Chemistry Biofuels: The carbon equation

[Pages:16]Chemistry

Biofuels: The carbon equation

This chemistry lesson is all about biofuels, as a sustainable alternative to fossil fuels, and the chemical reactions involved in making them. In this lesson you will investigate the following: ? Are biofuels really that different from fossil fuels? ? What makes ethanol an alcohol? ? How does the carbon cycle illustrate the law of conservation of matter? ? What makes yeast ferment? This lesson will sustain your interest and renew your enthusiasm for chemistry.

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Introduction: Biofuels (P1)

Making ethanol from plants to use as fuel for cars and trucks sounds like it should be an eco-friendly thing to do, but so far it's been disappointing. One reason is you need to grow a lot of crops to make the fuel ? crops that could otherwise be used for food. This type of fuel is called bioethanol, and it produces too little energy to justify the cost of producing it ? dirty old fossil fuels are a lot cheaper. But scientists have just worked out how they might be able to convert plants to ethanol a lot more efficiently, and that could tip the scales to make it a viable alternative to petrol. Making bioethanol is all about chemistry. Plants contain cellulose, which can be broken down into sugars. Those sugars can then be fermented and turned into ethanol ? a type of alcohol. But the biggest problem is getting to the cellulose in the first place, as it is cemented together by a stringy molecule called lignin that must be broken down first. Up until now, ethanol producers have tried using plants with cellulose that is easier to get to, but a new group of researchers is now looking at the problem in a different way. They have modified the genes of a plant to produce a form of lignin that breaks down more easily than the natural version, making the cellulose more accessible, which could produce more energy more efficiently. Their technique is still a work in progress, but the early tests are promising.

Read or listen to the full Cosmos magazine article here.

Credits: iStock.

Question 1

List: Think about your typical weekday morning routine, from when you get up to arriving at school. Identify as many activities as you can that require you to use a fuel, either directly or indirectly.

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Gather: Biofuels (P1)

Credits: iStock & Chappatte in "International Herald Tribune".

Question 1

Match: Draw lines to match the following terms from the article with their definition.

Fuels

The two short media clips below provide useful information about biofuels and fossil fuels. The ideas presented in the media clips will help you compare and contrast these two types of energy resources.

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Credit: Biofuels - the Green alternative / YouTube.

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Credit: Formation of Fossil Fuels / YouTube.

Question 2

Identify: Use the information in the media clips to help you decide whether the statements in the table below apply either to biofuels or fossil fuels, or both. Indicate your choices by typing "yes" or "no" into the relevant columns.

Statement Renewable Non-renewable Formed over millions of years Formed from currently living plant material or animal wastes Needs to be refined or processed Natural gas, oil, coal Ethanol, biodiesel, biogas Supplies energy for more than 90% of the world's energy needs Helps reduce greenhouse gas emissions Requires more sophisticated technology Technology already very well established around the world Undergoes combustion to release carbon dioxide, water and heat energy Production can have damaging environmental consequences

Applies to fossil fuels

Applies to biofuels

Alcohols

Ethanol is a member of a large family of chemical compounds called alcohols. It is a simple molecule containing only nine atoms. It has many everyday uses ranging from solvents, cleaning products, fuel for camping stoves, industrial applications and even medical uses. The International Union of Pure and Applied Chemists (IUPAC) prescribes the following rules for naming alcohols:

The number of carbon atoms in the chain is described by a special prefix, as shown in the following table:

Number of carbon

1

2

atoms

Prefix

meth

eth

3

4

prop

but

5

6

pent

hex

7

8

hept

oct

The presence of the ?OH group, substituting for an H atom on one of the carbons, is indicated by the suffix 'ol' and makes the molecule a member of the alcohol family.

The middle syllable 'an' indicates the fact that the carbon atoms are `saturated' so that all of the carbon atoms are bonded to each other by a single covalent chemical bond.

A number is used to indicate which carbon has the ?OH group attached to it, when there is a choice (start counting from the end of the molecule which results in the smallest number).

Question 3

Name: Complete the gaps to correctly name the following alcohol molecules.

Question 4

Solve: Assume that using conventional petrol as a fuel in a motor vehicle results in 90 units of CO2 emissions for a given amount of use. The use of E10 fuel in the same vehicle and the same amount of use results in only 71 units of CO2 emissions (E10 is formulated with 90% conventional petrol and 10% ethanol).

Calculate the percentage reduction in CO2 emissions achieved by using the E10 fuel over conventional petrol for the vehicle. Show your working.

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Process: Biofuels (P1)

Credits: iStock.

From photosynthesis to combustion

There are three main chemical processes associated with generating and burning ethanol as a biofuel. These are: Photosynthesis, the process by which plants and algae use sunlight to synthesise carbohydrates from carbon dioxide and water. Fermentation, the process by which bacteria and yeast convert carbohydrates into carbon dioxide and ethanol (and sometimes other chemicals). Combustion, the process by which a fuel (such as ethanol) reacts with oxygen to produce carbon dioxide, water and heat energy.

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