A fuel is any compound that has stored energy that can be released when the compound is burned.

Wood, petrol, coal, peat and a number of other fuels have energy-rich chemical bonds created using the energy from the Sun.

Energy is captured in chemical bonds through processes such as photosynthesis and respiration.

Combustion

Combustion is the reaction of burning a compound in oxygen. Hydrocarbons burn in oxygen to produce carbon dioxide and water. This is known as complete combustion.

Combustion Reactions

The experiment below is used to demonstrate the products formed by the complete combustion of a hydrocarbon. The oxygen combines with the carbon to form carbon dioxide and the hydrogen to form water.

hydrocarbon + oxygen → carbon dioxide + water

Carbon dioxide gas turns limewater from colourless to cloudy/ chalky. The water produced condenses on the sides of the gas jar. Water turns cobalt chloride paper from blue to pink.

Products of combustion experiment.

Set up the equipment as shown in the diagram below.

Instructions

1. Once the equipment has been set up, light the candle and place the gas jar back over the candle.
2. When the candle goes out, lift the gas jar up with out tilting and place the lid on the bottom of the jar.
3. Test to see if the candle made water by adding a piece of blue cobalt chloride paper, test the sides of the jar. If it turns pink, water is present.
4. Repeat steps 1 and 2.
5. This time, test to see if carbon dioxide was produced. Pour a little limewater into the gas jar. Swill it around a little. If carbon dioxide is present, the limewater turns cloudy.

Questions

1. What is the gas that reacts with the hydrocarbon when it burns?
2. What gases does the candle produce when it burns?
3. Name another fuel that produces the same gases when it burns.

Fractional Distillation

Hydrocarbons are fuels that are obtained from crude oil.

Hydrocarbon molecules contain carbon and hydrogen only.

The alkanes are a subset of hydrocarbons and are identified from the '-ane' ending. The alkanes are said to be saturated as they only contain carbon to carbon single bonds.

The full structural formula and molecular formula of the first two members of the alkanes have been done for you. The boiling point of the alkanes can be found on p8 of the Data Book.

Copy and complete the table into your jotter.

Name	Full Structural Formula	Molecular Formula	Boiling Point
Methane		CH4
Ethane		C2H6
Propane
Butane
Pentane
Hexane
Heptane
Octane

The alkenes are also a subset of hydrocarbons. An alkene can be identified from the carbon-to-carbon double bond and '-ene' ending. Alkenes are said to be unsaturated due to the presence of the C=C double bond.

The full structural formula and molecular formula of the first two members of the alkenes have been done for you. The boiling point of the alkenes can be found on p8 of the Data Book.

Copy and complete the table into your jotter.

Name	Full Structural Formula	Molecular Formula	Boiling Point
Ethene		C2H4
Propene		C3H6
Butene
Pentene
Hexene
Heptene
Octene

This is the content of Lesson 5.

When hydrogen burns in oxygen water is produced. This can be represented by the word equation and balanced formula equation below.

Word Equation:

hydrogen + oxygen → water

Balanced Formula Equation:

2H₂+ O₂ → 2H₂O

When hydrogen burns in oxygen lots of energy is released. We can detect this from the 'pop' sound that is made when it burns.

As hydrogen releases lots of energy on combustion this makes it suitable for use as a fuel.

Black Black Oil

Carbohydrates are compounds made from Carbon, Hydrogen and Oxygen.

Plants produce the carbohydrate glucose during photosynthesis. They store this in their leaves as starch.

Experiment - Testing for Starch

You are know going to carry out an experiment to show that plants store food (starch) in their leaves.

Using iodine (which turns blue / black if starch is present) we can test leaves to see if they have been making food.

Collect

• Safety mat and goggles
• tripod stand
• Bunsen burner
• small beaker
• small test tube
• white tile
• test tube rack

Instructions

1. Cut a leaf disc from the plant.
2. Boil 50cm³ of water in a small beaker.
3. Add the leaf disc and boil for 1 minute.
4. Turn your Bunsen burner flame off.
5. Using tongs, remove the leaf disc and place it in a test tube.
6. Take the test tube to the teacher to get alcohol.
7. Soak the test tube in the beaker.
8. When the leaf has turned white, tip the alcohol into the container at the front.
9. Rinse the leaf in hot water in the beaker.
10. Spread it on the tile and add a few drops of iodine.
11. Observe any colour changes to the leaf.

Making Food

Plants use a process called photosynthesis to make food.

The food plants make is a sugar called glucose. Glucose is a carbohydrate.

To store its food the plant joins the glucose molecules together to make starch.

Iodine is used to test for starch. It changes colour from orange to black in the presence of starch.

Beer and wine are produced by fermenting glucose with yeast. Yeast contains enzymes that catalyse the breakdown of glucose to ethanol and carbon dioxide.

In this experiment, a glucose solution is left to ferment.

Alcohol can be detected by its distinctive smell. The presence of carbon dioxide can be confirmed by testing with lime water.

Experiment- Fermentation

Equipment

• Boiling tube
• Test tube rack
• Limewater
• Conical flask (100 cm³)
• Measuring cylinder (50 cm³)
• Glucose, 5g
• Cotton wool
• Yeast (as fast acting as possible), 1 g

Instructions

2. Put 5g of glucose in the conical flask and add 50 cm³ of warm water. Swirl the flask to dissolve the glucose.
3. Add 1g of yeast to the solution and loosely plug the top of the flask with cotton wool.
4. Wait while fermentation takes place.
5. Remove the cotton wool and pour the invisible gas into the boiling tube containing limewater. Take care not to pour in any liquid as well.
6. Gently swirl the limewater in the boiling tube and note what happens.
7. Replace the cotton wool in the top of the flask.

Fermentation

When yeast is added to warm glucose solution, alcohol and carbon dioxide are produced.

When carbon dioxide is bubbled through lime water it turns the lime water cloudy.

When alcohols burn they release energy to their surroundings. This can be measured by measuring the temperature of water in a can before and after burning an alcohol.

Experiment- Energy released from burning alcohols

Equipment

• thermometer
• clamp stand
• copper can
• spirit burner containing alcohol
• measuring cylinder
• bunsen burner
• heat proof mat
• timer

Diagram

Instructions

1. Set the equipment up as shown in the diagram above.
2. Use the measuring cylinder to add 100cm³ to the copper can.
3. Add the thermometer and record the starting temperature of the water.
4. Place the spirit burner under the copper can. Remove the cap and light it.
5. Heat the can for two minutes then replace the cap on the burner.
6. Record the final temperature of the water.