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© Boardworks Ltd 20083 of 40

Some chemical reactions give out energy. For example:

Energy changes in chemical reactions

Magnesium burning – this reaction gives out heat and light energy.

A firework or bomb – this reaction gives out light, heat and sound energy.

A battery – this reaction gives out electrical energy.

A candle burning – this reaction gives out heat and light energy.

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Reactions that give out heat energy are called exothermic.

Exothermic reactions

Combustion is an example of an exothermic reaction – for example, when a piece of wood is burned, heat is produced.

Reactions that take in heat energy are called endothermic.

Sherbet dissolving on your tongue is an example of an endothermic reaction.

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An exothermic reaction

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An endothermic reaction

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Endothermic or exothermic?

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There are many different types of chemical reaction.

Types of chemical reactions

For example, oxidation is the reaction of a substance with oxygen.

What is the word equation for the oxidation of copper?

Reduction is the opposite of oxidation. What do you think is formed in the reduction of magnesium oxide?

magnesium oxide magnesium oxygen+

copper oxygen copper oxide+

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When you heat copper carbonate, it breaks up to form copper oxide and carbon dioxide.

Heating copper carbonate

What is the word equation for this reaction?

This reaction is called thermal decomposition. Can you explain why?

Thermal means heat, and decomposition means breaking into smaller pieces.

So in a thermal decomposition reaction, heat is used to break a substance into smaller pieces.

copper carbonate copper oxide carbon dioxide+

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Combustion

A common combustion reaction is petrol burning with oxygen in a car engine.

What is the word equation for this combustion reaction?

Combustion involves reacting with oxygen, so it can also be classified as another type of reaction. Which one?

petrol oxygen carbon dioxide+ water+

The products of this reaction are carbon dioxide and water.

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Displacement reactions are commonly used to obtain pure metals from their ores.

Types of chemical reaction

Displacement means replacing something with something else.

For example, copper can be displaced from copper sulfate by reacting it with zinc.

What would be displaced in the reaction between zinc sulfate and magnesium?

zinc sulfate magnesium magnesium sulfate+ zinc+

copper sulfate zinc zinc sulfate+ copper+

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What is neutralization?

Types of chemical reaction

Neutralization is the reaction between an acid and an alkali to make a neutral substance.

In the neutralization reaction between hydrochloric acid and sodium hydroxide, the products are sodium chloride and water.

What is the word equation for this reaction?

hydrochloricacid

sodiumhydroxide

sodiumchloride+ water+

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Which type of reaction?

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Hydrogen, oxygen and carbon dioxide are very useful gases.

Making useful gases

Hydrogen is used to make margarine and as a rocket fuel.

These gases are made in factories using chemical reactions.

Carbon dioxide is used in fire extinguishers.

Oxygen is used to help things burn at a high temperature, like welding torches.

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Making and identifying hydrogen

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Making and identifying oxygen

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Making and identifying carbon dioxide

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Word equations for making gases

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Which gas?

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Which test?

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What is combustion?

Combustion is the scientific word for burning.

It is the chemical reaction that takes place when a substance burns and reacts with oxygen to produce heat and light energy.

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Bell jar experiment

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The fire triangle

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Putting out fires

If a fire starts accidentally, it is important to be able to put it out quickly and safely.

For example: putting water on burning

wood cuts off the heat.

To put out a fire, you can cut off the supply of any side of the fire triangle.

cutting down trees in a forest fire cuts off the supply of fuel.

putting sand on burning oil cuts off the oxygen.

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Products of combustion

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Combustion of methane

Natural gas, methane, is often burned for cooking. Methane is made up of carbon and hydrogen.

What gas does methane burn with?

What substance will the carbon in methane change into when it burns?

What substance will the hydrogen in methane change into when it burns?

What is the word equation for the combustion of methane?

methanecarbondioxide water+ oxygen +

carbon dioxide

water

oxygen

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Word equations for combustion

When substances combust, the new substances formed are called ‘oxides’.

What are the word equations for these combustion reactions?

1. Coal (made from carbon) burns and forms carbon dioxide.

2. Hydrogen burns and forms dihydrogen oxide (i.e water!).

3. Methane burns and forms carbon dioxide and water.

methane oxygen carbon dioxide water+ +

hydrogen oxygen water+

carbon oxygen carbon dioxide+

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Symbol equations for combustion

What are the symbol equations for these combustion reactions?

1. Coal (made from carbon) burns and forms carbon dioxide.

2. Hydrogen burns and forms dihydrogen oxide (i.e water!).

3. Methane burns and forms carbon dioxide and water.

methane oxygen carbon dioxide+ water+CH4 2O2 CO2+ 2H2O+

hydrogen oxygen water+

2H2 O2 2H2O+

carbon oxygen carbon dioxide+

C O2 CO2+

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Complete and incomplete combustion

Fuels like methane, butane and petrol are hydrocarbons. Hydrocarbons are compounds made of carbon and hydrogen.

When these hydrocarbon fuels burn in a good supply of oxygen, they burn completely to make carbon dioxide and water.

If there is a lack of oxygen, incomplete combustion takes place. This means that the fuel burns to produce water, carbon monoxide and carbon particles.

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Incomplete combustion

Incomplete combustion is a hazardous problem because the products of the process are toxic.

Carbon monoxide prevents people’s blood from carrying oxygen.

What precautions do you think people take in their homes to prevent incomplete combustion?

Carbon particles make city buildings very dirty, and they can get into people’s lungs and cause breathing problems.

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Using fossil fuels

Coal, oil and natural gas are called fossil fuels.

Burning fossil fuels, like coal, petrol and natural gas, provides more than 90% of the energy we need for transport, factories and in our homes.

How different would life be without fossil fuels?

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Crude oil and fractional distillation

The oil that comes out of the ground is called crude oil. It cannot be used as it is, because it is a mixture of many different substances with different boiling points.

How can you separate a mixture of substances with different boiling points?

To separate out the useful substances, a process called fractional distillation is carried out.

big molecules boil at a high temperature

small molecules boil at a lower temperature

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Fractional distillation

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What’s the best fuel?

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Glossary

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Anagrams

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Multiple-choice quiz