9.2.C - Ethanol


Humans discovered ethanol not long after they figured out how to put fire to good use. Today, ethanol has many uses: we drink it; disolve solutes in it, use it in food, industry and other manufacturing; and blend it with petrol to make a sustainable and renewable transport fuel.

Ethanol belongs to the homologous series of alkanols, each with a hydroxyl (-OH) functional group. It can be made by fermenting the sugars found in plants using yeasts (funghi) and bacteria. Ethanol can be used as a fuel for vehicles in its pure form as a replacement for petrol, but it is usually blended with petrol so as to improve vehicle emissions and make motor fuel more sustainable.

In this topic students:

  • Describe the properties of ethanol and relate them to its use,
  • Describe and write equations for the dehydration, fermentation and combustion of ethanol, and
  • Assess the potential of ethanol as an alternative to petrol.


Properties

The molecular structure of ethanol.

Ethanol belongs to the homologuos series of carbon-based compounds called alkanols. The alkanols also bear the more common name of alcohols. Alkanols have a hydroxyl (-OH) group somewhere on their carbon chain which is their functional group and identifies them as a member of the homologous series of alkanols.

Ethanol has two carbon atoms in its carbon chain and has the chemical formula C2H6O or to make it easier to recognise, C2H5OH.

The presence of the hydroxyl group creates an uneven distribution of electrons in the molecule due to the high electronegativity of the the oxygen atom and this makes the molecule polar. The hydroxyl group is also responsible for the hydrogen bonds that form between adjacent molecules. These relatively strong intermolecular forces give ethanol its reasonably high melting point (-114°C) and boiling point (78°C) and make it a liquid at room temperature.

 

Ethanol as a Solvent

The polarity of the ethanol molecule which makes it
siuch a widely used solvent.

If you look carefully at the molecular structure of ethanol, you can infer one of two things about its solubility: either it will be soluble in a lot of things or it won't be soluble in much of anything. You shoudl recall at this point that polar solutes generally dissolve in polar solvents and non-polar solutes dissolve in non-polar solvents - like dissolves like. At one end of the ethanol molecule is a polar hydroxyl group (-OH) which should dissolve well in water because it is also polar. At the other end is a typical hydrocarbon chain (CH2-CH3-) which is non-polar and would make it miscible with other non-polar hydrocarbons such as hexane. One possibility is that these two factors will cancel each other out, making ethanol insoluble in everything, but that's not the case. Ethanol is in fact very soluble in both hexane and water. If the hydrocarbon chain is lengthened to five carbon units (pentanol) it will become insoluble in water If it is shortened to one (methanol), it becomes much less soluble in hydrocarbons.

Ethanol has many industrial uses as a solvent, due to its relatively high affinity both for water and a great range of non-polar organic compounds. Products largely based on ethanol include some solvent-based paints, lacquers, inks, household cleaning products, external pharmaceuticals (rubbing alcohol) and perfumes.

Since the hydrocarbon chain in ethanol is relatively short, the polar nature of the -OH group is still "visible" to other molecules. This makes the ethanol molecule polar overall, allowing its solubility in water.

 

Reactions

Ethanol can be produced by two different methods. The first is by the hydrolysis of ethene which comes from petrochemicals and is non-renewable. The second process involves the fermentation of sugars found in plants and is a signfiicantly more renewable source of ethene.

Hydration of Ethene

Industrial ethanol is usually produced by the acid-catalysed addition of water to ethene. It is usually carried out in the presence of a dilute phosphoric or sulfuric acid as a catalyst and at temperatures of about 300°C. This is a type of addition reaction and it is the reverse of the dehdration of ethanol (see below).

C2H4 (g) + H2O (g) → C2H5OH (g)


Fermentation of Sugars

Glucose can be converted to ethanol and carbon dioxide in a fermentation reaction. Microraganisms such as yeast and bacteria produce enzymes which catalyse the reaction. This process is exothermic and is most efficient when the temperature is 35°C and the environment is moist and humid. While yeast can function aerobically and anaerobically, fermentation is usually carried out on an industrial scale with oxygen excluded. The equation can be written as:

C6H12O6 (g) → 2C2H5OH (aq) + 2CO2 (g)


Ethanol is made largely from corn and from sugar cane, although it is also produced from many other sources, including waste matter from cheese production. While the reaction above is the most common example of fermentation, there are many other fermentation reactions, all of which allow micro-organisms to obtain energy from chemicals in their environment when oxygen is unavailable. Among the products of the biological reactions we now call fermentations (all of which produce carbon dioxide) are acetone, butanol, glycerol, and hydrogen gas. The physical process in which ethanol is produced from corn starch is outlined below. You should recall from the previous section of work on biological polymers that ethanol can also be produced from cellulose.

  1. Grind the corn up and mix it with water and with alpha-amylase, the enzyme that breaks down the bonds between glucose molecules in starch to generate maltose. This reaction is carried out at high temperatures to discourage bacteria from settling down for a good feed themselves.

  2. Glucoamylase is then added - this enzyme can attack all kinds of glucose-glucose bonds, but is most effective in splitting the maltose dimer to give glucose.

  3. Yeast is added to the mash to convert glucose to carbon dioxide and ethanol, and it passes through a series of fermenters from which air is excluded at about 35°C. This gives a mixture containing about 10% alcohol after about 24 hours.

  4. Finally this mash is distilled to give 96% alcohol, dried using molecular sieves to remove the excess water, and sent out to for use mixed with a small amount of unpleasant smelling and tasting organic chemical to make it unfit for human consumption.

Before biomass (or cellulose) can be used to ferment glucose, it must first be treated to break up the cellulose polymer into its glucose polymer units.

Click here to see more on the biochemistry of the fermentation process.

Dehydration of Ethanol

Before ethene became readily available through the catalytic cracking of petroleum fractions, it was produced from ethanol. Industrially, it can be produced by heating ethanol vapour over an aluminium oxide catalyst at 350°C. In the laboratory a concentrated sulfuric acid catalyst can be used. A molecule of water is liberated in this process which is why it is called a dehydration reaction. The equation can be written as:

C2H5OH (g) → C2H4 (g) + H2O (g)


Combustion of Ethanol

Ethanol undergoes combustion in the presence of oxygen to produce carbon dioxide and water. This equation opens up the way for the use of ethanol as a fuel.

C2H5OH (l)  +  3O2 (g) → 2CO2 (g)  +  3H2O (l)  ΔH = -1367 kJ mol-1  


At high temperatures in the presence of oxygen, all organic compounds will decompose to give carbon dioxide and water . This process is called combustion, and is usually accompanied by cheery flames and loud noises that many of us find rather pleasing! Ethanol can burn nicely, is as safe to handle and transport as petrol and can be used in internal combustion engines without requiring extreme redesign. As far back as the beginning of last century, the Model T Ford was designed to operate on ethanol.

The combustion of ethanol generates 1367 kilojoules of heat per mole of ethanol. This means that ethanol has the potential to be a quite useful fuel. One of the other things about using ethanol is that the presence of oxygen within the fuel itself (i.e. the O in the OH group) leads to cleaner combustion and less soot formation. This is a particular advantage as we will see later. Furthermore, if ethanol is produced by fermentation of sugars, it has considerable potential as a fuel because it becomes a renewable resource.

We have already seen that burning one mole of ethanol releases 1367 kilojoules of heat energy. This value - the amount of heat generated by the complete combustion of one mole of a substance - is called the molar heat of combustion. The molar heat of combustion is the amount of heat released by the complete combustion (reaction with oxygen gas to form water and carbon dioxide) of one mole of a substance. The molar heat of combustion is usually expressed in kJ mol-1.

Measuring the Heat of Combustion of a Fuel

Measuring the heat of combustion of a fuel using apparatus found in a typical
school laboratory.

Calorimetry is the area of chemistry concerned with the measurement of the energy produced or absorbed in chemical reactions. Where combustion is concerned, calorimetric processes measure the amount of heat produced when one mole of a fuel is burned.

In the school laboratory, the combustion reaction takes place in the wick of the spirit burner and this is the source of heat energy. We assume that all of the energy produced by the burner is absorbed by some water in a copper can just above the burner. The energy abosrbed by the water can be calculated using the equation below if the water's mass and change in temperature are measured.

ΔH = mcΔT

Where:
ΔH = energy absorbed by the water in joules (J)
m = mass of the water in kilograms (kg)
c = 4.18 which is the specific heat capacity of water in joules per kilogram per kelvin (J kg-1 K-1)
ΔT = change in temperature of the water in kelvin (K) - note that 1 K = 1°C.

There is a negative sign in the formula in the syllabus which can be confusing. I suggest you ignore it until you have finished you calculations and then put it in front of your calculated heat of combustion to show that it is an exothermic reaction.

Now we know how much energy was absorbed by the water and we presume that to be the same as the energy produced by the burner. If we measure the change in mass of the burner, that will tell us how much fuel was burnt to produce the calculated amount of energy and can deduce an answer in joules or kilojoules per gram. Converting grams to moles we can then work how much energy is produced per mole of fuel and this is the heat of combustion for the reaction.

Typically this experiment with this apparatus will produce a result which is about half of the result you would find in the literature for each fuel. There are several reasons for this and we call these collectively compromises to validity. They are outlined below.

  1. There is considerable energy loss from the burner to the environment so only a fraction of the energy produced by the burner is absorbed by the water. This is the most significant compromise to validity and accounts for most of the heat lost. The experiment can be improved in this regard by placing a reflective metal shield around the burner to funnel more heat up towards the can. This can also promote incomplete combustion so a source of excess oxygfen would also be necessary.

  2. There is also some heat loss from the can to the surroundings. This would also contribute to a measured value for the heat of combustion that was lower than the literature value. This can be mitigated against by insulating the can and using a lid.

  3. If the flame from the burner is touching the base of the tin, black soot appears on the tin which indicates that some incomplete combustion is occurring. Incomplete combustion produces less energy per mole of fuel than complete combustion and this would also contribute to a lower than expected value for the heat of combustion. Ensuring there is a small gap between the top of the flame and the can as well as a source of excess oxygen can help to remedy this problem.

A bomb calorimeter used to accurately measure heat of combustion.

When measuring the literature values, a bomb calorimeter is used in professional laboratories which is designed so that all of the heat produced by the combustion reaction is absorbed by the water. The combustion chamber is called a bomb and is usually made of steel. The bomb is completely surrounded by water so that no heat can escape to the environment. The combustion reaction starts using electricity provided by a battery and an gas inlet provides the excess oxygen needed for complete combustion.

These are not readily available in schools so be careful about suggesting this as an improvement to the experiment when the question asked for practical improvements that are available in school laboratories.

 

Potential as a Fuel

Ethanol was first used as a fuel in the late 1800s, but as fuel obtained from petroleum became cheaper, its use declined. The growing realisation that petroleum reserves are limited has forced a rethink. Ethanol now competes with hydrogen as the fuel of the future. Another factor is the enhanced greenhouse effect, or global warming. The continued use of fossil fuels poses a serious threat to the world's environment. As ethanol is a renewable resource, as much carbon dioxide is taken from the atmosphere by plants as is returned when the ethanol is burnt. Alcohol can be used as the fuel to power cars. In Brazil, alcohol fuel that is 95% ethanol and 5% water is used in some areas. The motor vehicle engines have to be modified so the water content does not cause corrosion. In other parts of Brazil the fuel is 78% petrol and 22% ethanol. In the United States and Australia, 'gasohol' or E10 blends are about 90% petrol and 10% alcohol.



Video from drivingethanol.org on how ethanol is made from corn starch.

 

Advantages

  • Energy Dense
    Ethanol is 'energy-dense', releasing 24 MJ L-1 (1368 kJ mol-1) of heat energy when burnt. Pure octane releases 38 MJ L-1 (5471 kJ mol L-1) while the mixture of hydrocarbons in petrol have a slightly lower energy density of 34 MJ L-1. Ethanol does have a lower energy density than octane but it is still quite high compared to other viable fuels.

  • Renewable
    The plant material needed to produce ethanol is a renewable resource. This is a two-edged sword. On the positive side, if cost-efficient ways could be found to convert cellulose from trees and crop residues then it may be possible to supply most of our fuel needs from ethanol.

  • Carbon Neutral
    Plants take carbon dioxide from the atmosphere to make sugars. We then make alcohol from these sugars. When this alcohol is burnt the carbon dioxide is returned to the atmosphere. This is not true for petrol or diesel, which add carbon dioxide to the atmosphere.

  • Cleaner Burning
    When ethanol is added to petrol, or used on its own, the amount of polluting gases such as carbon monoxide (CO) and hydrocarbons is reduced. Ethanol needs less oxygen per mole of fuel than octane so it is more likely to combust completely in a car engine.

 



Ethanol acting as a carbon neutral fuel. Of the six carbon atoms absorbed during photosynthesis, two are released
back into the atmosphere during fermentation and another four during combustion. This cycle could take place within
one year and it is this short time frame that makes ethanol carbon neutral. Petrol comes from fossil fuels which have
taken millions of years to form so the carbon put into the atmopshere during combustion does not become usable fuel
again for a very long time. This is why fossil fuels are not carbon neutral when combusted.

 

Disadvantages

  • The energy needed to produce the ethanol is substantial, particularly the distillation needed to separate alcohol from water. Until recent years it took more energy to make the alcohol than was able to be released during combustion. Improved technology, and the use of waste plant fibres to provide heat has seen the figures improve - alcohol can now release 20% more energy than is needed to make it.

  • To produce enough ethanol from crops such as corn, sugar cane or sugar beet would require an area of farmland twice that of the arable land being used for crops worldwide today. Thus ethanol derived from crops such as these will not become the major source of fuel in the future.

  • Motor vehicle engines need to be modified to operate on ethanol, especially if there is any water present. As a properly tuned engine gets only about 80% of the fuel consumption of a petrol engined car, you only get 80% of the range or you need a larger fuel tank.

  • Ethanol is more expensive than petrol.

Is it a Viable Replacement for Petrol and Other Fuels?

Given the issues of land available to grow crops to produce ethanol, the engine modification requirements and the cost compared to petrol, ethanol is currently not a viable replacement for petrol or other fuels. It is, however, a good addition to petrol to enhance its renewability and cleaner combustion. Future research could enhance current technology and improve the efficiency of sustainable distillation processes. Genetic engineering of bacteria could increase the concentration of ethanol produced in fermentation to higher than 15%. Developing mechanisms for the decomposition of cellulose to produce glucose could also help to reduce its cost. Further research and development on the production of ethanol from renewable resources could enhance its suitability as a replacement for petrol and other fuels in the future.



Scientist David Fridley explains the inherent cost and production problems with ethanol and similar biofuels.