fermentation

Definition

Fermentation

The conversion of aqueous glucose into ethanol and carbon dioxide by enzymes in yeast in the absence of oxygen.

Ethanol, $C2H5OH\text{C}_2\text{H}_5\text{OH}$ , is made on a large scale by two different methods: fermentation of glucose and catalytic addition of steam to ethene.
Fermentation starts with aqueous glucose, which is glucose dissolved in water, usually obtained from sugar-rich crops such as sugar cane or sugar beet.
Yeast is added to the glucose solution, because yeast cells contain enzymes that catalyse the breakdown of glucose.
Three conditions are needed for fermentation.
1. Yeast must be present to supply the enzymes.
2. The temperature is kept between $∘C25\ ^\circ\text{C}$ and $∘C35\ ^\circ\text{C}$ .
3. Oxygen is kept out, so the process is anaerobic.
Below $∘C25\ ^\circ\text{C}$ the enzymes work slowly, so fermentation takes much longer.
Above about $∘C35\ ^\circ\text{C}$ the enzymes start to become denatured, so the rate falls and fermentation stops.
Oxygen must be excluded because, with air present, the yeast respires aerobically to make carbon dioxide and water instead of ethanol, and bacteria can oxidise the ethanol that has formed.
The fermentation vessel is fitted with an airlock, which lets carbon dioxide escape but stops air getting in.
Glucose is broken down into ethanol and carbon dioxide.
$\text{glucose} \rightarrow \text{ethanol} + \text{carbon dioxide}$
The balanced symbol equation shows that one glucose molecule gives two ethanol molecules and two carbon dioxide molecules.
$\text{C}_6\text{H}_{12}\text{O}_6\text{(aq)} \rightarrow 2\text{C}_2\text{H}_5\text{OH(aq)} + 2\text{CO}_2\text{(g)}$
Fermentation is slow and takes several days, with bubbles of carbon dioxide given off the whole time.
Fermentation stops when the glucose is used up or when the ethanol concentration reaches about $15%15\%$ , because ethanol at this concentration kills the yeast.
The product is a dilute aqueous solution of ethanol, so the yeast is removed by filtration and the ethanol is separated from the water by fractional distillation.

Example

Balancing the fermentation equation

Glucose, $C6H12O6\text{C}_6\text{H}_{12}\text{O}_6$ , is the only reactant, and the products are ethanol, $C2H5OH\text{C}_2\text{H}_5\text{OH}$ , and carbon dioxide, $CO2\text{CO}_2$ .

\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow \text{C}_2\text{H}_5\text{OH} + \text{CO}_2

Glucose has 12 hydrogen atoms and each ethanol molecule has 6, so 2 ethanol molecules are needed.
Two ethanol molecules hold 4 carbon atoms, so the remaining 2 carbon atoms go into 2 carbon dioxide molecules.

\text{C}_6\text{H}_{12}\text{O}_6 \rightarrow 2\text{C}_2\text{H}_5\text{OH} + 2\text{CO}_2

Checking oxygen, the left side has 6 atoms and the right side has $2 + 4 = 6$ atoms, so the equation is balanced.

Practical

Fermenting a glucose solution

Aim: to make ethanol by fermenting aqueous glucose with yeast and to identify the gas given off.
Apparatus: $cm3250\ \text{cm}^3$ conical flask, $g10\ \text{g}$ of glucose, $cm3100\ \text{cm}^3$ measuring cylinder, warm water, $g1\ \text{g}$ of dried yeast, bung fitted with a delivery tube, boiling tube of limewater, thermostatically controlled water bath set to $∘C30\ ^\circ\text{C}$ , filter funnel and filter paper, and eye protection.
Method:
- Dissolve $g10\ \text{g}$ of glucose in $cm3100\ \text{cm}^3$ of warm water in the conical flask and swirl until no solid remains.
- Stir in $g1\ \text{g}$ of dried yeast.
- Fit the bung and delivery tube so that the end of the tube dips into limewater, which lets gas out but keeps air away from the mixture.
- Stand the flask in the water bath at $∘C30\ ^\circ\text{C}$ and leave it for several days.
- When bubbling stops, filter the mixture to remove the yeast and keep the filtrate for fractional distillation.
Results:
- Within about half an hour, bubbles rise steadily through the mixture and a froth forms on the surface.
- The limewater turns milky, showing that the gas is carbon dioxide.
- The filtrate smells of alcohol, and bubbling slows and stops after a few days.
Conclusion: yeast ferments glucose into ethanol and carbon dioxide when it is kept warm and away from oxygen.
Safety: wear eye protection, keep the ethanol-containing filtrate away from naked flames because ethanol is flammable, and never taste the product.

Steam addition to ethene

Definition

Catalyst

A substance that increases the rate of a reaction and is chemically unchanged at the end of the reaction.

The second method reacts ethene, $C2H4\text{C}_2\text{H}_4$ , with steam, $H2O\text{H}_2\text{O}$ , to make ethanol.
The ethene comes from cracking large alkane molecules obtained from petroleum.
Three conditions are used for this reaction.
1. A temperature of about $∘C300\ ^\circ\text{C}$ .
2. A pressure of about $kPa6000\ \text{kPa}$ , which is the same as $atm60\ \text{atm}$ .
3. An acid catalyst, usually phosphoric acid.
Steam adds across the $C=C\text{C=C}$ double bond, so ethene and steam join together to form ethanol as the only product.
$\text{ethene} + \text{steam} \rightarrow \text{ethanol}$
The balanced symbol equation is shown below.
$\text{C}_2\text{H}_4\text{(g)} + \text{H}_2\text{O(g)} \rightarrow \text{C}_2\text{H}_5\text{OH(g)}$
The reaction is reversible, so only a small fraction of the ethene reacts each time the gases pass over the catalyst.
The ethanol is condensed by cooling, and the unreacted ethene and steam are recycled back through the reactor.
The process runs continuously, with fresh ethene and steam fed in while ethanol is removed.

Combustion of ethanol

Ethanol is a flammable liquid that burns readily in air with a pale blue flame.
Combustion is exothermic, so burning ethanol releases a large amount of thermal energy.
In a plentiful supply of oxygen, ethanol undergoes complete combustion to form carbon dioxide and water only.
$\text{ethanol} + \text{oxygen} \rightarrow \text{carbon dioxide} + \text{water}$
Each ethanol molecule needs three oxygen molecules for complete combustion.
$\text{C}_2\text{H}_5\text{OH(l)} + 3\text{O}_2\text{(g)} \rightarrow 2\text{CO}_2\text{(g)} + 3\text{H}_2\text{O(l)}$
Ethanol already contains an oxygen atom in its molecule, so it needs less oxygen from the air than a hydrocarbon with the same number of carbon atoms.
Ethanol burns with a clean flame that produces very little soot.
In a limited supply of oxygen, incomplete combustion can also produce toxic carbon monoxide.

Example

Balancing the combustion of ethanol

Start with the unbalanced equation for complete combustion.

\text{C}_2\text{H}_5\text{OH} + \text{O}_2 \rightarrow \text{CO}_2 + \text{H}_2\text{O}

Ethanol has 2 carbon atoms, so 2 carbon dioxide molecules are formed.
Ethanol has 6 hydrogen atoms, so 3 water molecules are formed.
The products now contain $\times 2) + 3 = 7$ oxygen atoms.
Ethanol supplies 1 of these oxygen atoms, so the other 6 come from 3 oxygen molecules.

\text{C}_2\text{H}_5\text{OH} + 3\text{O}_2 \rightarrow 2\text{CO}_2 + 3\text{H}_2\text{O}

Uses of ethanol

Ethanol as a solvent
1. Ethanol dissolves many substances that are insoluble in water, such as oils, resins and many dyes.
2. Ethanol also mixes with water, so it can carry both kinds of substance in one solution.
3. Ethanol evaporates quickly at room temperature, so it leaves a surface dry soon after it is applied.
4. These properties make ethanol a common solvent in perfumes, aftershaves, medicines, inks, varnishes and hand sanitisers.
Ethanol as a fuel
1. Ethanol burns easily and releases a large amount of thermal energy, so it is a useful fuel.
2. Complete combustion gives only carbon dioxide and water, and the clean flame produces little soot.
3. Ethanol is mixed with petrol to fuel cars, and it is used on its own in some vehicles, in spirit burners and in camping stoves.

Common Mistake

Do not give a fermentation temperature above $∘C35\ ^\circ\text{C}$ , because the enzymes in the yeast are denatured at higher temperatures.
Do not leave carbon dioxide out of the products of fermentation, because two molecules form for every glucose molecule.
Do not mix up the raw materials, because fermentation uses glucose while the catalytic process uses ethene and steam.
Do not write "no air" for steam addition, because the absence of oxygen is a condition for fermentation only.

Exam technique

Stating conditions and equations

When asked for the conditions for fermentation, give all three: yeast, a temperature of $∘C25\text{–}35\ ^\circ\text{C}$ , and absence of oxygen or anaerobic.
When asked for the conditions for steam addition, give $∘C300\ ^\circ\text{C}$ , $kPa6000\ \text{kPa}$ or $atm60\ \text{atm}$ , and an acid catalyst, always with units.
Write "aqueous glucose" rather than "sugar" when describing the starting material for fermentation.
Check that the combustion equation has 3 oxygen molecules, 2 carbon dioxide molecules and 3 water molecules before moving on.
For a use of ethanol, name the use and give its reason, such as "a fuel because it releases a lot of energy when it burns".

Self review

What three conditions are needed to make ethanol by fermentation?
Why does fermentation slow down and stop above about $∘C35\ ^\circ\text{C}$ ?
What temperature, pressure and catalyst are used to add steam to ethene?
Write the balanced equation for the complete combustion of ethanol.
Give two uses of ethanol.

11.6.1 Ethanol: manufacture, combustion and uses

Fermentation

Balancing the fermentation equation

Fermenting a glucose solution

Steam addition to ethene

Combustion of ethanol

Balancing the combustion of ethanol

Uses of ethanol

Stating conditions and equations

Review 11.6.1 Ethanol: manufacture, combustion and uses by teaching Genie

11.6.1 Ethanol: manufacture, combustion and uses

Fermentation

Balancing the fermentation equation

Fermenting a glucose solution

Steam addition to ethene

Combustion of ethanol

Balancing the combustion of ethanol

Uses of ethanol

Stating conditions and equations

Review 11.6.1 Ethanol: manufacture, combustion and uses by teaching Genie

11.6.1 Ethanol: manufacture, combustion and uses

Fermentation

Balancing the fermentation equation

Fermenting a glucose solution

Steam addition to ethene

Combustion of ethanol

Balancing the combustion of ethanol

Uses of ethanol

Stating conditions and equations

Review 11.6.1 Ethanol: manufacture, combustion and uses by teaching Genie

11.6.1 Ethanol: manufacture, combustion and uses Revision Guide

11.6.1 Ethanol: manufacture, combustion and uses

Fermentation

Balancing the fermentation equation

Fermenting a glucose solution

Steam addition to ethene

Combustion of ethanol

Balancing the combustion of ethanol

Uses of ethanol

Stating conditions and equations

Review 11.6.1 Ethanol: manufacture, combustion and uses by teaching Genie

11.6.1 Ethanol: manufacture, combustion and uses Revision Guide