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9.2 Production of materials: 3. Renewable ethanol

Syllabus reference (October 2002 version)
3. Other resources, such as ethanol, are readily available from renewable resources such as plants	Students learn to: describe the dehydration of ethanol to ethylene and identify the need for a catalyst in this process and the catalyst used   describe the addition of water to ethylene resulting in the production of ethanol and identify the need for a catalyst in this process and the catalyst used   describe and account for the many uses of ethanol as a solvent for polar and non-polar substances    outline the use of ethanol as a fuel and explain why it can be called a renewable resource   describe conditions under which fermentation of sugars is promoted   summarise the chemistry of the fermentation process   define the molar heat of combustion of a compound and calculate the value for ethanol from first-hand data    assess the potential of ethanol as an alternative fuel and discuss the advantages and disadvantages of its use identify the IUPAC nomenclature for straight-chained alkanols from C1 to C8	Students: process information from secondary sources such as molecular model kits, digital technologies or computer simulations to model: the addition of water to ethylene the dehydration of ethanol   process information from secondary sources to summarise the processes involved in the industrial production of ethanol from sugar cane   process information from secondary sources to summarise the use of ethanol as an alternative car fuel, evaluating the success of current usage   solve problems, plan and perform a first-hand investigation to carry out the fermentation of glucose and monitor mass changes   present information from secondary sources by writing a balanced equation for the fermentation of glucose to ethanol   identify data sources, choose resources and perform a first-hand investigation to determine and compare heats of combustion of at least three liquid alkanols per gram and per mole

Extract from Chemistry Stage 6 Syllabus (Amended October 2002). © Board of Studies, NSW.
[Edit: 7Jul09]

Prior learning: Preliminary modules 8.4.3, 8.5.4

Background: Much of our energy comes from chemicals formed millions of years ago, e.g. coal and oil, which are termed fossil fuels. Research into alternative energy sources continues, with interest in solar energy, tidal energy and so forth. Chemists too are involved in this search and look to the chemicals in the biomass for renewable sources of energy. As fossil fuel supplies are depleted, ethanol, obtained from renewable plant resources, will be of increasing importance.

process information from secondary sources, such as molecular model kits, digital technologies or computer simulations to model

• the addition of water to ethylene
• the dehydration of ethanol

• Process information from a variety of sources to reliably describe changing ethylene to ethanol and ethanol to ethylene. Use a molecular model kit (or if you don't have one, use toothpicks for bonds and pieces of marshmallow or bits of jelly snake for atoms) to model the two reactions. You can do this by constructing “ethanol” from “ethylene” and “water” molecules, then reconstructing “ethylene” and “water” molecules from the “ethanol”.

describe the dehydration of ethanol to ethylene and identify the need for a catalyst in this process and the catalyst used

describe the addition of water to ethylene resulting in the production of ethanol and identify the need for a catalyst in this process and the catalyst used

The following information addresses the above two syllabus points together.

• ethylene and ethanol are easily interchanged by addition of water (hydration) and removal of water (dehydration). Catalysts such as sulfuric acid, phosphoric acid or heated ceramic solids can be used to catalyse these dehydration and hydration reactions.

   

     

  Additional background information

  Countries rich in petroleum or natural gas, e.g. around the Persian Gulf, or petroleum refining and cracking facilities, e.g. Singapore, can make ethanol by hydration of ethylene.

  Countries rich in land and climate suitable for growing crops that could be used to produce ethanol, e.g. Brazil, can make ethylene by dehydration of ethanol.

Dehydration of ethanol and Hydration of ethylene to ethanol ,
Key Centre for Polymer Colloids, University of Sydney, Australia

describe and account for the many uses of ethanol as a solvent for polar and non-polar substances

• Ethanol is used as a solvent in dissolving medicines and food flavourings and colourings that do not dissolve easily in water. Once the non-polar material is dissolved in the ethanol, water can be added to prepare a solution that is mostly water.
   
• The ethanol molecule has a water loving (hydrophilic) -OH group that helps it dissolve polar molecules and ionic substances. This occurs through hydrogen bonding, dipole-dipole attraction or ion-dipole attraction. The short, water fearing (hydrophobic) hydrocarbon chain CH₃CH₂- can attract non-polar molecules. The non-polar component of an ethanol molecule bonds to non-polar molecules through dispersion forces. Thus ethanol can dissolve both polar and non-polar substances.
   
• Industrially and in consumer products, ethanol is the second most important solvent after water. Ethanol is the least toxic of all the alcohols as it is poisonous in moderate amounts rather than small amounts. Consumer products listed as containing alcohol practically always contain ethanol as the alcohol.

Ethanol as a solvent Key Centre for Polymer Colloids, University of Sydney, Australia

process information from secondary sources to summarise the processes involved in the industrial production of ethanol from sugar cane

• Identify reliable information by considering it from various sources. Continue processing the information by creating summary notes of the industrial production processes used to produce ethanol from sugar cane.

  The general process:

  1. Sugar cane fermentation  an ethanol and water mixture
  2. Distillation of the ethanol and water mixture separates ethanol from the water.

  The future . . . 

  Distillation of the aqueous ethanol product (96% ethanol and 4% water) to obtain almost pure ethanol can take half as much energy as that released when the ethanol is burnt. Distillation is being replaced by low energy methods such as passing the aqueous ethanol through special zeolite filters that act as molecular sieves. The more polar water molecules are strongly attracted to polar parts of the zeolite while the less polar ethanol passes through thus separating pure ethanol. 

process information from secondary sources to summarise the use of ethanol as an alternative car fuel, evaluating the success of current usage

• Identify reliable information by considering it from various sources. Continue processing the information by creating summary notes of how well ethanol functions as an alternative car fuel.
• An evaluation is a judgement based on criteria. In your evaluation, list the criteria you used. Consider the following issues.
  • The energy and financial cost of separating ethanol from the aqueous ethanol produced in fermentation.
  • What percentage of ethanol can be added to petrol without engine modification?  In Australia there is concern about the sale of 20%ethanol-80%petrol mixtures as petrol.  The federal government is planning to restrict petrol to contain no more than 10% ethanol.
  • Requirements of engines using ethanol only.
  • The effect on exhaust pollutants of using ethanol fuel.
  • The economic cost at different prices for oil, costs and subsidies, and different rates of taxation levied by governments, on different fuels.

solve problems, plan and perform a first-hand investigation to carry out the fermentation of glucose and monitor mass changes

This activity could be an open-ended investigation.

• The analysis of mass changes provides a means to investigate phenomena. However, there are problems to solve when a process involves unintended mass losses due to the production of gases. The fermentation of glucose in water releases gaseous carbon dioxide as a product. Fermentation occurs best at about 30^oC. Above this elevated temperature, significant evaporation of water, and hence more mass loss occurs.
   
• Plan your investigation carefully. Mass losses due to release of carbon dioxide and evaporation of water can be allowed for in planning and performing this first-hand investigation. Consider controls you could put in place to account for these mass losses.
   
• Here is one approach that could be used to solve some of the problems. Carry out the fermentation in a gas tight container with a pipe or bendy straw leading into a beaker of limewater (saturated Ca(OH)₂ solution).

  Carbon dioxide released will react:

  

  The carbon dioxide will be trapped in the limewater. Water passing out of the fermentation container will also be collected in the limewater beaker.

• When performing the investigation, compare the loss in weight of the fermentation container with gain in weight of the limewater beaker. If they differ in magnitude you have another practical problem to solve to improve your investigation. A good scientist would realise that all carbon dioxide passing out of the fermentation container should be trapped in the limewater. If this does not happen for your equipment then attempt to improve your design.

present information from secondary sources by writing a balanced equation for the fermentation of glucose to ethanol

• Present information in the most appropriate way. Insert coefficients in front of the formulas for the two products so that the equation is balanced. A balanced equation is one in which there is the same number of each kind of atom on both sides of the equation.

  

describe conditions under which fermentation of sugars is promoted
 

• The conditions that promote the fermentation of sugar are:
  • a suitable micro-organism such as yeast
  • water
  • a suitable temperature for the fermenting yeast
  • low oxygen concentrations favouring the fermenting yeast
  • a small amount of yeast nutrients such as phosphate salt.
• Once the ethanol concentration reaches 14-15% by volume, the yeast cannot survive, and the fermentation process stops.

Conditions for fermentation Key Centre for Polymer Colloids, University of Sydney, Australia

summarise the chemistry of the fermentation process

• Cane sugar waste, such as molasses, is rich in sucrose (C₁₂H₂₂O₁₁), however, it is uneconomic to separate.
   
• If water and yeast is added, the sucrose reacts with water producing glucose and fructose, both of which have the molecular formula C₆H₁₂O₆.

   

• Fermentation can then occur:

  

outline the use of ethanol as a fuel and explain why it can be called a renewable resource

• Ethanol combusts in air, releasing carbon dioxide, water and heat. Because the ethanol molecule contains an O atom, the combustion is practically always complete. There is hardly any formation of the polluting CO or C forms, which form from the incomplete combustion of many other hydrocarbons.

• Ethanol can be called a renewable resource because ethanol can be made from plant material and the products of its combustion, carbon dioxide and water, are the reactants needed by plants for photosynthesis.

The use of ethanol as a fuel Key Centre for Polymer Colloids, University of Sydney, Australia

identify the IUPAC nomenclature for straight-chained alkanols from C1 to C8

IUPAC nomenclature for alkanols refers to the International Union of Pure and Applied Chemists' (IUPAC) way of naming alkanols.
You are only required to deal with straight chained alkanols with up to, and including, 8 carbon atoms.

For straight chained alkanols (those without side branches) the number of carbon atoms in the chain is given by the prefix as follows:

The presence of the -OH, substituting for an H, on one of the carbons is indicated by the suffix 'ol'.
The middle syllable 'an' indicates the fact that the carbon atoms are saturated (There are no double or triple bonds) therefore without the -OH functional group it would be an alkane.

A number is used to indicate which carbon has the -OH attached to it. Of course you can usually get two numbers for such a carbon, depending on which end of the chain you start from. Simply use the smallest number you can. (No number is needed for methanol or ethanol as the -OH can only be on an end carbon, when there is only one or two carbons in the molecule.)

Numbers and letters in IUPAC nomenclature are linked with a hyphen.

Exercises

1. CH₃OH  has one carbon. It is called methanol.

2. CH₃CH₂OH has ...... carbons. It is called   .......anol.

3. CH₃CH₃CH₂OH has .......................  It is called .....................

4. CH₃CH₂CHOHCH₃   has 4 carbons so it is a butanol, but as the -OH is on the second carbon from the right (or the 3rd from the left) it is correctly called 2-butanol.

5. CH₃CH₂CH₂CH₂CHOHCH ₃   has ........ carbons with the -OH on the ........ one from the nearest end (not the .......... five from the furthest end). It is called ............................

6. CH₃CH₂CH₂CH₂CH ₂CH ₂CHOHCH₃  has ........ carbons with the -OH on the ........ one from the nearest end (not the .......... seven from the furthest end). It is called ............................

7.   CH₃CH₂CHOHCH₂CH₂CH ₃   has ........ carbons with the -OH on the ........ two from the nearest end (not the .......... three from the furthest end). It is called ............................

[Answers: 2. ethanol; 3. propanol (more correctly it is 1-propanol); 5. 2-hexanol; 6. 2-octanol; 7. 3-hexanol]

identify data sources, choose resources and perform a first-hand investigation to determine and compare heats of combustion of at least three liquid alkanols per gram and per mole 
 

• Consider the type of data that must be collected. You will need to use the mathematical relationship, H = –mCT. Consider using a table or graph to compare results. Use ethanol as one of the alkanols so you can use your data in the next part.
   
• This is a quantitative investigation so you will need to either measure liquid masses or, alternatively, calculate masses from measurements of volumes and a knowledge of densities of the liquids.
   
• When choosing equipment and resources, carry out a risk assessment of your intended experimental procedures to identify and address potential hazards. Be aware that alkanols burn with a less luminous flame than hydrocarbons. This flame is much more difficult to see. You must ensure there is no alkanol flame when you add alkanol to a burner.
   
• Perform the investigation carefully to ensure your safety.

Using calorimetry and Calculating Molar Heat of Combustion
Key Centre for Polymer Colloids, University of Sydney, Australia

define the molar heat of combustion of a compound and calculate the value for ethanol from first-hand data

• The molar heat of combustion is the heat change when one mole of the substance is combusted to form products in their standard states (that is, solid, liquid or gas) at 10⁵ Pa (100 kPa) and 25^oC (298K).
   
• To calculate the value for ethanol from first hand data:
  1. Write a balanced equation for the complete combustion of ethanol including states of matter required for the molar heat of combustion definition.
  2. Use your measurement of heat released per gram for ethanol as your first-hand data to calculate the molar heat of combustion for ethanol.
  3. Compare your calculated value with a published value from a text or data book. Explain any difference in measured value and published value by considering how you carried out the measurement.

  An example . . . 

  A student measures the temperature rise of a measured mass of water heated by a burner containing ethanol. Weighing of the burner before and after use, gives a difference equal to the mass of ethanol that burned.

  The student does a calculation (H = –mCT) to find the amount of heat released. From these two measurements, the heat released per gram and then per mole is calculated.

Molar Heat of Combustion Key Centre for Polymer Colloids, University of Sydney, Australia

assess the potential of ethanol as an alternative fuel and discuss the advantages and disadvantages of its use

An assessment is a judgement of value, quality, outcomes, results or size. Make sure you include advantages and disadvantages in your assessment of the potential of ethanol as an alternative fuel.

• Ethanol can be used in internal combustion engines if it can be economically produced from renewable resources or subsidised as a fuel to reduce air pollution.
   
• The advantages of using ethanol include its complete combustion with minimal pollution. It can also be made in a number of ways. See the diagram below.

  

• The disadvantages of using ethanol include the need to modify fuel lines and even the engine if the ethanol is more than 10-15% when mixed with petrol. Another disadvantage is the low price of still readily available petroleum. Large tracts of land would need to be allocated to growing plants to use in the production of ethanol rather than food.
   

Ethanol as a fuel: Advantages and disadvantages Key Centre for Polymer Colloids, University of Sydney, Australia