S3 Chemistry elective

Fuels

The work of this topic builds on what you learned about fuels in the S2 Science course.

This topic is about 3 very important raw materials that we use in the modern world - coal, oil and natural gas. In this topic you will also learn about the different families of compounds we obtain from these raw materials. Many of these compounds are used in our everyday life as fuels.

A raw material is a basic natural material from which a product is made.

A fuel is a substance which burns giving out energy.

The type of chemical reaction which takes place when a fuel is burned is called combustion. Combustion is the reaction of a substance with oxygen giving out energy. The fuel itself and oxygen are two of the three requirements necessary for a combustion reaction to take place. A source of ignition, often a spark or a little heat, is the third requirement. You will remember from your S2 Science rotation class that these 3 requirements are sometimes said to form the 3 sides of the Fire Triangle.

A Fire Triangle diagram can be drawn like this.

Combustion reactions, where energy is given out during the reaction are called exothermic reactions. Most of the energy released is in the form of heat energy.

A good fuel is one which is cheap and readily available, which burns easily to produce lots of energy and which produces little or no pollution.

A fuel burned in pure oxygen will burn better than the same fuel burned in air. Air is composed of about 1/5th oxygen and 4/5th nitrogen. Although air contains the oxygen necessary for combustion the lower percentage of oxygen in air means fewer collisions and a slower reaction. The test for oxygen gas is that it will relight a glowing splint.

What is a raw material?

What is a fuel?

Give some examples of fuels used in everyday life.

What is a combustion reaction?

What are the three requirements of any combustion reaction?

What is an exothermic reaction?

List some characteristics of a good fuel.

Why do fuels burn better in pure oxygen than in air?

What is the test for oxygen?

Many different fuels are forms of the element carbon or are compounds containing carbon. Charcoal, which is commonly used as a fuel on a barbecue, is a form of the element carbon and is a good fuel. Natural gas is the gas we use in our homes, schools and factories and is one of the commonest fuels in use today. Natural Gas is 96% methane. Methane is the simplest hydrocarbon and has the chemical formula CH4. Hydrocarbons are compounds which contain hydrogen and carbon only.

What element do most fuels contain?

Give examples of fuels containing this element.

What is the main component of natural gas?

What are hydrocarbons?

We will carry out some burning experiments on charcoal and methane to find out a bit more about combustion reactions.

Activity 1

Burning of charcoal

GOGGLES MUST BE WORN

DO NOT TOUCH THE BURNING SPOON AFTER IT HAS BEEN USED IN THE BUNSEN BURNER

1. Collect a gas jar containing oxygen gas.

2. Adjust a burning spoon so that it will fit the gas jar.

3. Put a small amount of charcoal powder on the burning spoon and heat it in a Bunsen burner until it begins to glow red hot.

4. Take the lid off the gas jar and carefully lower the burning spoon with the charcoal into the jar. Make sure that no powder falls off the burning spoon.

5. When burning stops, remove the burning spoon, pour a little lime water into the gas jar and replace the lid.

6. Hold the lid of the gas jar and shake the gas jar with the lime water and observe any change.

When the charcoal burns, the carbon atoms combine with oxygen to form carbon dioxide. Carbon dioxide gives a cloudy/milky colour when tested with lime water.

Write a short report on your experiment, including a suitable title, a description of what you did and what happened.

Write a word equation for the combustion reaction which took place?

What is the test for carbon dioxide?

Methane can be burned in the laboratory using a Bunsen burner. Because methane contains carbon and hydrogen atoms, the oxygen required for combustion can combine with both types of atom to form two different products.

The carbon atoms combine with oxygen to form carbon dioxide and the hydrogen atoms combine with oxygen to form water. You carried out an experiment in your S2 Science rotation class to show that these were the products when natural gas is burned using a Bunsen burner.

The lime water produced turns milky indicating that carbon dioxide has formed, and the colourless liquid collected was found to boil at 100 C and to freeze at 0 C. This is the test for water.

Explain why both carbon dioxide and water are formed when methane is burned?

Write a word equation for the combustion of methane.

What are the tests for carbon dioxide and water?

Methane, if used properly, is a safe fuel to burn. However, like any other fuel, it can be dangerous. Methane explosion can have a disastrous effect on houses or other buildings. Your teacher will show you an experiment which demonstrates the potential danger of even a small amount of methane.

Activity 2

Methane explosion (Teacher demonstration)

GOGGLES MUST BE WORN

1. Your teacher will fill a large metal can, with a hole in the bottom and a hole in the lid, with methane.

2. The can will be placed on a tripod and the gas escaping from the hole in the top of the can will be ignited with a burning splint.

3. Stand back and wait!

Warning the flame may appear to go out it has NOT!

Write a short report on the experiment.

Fossil fuels

CRUDE OIL and NATURAL GAS

Earlier, you learned a little about methane (the simplest hydrocarbon) which is the main component of natural gas. Natural gas is an example of a fossil fuel and is often found with crude oil, another fossil fuel.

Crude oil is one of the most important sources of fuel in the world today. It is a mixture of many different chemical compounds, mostly hydrocarbons, which are found in fuels such as petrol, diesel and fuel oil.

Crude oil and natural gas are termed fossil fuels because they were formed from things that were living millions of years ago. Coal is a third very important fossil fuel.

Formation of crude oil

Crude oil was formed from the remains of dead sea plants and animals. The dead remains were covered by layers of sediment and over millions of years they were affected by heat, pressure and bacteria and converted into crude oil.

Your teacher may show you a video to further explain the formation and importance of fossil fuels such as crude oil.

Finite resources and the Energy Crisis

Natural gas and crude oil are finite resources. The word finite means that they cannot be replaced and will run out in time. Although there are greater reserves of coal, supplies of all three of these very important fossil fuels are eventually going to run out. This is such a major problem that it has been called the Energy Crisis.

One thing that could be done is to use energy sources that are not going to run out. These are called renewable sources e.g. wind power, wave power, solar power etc. They are called alternative energy sources, because they are alternatives to the fossil fuels which are running out.

A possible solution to the problem is to develop fuels which are made from plants. A good of example of an alternative fuel of this type would be ethanol. Ethanol is an alcohol which is made from sugar cane, or other sugar containing plants, which can be grown on a continuous basis. Ethanol can therefore be thought of as a renewable fuel and is not finite, unlike the fossil fuels.

Another area which is likely to be developed to provide an alternative to fossil fuels is the establishing of a Hydrogen Economy. You know from Topic 5.2, that hydrogen burns very easily and gives out a large amount of energy when it burns. The use of hydrogen as a fuel is likely to be of major importance in the future.

What is a fossil fuel?

Name the three most important fossil fuels.

What does crude oil contain?

Describe how crude oil was formed.

Why are the fossil fuels described as finite resources?

What is the Energy Crisis?

What are renewable energy sources?

Give three examples of renewable sources of energy.

What role might plants play in the replacement of fossil fuels?

What is the Hydrogen Economy?

COAL

Coal like crude oil and natural gas is also a fossil fuel.

Current information about the size of world reserves of coal and crude oil, and the speed at which we are using them up, would suggest that coal will last for longer than crude oil or natural gas. However, coal reserves will also eventually be used up. Coal is still burned extensively as a source of energy, particularly in power stations.

Burning coal to obtain energy is however rather a wasteful way of using it as it has many other uses, e.g. as a source of coke for use in blast furnaces in the steel industry or to produce a wide range of chemicals for making disinfectants, insecticides, wood preservatives etc.

Formation of coal

Coal is formed from the remains of dead vegetation (plants and trees). The dead remains were covered by layers of sediment and over millions of years they were affected by heat, pressure and bacteria and converted into coal.

Why is coal also described as a finite resource?

Where is coal still extensive