BTEC First Principles of Applied Science96

2.6 Chemical properties and uses of chemicals

In lesson 1.20 you discovered how indigestion remedies are used to treat stomach ache caused by too much acid inthestomach.Calciumcarbonate is used in many indigestion remedies.

Discuss why calcium carbonate is a suitable choice for an ingredient in indigestion remedies. You will need to think about its chemical properties.

Get started

Sodium is very reactive and reacts with water to form sodium hydroxide which is very corrosive. So airbag manufacturers mix the sodium azide with other chemicals to react with the sodium when it forms.

Safety and hazards

�Sodium azide in car airbagsNowadays cars are fitted with airbags which protect you if the car is involved in a collision. Each airbag contains about 100 g of a chemical called sodium azide. There is also a small amount of a detonator compound. If there is an accident, the shock of the collision causes sensors in the car to send an electrical signal to the detonator. This reacts and releases heat which causes the sodium azide to break down.

One hundred grams of sodium azide, which is enough to fill a small cup, produces over 50 litres of nitrogen gas. It does this incredibly quickly – the airbag inflates in less than 50 milliseconds.

Sodium azide is a white ionic solid which has the chemical formula NaN3. It looks just like ordinary table salt (sodium chloride) and normally it is quite stable and unreactive at room temperature. But when it is heated to 300 °C it breaks down in a reaction called thermal decomposition.

sodium azide → sodium + nitrogen 2NaN3 → 2Na + 3N2

�Argon for arc weldingArgon is an element from group 0 of the periodic table. The elements in this group are called the noble gases and they are very unreactive. This means argon will not react with oxygen or metals, even at very high temperatures.

Argon is used in arc welding. In this process, pieces of metal are joined together by melting them with an electric current and letting them cool. If hot metal is in contact with air it can react. This can be dangerous or affect the weld joint. To prevent this, a stream of argon is passed over the joint during welding.

�Carbon dioxide in fire extinguishersCarbon dioxide is a gas made up of covalent molecules with the formula CO2. Because the covalent bonds in each molecule are strong, carbon dioxide is quite unreactive, except at very high temperatures.

Carbon dioxide is used in fire extinguishers. Fires happen when materials like wood, paper or plastics react with the oxygen in the air at high temperatures. These substances do not react with carbon dioxide. So, when carbon dioxide is sprayed from a fire extinguisher over the burning material it covers the material, stopping oxygen reaching it. The fire will then quickly go out.

Airbags cushion passengers from impact in an accident.

Suggest another element which can be used instead of argon in arc welding. Explain your answer.

Activity A

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UNIT 2 Chemistry and Our Earth

A CO2 fire extinguisher in action.

You are a chemist working for a company which manufactures rocket fuel for rockets that launch communication satellites into space. You are considering developing new rocket fuels. There are three possible fuels to consider: hydrogen, hydrazine and gunpowder (a mixture of potassium chlorate, sulfur and carbon). Note that rockets carry a supply of oxygen to react with the fuel.

Research what properties are needed for a rocket fuel. Write a report on the properties of the three possible fuels, explaining which one will be the best choice.

Assessment activity 2.4 2B.P4 2B.P5 2B.M4 (part) 2B.D3 (part)

Grading tips

For 2B.P4 and 2B.P5 you will need to describe some physical properties and also some chemical properties of the three substances and explain how these properties are important when they are used as a fuel.

By explaining and discussing the link between the properties and uses, you will also provide part of the evidence needed for 2B.M4 and 2B.D3.

For 2B.D3 you will need to choose one fuel and explain why the properties of the fuel make it the most appropriate choice.

Lesson outcome

You should understand how the uses of chemicals are based on their chemical properties.

1 Look at the following descriptions of the properties of substances. Decide whether each property described is a physical or a chemical property.(a) Sodium reacts with water to produce hydrogen gas and sodium hydroxide.(b) Glycerol is a very viscous substance.(c) Ethanol dissolves in water.(d) Nitrogen is an unreactive gas.

Just checking

Carbon dioxide is denser than air, so when it is sprayed on a fire it will sink, pushing the air out of the way.

Did you know?

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BTEC First Principles of Applied Science98

2.7 Word equations and chemical equations

Chemicalreactionsarehappening all around you in your daily life. Discuss with other learners chemical reactions which you have seen happening today (outside your school or college laboratory).

Get started �Chemical reactionsYou have probably seen quite a lot of chemical changes happening in your school or college laboratory. But many chemical changes happen inside living organisms or in everyday situations like cooking.

You can recognise chemical changes because new substances are formed, with different properties. So you could see a colour change, a gas being given off or heat being given off when a fuel burns.

�Word equationsWhen sodium and chlorine react together a new compound, sodium chloride, is formed. Sodium is in group 1 of the periodic table and chlorine is in group 7 of the periodic table. Both are reactive elements, so this reaction is quite violent. The reaction can be shown as a word equation. The arrow shows you that a chemical change happens:

sodium + chlorine → sodium chloride reactants product

Be particularly careful about spellings: chlorine, chloride and chlorate all mean different things.

�Balanced chemical equationsChemists often write balanced chemical equations. These give more information about the reaction and:

• theformulaeofthereactantsandproducts

• thenumberofparticlesofreactantsandproductsinvolvedinthereaction

• thephysicalstateofeachsubstance,solid,liquid,gasordissolvedinwater,shownby the state symbols: (s), (l), (g) and (aq).

Reactants – The substances that react together in a chemical reaction.

Product(s) – The new substance(s) formed by a chemical reaction.

State symbol – Abbreviation used to show the physical state of each substance in a reaction:•(s),solid•(l),liquid•(g),gas•(aq),aqueous.

Key terms

Word equations always include the full chemical names of the reactants and products. So although sodium chloride is often called ‘salt’ we always use its chemical name.

Remember

This is sodium reacting with chlorine to make sodium chloride (salt).

Look at the descriptions of chemical reactions below. Use the descriptions to write word equations for the reactions.• Whensodiumisaddedtowateritgivesoffhydrogenandasolutionofsodium

hydroxide is left behind.• Chlorinedisplacesbrominefromasolutionofpotassiumbromide.Potassium

chloride is also formed.

Activity AThe ideas about equations in this section link with lesson 1.17.

Link

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UNIT 2 Chemistry and Our Earth

In a balanced chemical equation, the reactants must have the same number of atoms of each type as the products. Here is a balanced chemical equation for the combustion of hydrogen:

2H2 (g) + O2 (g) → 2H2O (l)

hydrogen 1 oxygen → water

�Reversible and irreversible chemical changesNitrogen dioxide (NO2) is a brown gas and is one of the pollutants in car exhausts. Two nitrogen dioxide molecules can join to make a new product called dinitrogen tetroxide (N2O4), a colourless gas. If N2O4 is heated, it splits back up into NO2 molecules. So we say that the reaction is reversible.

2NO2 N2O4

nitrogen dioxide > dinitrogen tetroxide brown colourless

Lesson outcomes

You should:• beabletowriteword

equations and simple balanced equations (including state symbols)

• recognisereactantsandproducts in a reaction

• knowthedifferencebetween reversible and irreversible changes.

1 Look at this balanced chemical equation:Cl2 (g) 1 2NaBr (aq) → Br2 (aq) 1 2NaCl (aq)

(a) Name the products of the reaction.(b) How many atoms of sodium are involved in this reaction?(c) Is the reaction reversible? Explain how you can tell.(d) In what state is the reactant sodium bromide?

Just checking

H

H

H H H

HO

OH H

OO

Drawing the molecules helps you to count the number of atoms of each type involved in the reaction.

The > symbol shows you that the reaction is reversible. Most reactions aren’t easily reversible which is why the forward arrow (→) is used in most chemical equations.

O

O

N

O

ON

O

ON

O

O

N>

Ethanol, a useful fuel, can be made from ethene and water under certain carefully controlled conditions. The balanced chemical equation is:

C2H4 (g) + H2O (g) > C2H5OH (g)

Write down everything that you can work out about the reaction from this equation.

Activity B

The balanced chemical equation for the reaction between magnesium and hydrochloric acid is:

Mg (s) + 2HCl (aq) → MgCl2 (aq) + H2 (g)

What does this tell you?• Thereare1Mgatom,2Hatomsand2Clatomsinvolvedinthereaction.• Thereactantsaresolidmagnesiumandhydrochloricaciddissolvedinwater.• Theproductsaremagnesiumchloridedissolvedinwaterandhydrogengas.

Worked example

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