Symbols for elements - Wikispaces Web viewBe able to write simple word equations that describe chemical reactions. ... Molybdenum. 42. 98. Tc. Technetium. 43. 101. Ru

SUSSEX HOUSE SCHOOL

SCIENCE DEPARTMENT

5th Form

TOPIC FOUR:

CHEMICAL REACTIONS

Fifth Form Topic Five: Chemical Reactions

Topic Overview

By the end of this unit, you will:

Know that elements are shown in an organised pattern called the Periodic Table.

Be able to state definitions of and know what is meant by the terms element, mixture and compound.

Know what is meant by the term chemical reaction. Be able to write simple word equations that describe chemical reactions. Know how to identify whether a substance is an acid or an alkali or is

neutral. Know what a neutralisation reaction is and how such reactions are useful. Know how acids react with other substances. Know and be able to test a gas to see if it is hydrogen, carbon dioxide or

oxygen. Understand how acid rain is produced and why it is a problem. Know the general types of chemical reaction and how they are beneficial

and harmful. Know that the metals can be placed in an order of reactivity called the

Reactivity Series and that this can explain why some metals are easier to mine than others.

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The Periodic Table

This is the Periodic Table. It is an important tool that helps chemists to understand the properties of materials and how substances will react together. There are over 100 elements and these are shown in the Periodic Table.

What is an element?To understand this question you must know that all substances are made up of atoms. Atoms can be joined to other atoms to make different substances. Atoms are made up of three types of other particle: electrons, neutrons and protons. Atoms of a particular type will have the same number of electrons and protons and a similar number of neutrons. For example, all atoms of carbon have 6 electrons and 6 protons. Most samples of carbon have 6 neutrons as well, but some have 8.

A substance that is made up of only one type of atom is called an element. There are over 100 different types of atom, so there are over 100 different elements. Aluminium is an element. It is made up of nothing but aluminium atoms. These will all act in the same way. They are different from the atoms in other elements. For example, lead atoms are heavier and larger than aluminium atoms. This is why lead is a heavier metal than aluminium.

Symbols for elementsIf you look at the table above, you will notice that the names of the elements are not written on the table – only symbols are given. Some of the symbols seem sensible e.g. S for sulphur, O for oxygen. Others seem less obvious e.g. Na for sodium and Hg for mercury.

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You should know the following elements and their symbol:

Element SymbolHydrogen HHelium HeLithium LiBeryllium BeBoron BCarbon CNitrogen NOxygen OFluorine FNeon NeSodium NaMagnesium MgAluminium AlSilicon SiPhosphorus PSulphur SChlorine ClArgon ArPotassium KCalcium CaIron FeNickel NiManganese MnCopper CuZinc ZnBromine BrSilver AgPlatinum PtGold AuLead PbUranium U

Why is the Periodic Table laid out like it is?One way to arrange all the elements would be alphabetically. However, this is not especially helpful. A much better way is to arrange them according to their similarities.

During the 19th Century a Russian chemistry professor named Dmitri Mendeléev tried to organise the known elements in a sensible fashion. Other scientists had noted that many elements reacted in similar ways. For example, some metallic elements readily reacted with water.

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Mendeléev used cards with the names and properties of elements on and arranged them as if he was playing the card game patience. He made a huge assumption – that not all the elements had been discovered! He left gaps in his table, but made predictions about the missing elements. He first published his Periodic Table of the Elements in 1871.

Other scientists laughed at his idea, until in 1886 the element germanium was discovered. It was located in the Periodic Table exactly where it should be, having similar chemical properties to silicon and almost exactly the same physical properties that Mendeléev predicted! Two more elements were discovered that Mendeléev predicted: gallium and scandium.

The modern Periodic Table is not just arranged according to properties – it is also arranged according to atomic number. The atomic number of an element tells you how many electrons that an atom of that element has. Hydrogen, the simplest element, has an atomic number of 1. Therefore, it only has 1 electron. Uranium has an atomic number of 92, therefore it has 92 electrons.

Groups and Periods and Metals and Non-metalsA group is a vertical column. It contains elements that have similar properties. For example, the first column is called Group 1. It contains the metallic elements lithium, sodium, potassium, rubidium, caesium and francium Group 7 consists of reactive non-metal elements fluorine, chlorine, bromine, iodine and astatine. As you go down the group, these elements get less reactive.

A period is a horizontal row. Elements in the period have only one thing in common: they have the same number of shells of electrons – this is quite complicated and you don’t have to worry about it until senior school!

The metallic elements are found on the left hand side of the Periodic Table; whilst the non-metallic elements are found on the right hand side.

There are three groups you should be particularly aware of:

Group 1 – The alkali metalsAll these metals react vigorously with water to release hydrogen gas and make a metal hydroxide. However, as you go down the table the reaction becomes more and more vigorous.

Group 7 – The halogensThese elements react with metals to produce different types of salt. Fluorine is extremely reactive but as you go down the group the halogens become less and less reactive.

Group 8 – The Noble gasesThese are helium, neon, argon, krypton, xenon and radon. They are all highly unreactive. However, they do have uses. For example, helium is used in lighter than air aircraft. Neon is used to make coloured lights.

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Year 7 Topic Five: Chemical ReactionsThe Periodic Table

GroupTitle

1 2 3 4 5 6 7 8Period

1

1H

Hydrogen1

Hydrogen isn’t really in any group!

4He

Helium2

Period 2

7Li

Lithium3

9Be

Beryllium4

11B

Boron5

12C

Carbon6

14N

Nitrogen7

16O

Oxygen8

19F

Fluorine9

20NeNeon

10

Period 3

23Na

Sodium11

24Mg

Magnesium12

27Al

Aluminium13

28Si

Silicon14

31P

Phosphorus15

32S

Sulphur16

35.5Cl

Chlorine17

40Ar

Argon18

Period 4

39K

Potassium19

40Ca

Calcium20

45Sc

Scandium21

48Ti

Titanium22

51V

Vanadium23

52Cr

Chromium24

55Mn

Manganese25

56FeIron

26

59Co

Cobalt27

59Ni

Nickel28

64Cu

Copper29

65ZnZinc

30

70Ga

Gallium31

73Ge

Germanium32

75As

Arsenic33

79Se

Selenium34

80Br

Bromine35

84Kr

Krypton36

Period 5

85.5Rb

Rubidium37

88Sr

Strontium38

89Y

Yttrium39

91Zr

Zirconium40

93Nb

Niobium41

96Mo

Molybdenum42

98Tc

Technetium43

101Ru

Ruthenium44

103Rh

Rhodium45

106Pd

Palladium46

108AgSilver

47

112Cd

Cadmium48

115In

Indium49

119SnTin

50

122Sb

Antimony51

128Te

Tellurium52

127I

Iodine53

131Xe

Xenon54

Period 6

133Cs

Caesium55

137Ba

Barium56

139La

Lanthanum57

178.5Hf

Hafnium72

181Ta

Tantulum73

184W

Tungsten74

186Re

Rhenium75

190Os

Osmium76

192Ir

Iridium77

195Pt

Platinum78

197AuGold

79

201Hg

Mercury80

204Tl

Thallium81

207PbLead

82

209Bi

Bismuth83

210Po

Polonium84

210At

Astatine85

222Rn

Radon86

Period 7

223Fr

Francium87

226Ra

Radium88

227Ac

Actinium89

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Year 7 Topic Five: Chemical Reactions

Elements, Mixtures and Compounds

All matter is made up of atoms. However, atoms can be arranged in many different ways. A substance could consist of just one type of atom. This would mean it was an element. On the other hand a substance could be made up of two or more types of atom that are bonded together. In this case the substance is known as a compound. Sometimes a group of substances are found together but not actually joined chemically. In this case they are known as a mixture.

Look at the diagrams below. Write the word element, mixture or compound underneath the correct diagram:

All around you today you will have seen a variety of materials. In the table below list some of them in the correct column.

Element Mixture Compound

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Naming Compounds

Naming chemical compounds can seem rather strange. However, there are some basic rules that can be followed to explain how to do it!

1. If the compound contains a metal, the metal goes first.e.g. sodium chloride

aluminium iodide

2. When non-metals react with metals to make a compound they change their name like this:

Element Compound name SymbolOxygen oxide OFluorine fluoride FChlorine chloride ClBromine bromide BNitrogen nitride NSulphur sulphide SIodine iodide I

e.g. lead sulphidesodium fluoridelithium chloride

3. Some compounds have oxygen and another non-metal combined with a metal. In this case the following names arise:

Non – metal and oxygen Name SymbolOne sulphur atom + three oxygens

sulphite SO3

One sulphur atom + four oxygens sulphate SO4

One nitrogen atom + two oxygens nitrite NO2

One nitrogen atom + three oxygens

nitrate NO3

One carbon atom + three oxygens

carbonate CO3

e.g. lead nitratesodium carbonatecopper sulphate

4. If oxygen reacts with a non-metal then it always goes after the non-metal.e.g. carbon dioxide

sulphur trioxide

5. If hydrogen reacts with a non-metal then it always goes before the non-metal.e.g hydrogen sulphide

hydrogen iodide

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What is a chemical reaction?

Physical ChangesYou will often see materials undergoing changes. Scientists class changes into two groups: physical and chemical. We examined physical changes in an earlier topic. These are ones where the chemical properties of the substance that changes are not altered. Any change of state is a physical change. For example, when water turns from ice to liquid water it is still water – a molecule of it is still made from two atoms of hydrogen and one atom of oxygen. Dissolving is also a physical change. Physical changes are easily reversed, there are no changes in colour, energy is generally not released and no new substances are made.

Chemical Changes (or reactions)In a chemical change one or more reactants combine to make one or more new products. For example, we have already seen that when iron and sulphur are heated together, a new material, iron sulphide, is produced. Chemical changes are usually very hard to reverse – it is difficult to separate the iron and the sulphur once they have formed iron sulphide. They are often accompanied by changes in colour and release of energy. The properties of the new material are different from the properties of the reactants.

Word Equations

A chemical reaction can be summarised by writing what is called a word equation.

All word equations must show:

The names of the materials that take part in the chemical reaction. These materials are called reactants. The reactants are written on the left-hand side of a word equation.

The names of the new materials that are produced by the chemical reaction. These materials are called products. The products are written on the right-hand side of a word equation.

Whether the reactants are solids, liquids, gases or in solution. Solids are indicated by writing the symbol (s) immediately after the name of those reactants and products that are solids. Liquids are shown using the symbol (l); gases with the symbol (g) and solutions with the symbol (aq).

Example 1

The production of a black solid compound called copper sulphide by heating a mixture of pink copper metal and yellow sulphur powder can be summarised by writing:

Copper (s) + sulphur (s) copper sulphide (s)

Example 2

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The violent production of a white solid compound called aluminium iodide by heating a mixture of solid black iodine and shiny aluminium powder can be summarised by writing:

Aluminium (s) + iodine (s) aluminium iodide (s)

Example 3

The production of colourless hydrogen gas and a clear colourless solution of magnesium chloride by adding shiny magnesium powder to clear colourless dilute hydrochloric acid can be summarised by writing:

Hydrochloric acid (aq) + magnesium (s) magnesium chloride (aq) + hydrogen (g)

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Practical: Comparing elements, mixtures and compounds

In this practical you will be comparing the differences between two elements, a mixture of the elements and a compound made from the elements.

The elements you will be using are iron (Fe) and sulphur (S). You will be given powders of each element.

Do not take the powders out of their bags unless instructed to.

METHOD

1. Examine each element. Note down the appearance of both powders in the results table.

2. Test for magnetism. Take a magnet and move it towards each element in turn. Keep the powders in the bags. Note down what happens in the results table.

3. Test the density. Take a spatula full of iron powder and place into a test tube of water. Shake the tube. Does the iron sink or float? If it sinks its density is greater than water; if it floats it is less dense than water. Repeat this for the sulphur powder. Note your results in the table.

4. Reaction with dilute hydrochloric acid. Put a spatula full of iron into a test tube. Add 2 cm3 of dilute hydrochloric acid. What happens? Hold a lit splint to the mouth of the tube. Does it pop? What gas is being produced?Note your results and try to write a reaction equation (if applicable). Repeat using sulphur.

Into a spare plastic bag put in roughly equal amounts of iron and sulphur. Make sure the bag is closed and mix thoroughly. Carry out the steps 1 - 4 (above) and complete the table.

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Demonstration: Reaction of iron and sulphur to make iron sulphide

Watch the demonstration of the formation of iron sulphide. Draw and annotate diagram of the apparatus below:

The same tests will be carried out on the compound as for the individual elements and the mixture. Make sure you note down the results.

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RESULTS

Material Appearance Magnetic? Density compared to water

Reaction with dilute HCl

Iron

Sulphur

Iron + Sulphur

Iron sulphide

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CONCLUSION

1. Are iron and sulphur different? Summarise how.

2. How does the mixture of sulphur and iron differ from sulphur on its own and iron on its own?

3. How does the compound (iron sulphide) differ from its constituent elements?

4. Write a word equation for the reaction of iron with sulphur:

5. Write down the definitions of an element, a mixture and a compound:

ELEMENT

MIXTURE

COMPOUND

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Air an important mixture!

Air is an important raw material. It is used to manufacture oxygen, nitrogen and noble gases for industrial processes. These gases are separated by liquefying air, then fractionally distilling the liquid air. As the liquid air warms, the different constituents boil off at different temperatures. The proportions of these gases in the air are shown in the pie chart below.

The main products from air are nitrogen, argon and oxygen. The important uses of these gases are summarised in the Table below.

Uses of nitrogen, argon and oxygen Gas Use Nitrogen Manufacture of ammonia, nitric acid, and fertilisers

Used as a cheap unreactive gas ‘blanket’ to stop materials reacting with oxygen (e.g. in petrol storage tanks)

Oxygen Manufacture of steelWelding and cutting – oxyacetylene flames are hot enough to melt metalBreathing apparatus, e.g. for fire fighting, deep-sea diving

Argon Used as an unreactive gas to stop materials reacting with air, e.g. in electric light bulbs where the very hot filaments would react with the nitrogen in air

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Acids and Alkalis

One way to classify the many chemicals in the universe is to separate them into acids and alkalis. We can test for acids and alkalis by using indicators. These change colour depending on how acid or alkali the chemical is.

Practical – Finding acids and alkalis

1. Test the following substances to see whether they are acid or alkali. You will use three different indicators – litmus, universal indicator and methyl orange.

2. When using the universal indicator you will have to use a colour chart. This will tell you the pH of the substance.

Substance Universal Indicator(Colour and pH)

Litmus Phenol-phthalein

Methyl Orange

pH meter

Acid/ Alkali/ Neutral

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Questions

1. What colours does universal indicator go if it is in an acid?

2. What happens to litmus if it is in a neutral substance?

3. Are acids always dangerous? Justify your answer?

4. Complete the chart below to show the correct colour for universal indicator at different pH values using coloured pencil.

1 2 3 4 5 6 7 8 9 10 11 12 13 14

4. What is a pH number?

Acids Alkalis

Neutral

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Natural AcidsThere are many acids found in nature. Ants produce formic acid. The Latin name for ant is Formica. Ants squirt formic acid at their enemies. Some ants bite, releasing the acid under the skin. Stinging nettles also contain formic acid, that’s why they hurt so much when you brush against them!

Vinegar contains ethanoic acid. Pickling food in vinegar works because living things are very sensitive to acids. If food is put in vinegar, then micro-organisms are unable to grow and destroy the food.

Your stomach contains hydrochloric acid. This helps digestion by helping enzymes to break up proteins. Citric acids in fruit help prevent the going off. Vitamin C is actually an acid – ascorbic acid. It is an antioxidant. Antioxidants prevent oxidation reactions.

Acid Where found UsesCitric Citrus fruits Cooking and drinksFormic Ants and nettles Cleaning pansEthanoic Vinegar Cooking and preserving foodHydrochloric Stomach Digestion. Industrial uses.Nitric Made from nitrogen gas Fertilisers. Explosives.Sulphuric Made from nitric acid Fertilisers, Batteries, Plastics.Carbonic Rainwater Fizzy drinks.

Strong and Weak AcidsAll acids contain hydrogen. Acids can be divided into two main groups – strong and weak. The difference between the two is not how dilute they are. A strong acid is one where the hydrogen it contains combines easily with other substances. The most common strong acids are: hydrochloric acid; sulphuric acid and nitric acid. Weak acids include ethanoic acid, formic acid and citric acid.

AlkalisIf you chemically join oxygen to a metal it makes a metal oxide. Many metal oxides are unable to dissolve in water – they are insoluble. An alkali is a solution of a soluble metal oxide. Just like acids there can be strong alkalis and weak alkalis. Sodium hydroxide, for example is a strong alkali. Strong alkalis are just as dangerous as strong acids – they are just as corrosive.

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Practical - Acids and Neutralisation

Here is a very important general chemical equation:

Acid + Base Salt + Water

This means that if you add any acid to any alkali it will make a salt and water. In the following practical you are going to investigate this in more detail.

ApparatusCollect the following equipment: a test tube rack, four boiling tubes tubes, 5cm3 of dilute sodium hydroxide, 10cm3 of dilute hydrochloric acid, a syringe, universal indicator and a pH test chart.

Method1. Pour 5cm3 of the dilute sodium hydroxide into a test tube. Place the test

tube in the rack.2. Add 3 drops of universal indicator to the dilute sodium hydroxide.3. Fill your syringe with the 10cm3 of dilute hydrochloric acid.4. Using the syringe, carefully add the dilute hydrochloric acid to the sodium

hydroxide in 1cm3 amounts. After each addition of 1cm3 observe any colour changes that occur.

5. Complete the table below to show your findings.

Diagram

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Results

Amount of Alkali Amount of Acid Colour of solution pH of solution5cm3 0cm3

5cm3 1cm3

5cm3 2cm3

5cm3 3cm3

5cm3 4cm3

5cm3 5cm3

5cm3 6cm3

5cm3 7cm3

5cm3 8cm3

5cm3 9cm3

5cm3 10cm3

Conclusion1. Describe what happened as you added acid to the alkali.

2. What does the term ‘neutralisation’ mean? When did neutralisation occur in your experiment?

3. What is the reaction equation for this reaction?

4. Are salts acid, alkali or neutral?

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You will also be given a printout from a demonstration of the pH meter measuring the change in pH as acid is added to alkali. Stick this into your folder below. Describe the curve that is shown.

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Practical – Making a Salt

Any neutralisation reaction will produce a salt. There are many different salts – don’t be confused by the fact that common salt is (to a chemist) sodium chloride.

When an alkali is exactly neutralised by an acid, a compound called a salt is made. You can’t see anything happen when you mix the acid and alkali. The acid and alkali are colourless solutions. The salt is colourless, too. But when you evaporate off the water, the solid salt is left.

Apparatus

Collect the following apparatus: 20cm3 of dilute hydrochloric acid, 20cm3 of dilute sodium hydroxide, a 100ml beaker, an evaporating basin, tripod, gauze, heatproof mat and Bunsen burner.

Method

1. First you must decide how much acid you should add to the sodium hydroxide. The hydrochloric acid and the sodium hydroxide are the same strength as used in the previous experiment. Write down how much of each you could use, explain your answer!

2. To the evaporating basin add the correct amounts of acid and hydroxide.3. Now evaporate the solution using your Bunsen, gauze and tripod. Do not

heat the solution too strongly – it may begin spitting!

Diagram

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Results

Write your observations below:

Conclusion

Write a sensible conclusion of your own:

Making different salts

We have looked at how hydrochloric acid and sodium hydroxide can be used to make sodium chloride and water. However, using different acids can produce different salts.

Making copper sulphate

Method

1. Put a spatula of black copper oxide powder into a boiling tube.2. Add 10cm3 of sulphuric acid to the copper oxide3. Heat gently

Diagram

Observations

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In summary acids produce the following

Hydrochloric acid always makes chloride salts.

Sulphuric acid always makes sulphate salts.

Nitric acid always make nitrate salts.

e.g.

1. sulphuric acid + sodium hydroxide sodium sulphate + water2. nitric acid + magnesium hydroxide magnesium nitrate + water3. hydrochloric acid + copper hydroxide copper chloride + water

Try these:

1. Hydrochloric acid + magnesium hydroxide

2. Hydrochloric acid + aluminium hydroxide

3. Sulphuric acid + potassium hydroxide

4. Sulphuric acid + magnesium hydroxide

5. Nitric acid + lead hydroxide

6. Nitric acid + lithium hydroxide

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Practical – Reactions of Acids

IntroductionWe have already looked at how acids react with alkalis. Remember that alkalis are metal oxides (a metal atom chemically joined to one or more oxygen atoms) and that the general equation for the reaction is:

Acid + base salt + water

In this practical we shall examine how acids react with metals and a group of substances called carbonates.

1. Reactions of acids with metals

ApparatusYou will need: a test tube rack, five test tubes, 50cm3 of dilute hydrochloric acid, samples of magnesium, aluminium, zinc, iron, copper and tin, Bunsen burner, heatproof mat, 2 or 3 splints.

MethodPut about 5cm3 of hydrochloric acid in each test tube. To each test tube add a sample of a metal. Observe what happens. Carry out the test for hydrogen gas.

ResultsMetal Observations

Magnesium

Zinc

Iron

Tin

Copper

Aluminium

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Conclusion

1. Write reaction equations for all the reactions you observed.

2. Explain how you tested for hydrogen gas.

3. Write a general equation for the reaction between an acid and a metal.

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2. Reactions of acids with metal carbonates

A metal carbonate is a metal atom joined with at least one carbon atom and three oxygen atoms.

ApparatusYou will need: a test tube rack, eight test tubes, 50 cm3 of dilute hydrochloric acid, samples of calcium carbonate, copper carbonate, sodium carbonate and sodium hydrogencarbonate, delivery tube and bung, 50cm3 of limewater.

Method1. Put two spatulas of one of the carbonate samples into a test tube.2. Into a second test tube add 5cm3 of limewater.3. Pour 5cm3 of dilute hydrochloric acid into the tube with the carbonate

sample. Quickly attach the delivery tube to this test tube.4. Bubble the gas produced through the test tube containing the limewater.5. Repeat for the other carbonate samples using fresh tubes.6. Record your observations in the table below.

Apparatus (Draw a diagram below)

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Results

Carbonate Observations

Calcium carbonate

Copper carbonate

Sodium hydrogencarbonate

Sodium carbonate

Conclusion

1. Write out reaction equations for each of the reactions you have carried out:

2. What does the limewater test for?

3. Write out a general equation for the reaction of acids with carbonates.

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Acid Rain

Rain is naturally slightly acid. It slowly attacks stone, paint and metalwork on buildings. Over the past two hundred years, the problem has got worse. This is because pollutants in the air have been dissolving in rain and making the acid stronger. Acid rain can damage trees and other plants. It can reach streams, rivers and lakes where it can poison fish and other forms of life.

1. How can acid rain get into a tree?

2. How can acid rain become concentrated in a lake?

The pH of pure water is 7. Pure rainwater has a pH of 5.5. Acid rain can vary from a pH of 5 to as low as pH 1. Remember that a pH of 1 is as acidic as hydrochloric acid.

You have been given four samples of water. Using Universal indicator measure their pH and fill in the results below:

Distilled water: Rainwater:

Tapwater: Pondwater:

In areas of high pollution, the damage to stonework is much worse than elsewhere. Stone like marble and limestone is especially at risk. These stones are made from calcium carbonate, which reacts readily with dilute acid.

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Tree damage

If a tree starts to die, it is not easy to tell whether acid rain is the cause. The photograph shows trees that may have been damaged by acid rain.

1. Describe the appearance of the trees.

2. Describe how you would expect normal trees to look after a long, warm, dry summer. How does this compare with the signs of damage by acid rain?

3. Why is it difficult for scientists to estimate the true scale of damage by acid rain pollution?

Looking for causesScientists think that the burning of fossil fuels causes acid rain. Power stations that burn coal and oil give out a gas called sulphur dioxide. Oil and coal both contain sulphur as an impurity. When the fuel burns the sulphur reacts with oxygen.

Sulphur + oxygen sulphur dioxide

Car exhausts give out other gases called nitrogen oxides. Both of these chemicals dissolve in rainclouds to form sulphuric and nitric acids. These then fall as rain.

Use the map below and an atlas to answer the questions.

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Year 7 Topic Five: Chemical Reactions 31


1. Label the following countries or regions on the map: UK, France, Germany, Poland, Norway, Sweden Spain, Switzerland, Romania, Serbia, Croatia, Bosnia.

2. List the countries which produce the greatest amounts of sulphur dioxide.

3. List the countries which are most affected by acid rain.

4. How do you think pollutants from one country could affect the rain in another?

5. Explain whether you think that industrial pollution could be responsible for the damage to forests, lakes and streams.

6. What type of damage from acid rain is not shown on the map?

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Types of Reactions 1 – Thermal Decomposition

Chemical reactions are categorised into several types. You should be able to recognise them and be able to apply the ideas to reactions you may not have seen before. You should also be able to recognise whether the reactions are useful or harmful.

Thermal decomposition

If a new material is formed as a result of a material breaking down when heated the chemical reaction is called thermal decomposition. Several (but certainly not all!) materials will undergo thermal decomposition.

Practical – Investigating thermal decomposition

1. Thermal decomposition of hydrated crystals

ApparatusYou will need the following apparatus: one test tube, Bunsen burner, test tube holder, heatproof mat, sample of copper sulphate.

Method1. Place three or four spatulas of copper sulphate into a boiling tube.2. Using a roaring Bunsen flame heat the sample, holding the tube with the test

tube holder!3. Observe the tube carefully.4. You will observe Dr H heating cobalt chloride.

Diagram

33


Results

Conclusion

1. Write out reaction equations for the two reactions.

2. What is meant by the term ‘anhydrous’?

3. Why can anhydrous cobalt chloride or anhydrous copper sulphate be used to test for water?

2. Thermal decomposition of metal carbonates

ApparatusYou will need: six test tubes, delivery tube, 50cm3 of limewater, Bunsen burner, heatproof mat, test tube holder, samples of calcium carbonate, copper carbonate, sodium carbonate.

Method1. Put a 5 or 6 spatulas of calcium carbonate into a test tube.2. Attach the delivery tube to the tube.3. Put 5cm3 of limewater into a second test tube.4. Strongly heat the calcium carbonate with the Bunsen.5. Allow any gas produced to bubble through the test tube of limewater.

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6. Repeat with sodium carbonate and copper carbonate. WHEN HEATING THE COPPER CARBONATE TAKE ESPECIAL CARE – IT DOESN’T NEED TO BE STRONGLY HEATED!

Diagram

Results

Carbonate Observations

Calcium carbonate

Sodium carbonate

Potassium carbonate

Copper carbonate

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Conclusion

Write out reaction equations where appropriate.

3. Thermal decomposition of potassium permanganate (demonstration)

Purple potassium permangante (VII) decomposes when heated in a boiling tube to produce two solids and a gas.

Diagram

Results

36


Conclusion

1. Write out the reaction equation.

2. How do we test for the presence of oxygen gas?

37


Limestone

Many millions of years ago a lot of Britain was submerged beneath a shallow warm sea. The weather would be described as tropical and would have been similar to that of the Caribbean today. The conditions were ideal for the growth of tiny organisms called diatoms. These are tiny sea creatures that are single celled but develop a minute shell made of calcium carbonate. When these creatures died they fell to the bottom of the sea where they were eventually covered by diatoms that died after them. Millions upon millions of these tiny creatures died forming thick layers of calcium carbonate beneath the sea. Eventually conditions changed and layers of sand fell on top of the calcium carbonate.

With time these layers of chalk were exposed to the air by erosion of the ground above and uplifting caused by the movements of the Earth’s crust. Sometimes little had happened to the calcium carbonate and it can now be found as chalk, examples of which can be found at Dover and all along the SE Coast of England. Sometimes layers lying above it squashed the calcium carbonate, this caused it to turn into limestone, examples of which can be found in Yorkshire.

Both limestone and chalk are known as sedimentary rocks as they are formed from sediments. They can both be turned into the rock marble if they are exposed to heat, as well as pressure. Marble is known as a metamorphic rock as it has changed from one form to another.

Uses of LimestoneLimestone has many uses. It was first used as a building material. York Minster Cathedral was built of limestone.

Chemical reactions are able to turn limestone into an even more useful material. By carrying out thermal decomposition you can produce carbon dioxide. This leaves a white powder called quicklime. Quicklime (an alkali) is spread on soil by gardeners and farmers to combat acid conditions, since plants prefer alkali soils. Quicklime is also one of the ingredients of cement. Quicklime is used to make limewater. This is done simple by adding the quicklime to excess water and then filtering. You are going to try to make your own limewater!

On the next page describe what you did using diagrams if you like. Explain your results and observations – use the usual sub-headings!

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Making Limewater

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Types of Reaction 2

Oxidation and Reduction

Simple Oxidation

Oxidation is simply chemically joining oxygen atoms to atoms of another element. Combustion is an example of oxidation as oxygen atoms combine with carbon atoms and hydrogen atoms to make carbon dioxide and water.

MagnesiumDiagram:

Describe what you observe when magnesium is reacted with oxygen:

Write a reaction equation:

IronDiagram:

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Describe what you observe when iron is reacted with oxygen:


CarbonDiagram

Describe what you observed:


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CopperDiagram

Describe your observations

Write a word equation for the reaction

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ReductionReduction is the opposite of oxidation; it is the removal of oxygen from a compound. When this happens the compound containing oxygen is reacted another compound. The second compound gains the oxygen – so it becomes oxidised! Whenever reduction occurs, so does oxidation, this means that it is more usual to call these reactions ‘redox’ reactions.

The reaction between copper oxide and carbon

1. Collect a boiling tube, test tube holder, goggles, Bunsen burner and heatproof mat.

2. Into the boiling tube put 5 spatulas of copper oxide powder.3. Now add 10 spatulas of carbon powder. Mix the powders thoroughly.4. Heat the mixture strongly.

Diagram

Results

Conclusion

1. Write a reaction equation

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2. The reaction between copper oxide and carbon works because carbon is a much more reactive element than copper. The carbon is able to ‘grab’ the oxygen from the copper. You can also use carbon to react with lead oxide to extract lead. Suggest a reaction equation for the reduction of lead oxide with carbon:

3. (a) In your experiment, which substance has been oxidised?

(b) In your experiment, which substance has been reduced?

INFORMATION: Oxides of elements that are more reactive than carbon are able to hold onto their oxygen, rather than give it away. So you can’t use carbon to extract aluminium from aluminium oxide (bauxite).

4. Malachite is copper ore. An ore is a mineral that occurs naturally in the Earth’s crust and contains enough metal to make it worthwhile extracting the metal from it. Malachite is mostly copper carbonate. The Ancient Egyptians were able to extract copper from it – can you suggest how they did it? (Hint: Think about thermal decomposition and ‘redox’)

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Rusting

IntroductionWe have observed several useful chemical reactions. However, not all reactions are useful. A particularly annoying reaction is rusting. This is a form of oxidation that affects the element iron. Since iron is used for many jobs (e.g. building, motor cars etc.) it is very important to understand what causes rusting. In this practical you will observe the conditions that are required to make iron rust and then suggest ways of stopping rusting.

MethodSix conical flasks were set up. Each flask was filled differently:

Flask 1: Nail in a very damp atmosphereFlask 2: Nail half in and half out of waterFlask 3: Nail submerged in tap waterFlask 4: Nail submerged in boiled waterFlask 5: Nail in salty waterFlask 6: Nail in flask with a drying agent (silica gel)

Explain what each treatment is doing to the nail:

Diagram

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ResultsFlask Treatment Observations

1 Very damp atmosphere

2 Nail half in water

3 Nail submerged in water

4 Nail submerged in boiled water

5 Nail in salty water

6 Nail and silica gel

Conclusion1. What two substances must be present in order for rusting to occur? Which

tubes demonstrate this?

2. Write a reaction equation for rusting.

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3. How would you prevent iron objects from rusting?

More on oxidation

Prevention of Rusting

To stop iron from reacting with oxygen and water and producing rust, it is necessary to coat the metal with a non-reactive barrier. This can be done in several ways:

1. Covering the iron with oil or paint. This will prevent air and water from reaching the iron.

2. Coat the iron with a more reactive metal. The more reactive metal will react with oxygen to form a very stable (non-reactive) oxide which protects the iron from attack. Zinc is often used to coat iron. If iron is coated in zinc it is said to be galvanised.

3. Mixing the iron with another metal can make an alloy that does not rust as easily as pure iron. Adding chromium to iron makes stainless steel.

Food spoilage

Bacteria or other micro-organisms can make food unpleasant or poisonous to eat. The poisons are often waste products excreted by micro-organisms when they feed. However, food spoilage is not always due to micro-organisms. Oxygen in the air can react with food. There are two ways that such spoilage can be prevented. Firstly, food can be packaged in a ‘protective atmosphere’. Foods such as crisps are put into bags in rooms full of nitrogen – nitrogen is very unreactive. Secondly, antioxidants are added to foods. These chemicals react with oxygen very easily, so they remove the oxygen before it can react with the food.

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Using rusting to calculate the percentage of oxygen in the air

The process of rusting can be used to calculate how much oxygen is in a sample of air.

MethodA gas syringe is filled with 100cm3 of atmospheric air. The syringe is then connected by a short delivery tube to an ignition tube (a small test tube). This tube has a few drops of water in it and some fine iron filings. The tube is swirled so that the iron filings and water cover as much of the surface of the tube as possible. This means there is a big surface area of iron and water to react with any oxygen in the atmosphere, so the experiment will be over quicker. The apparatus is left for four days.

Draw a diagram of the apparatus used below:

Results

Conclusion

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Combustion

Combustion is the process by which heat (and often light) energy is released from a fuel. It is a form of oxidation as combustion can only occur in the presence of oxygen.

Demonstration: To show that the products of combustion are carbon dioxide and water.

Method:Draw a fully labelled diagram of the apparatus used and explain how it works.

Results:What do you observe?

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Conclusion:1. How do we know that combustion produces carbon dioxide and water?

2. Write a general reaction equation for combustion.

3. What is a hydrocarbon?

4. Give 5 examples of fuels.

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Types of Reactions 3

Displacement Reactions

IntroductionIn a displacement reaction a more reactive metal displaces a less reactive metal from a compound. Metals can be placed in an order of reactivity called the reactivity series. The most reactive metals are found at the top of the reactivity series, whilst the least reactive are found at the bottom.

If a piece of metal is dropped into a solution of another metal salt then a chemical reaction may occur. The reaction will only occur if the metal dropped into the solution is more reactive than the metal that is in the metal salt.

This practical investigates displacement reactions

Method1. Collect samples of the following metals: magnesium, zinc, iron, copper2. Collect four test tubes3. Using the table in the results section, select a metal salt solution and place

about 5cm3 of solution in each tube4. Record your observations in the results table5. Repeat for another salt solution, until your table is complete6. There will also be a demonstration of how these four metals react with silver

nitrate make a note of these results as well.

Results

Magnesium sulphate

Zinc sulphate

Iron sulphate

Copper sulphate

MagnesiumNo reaction

ZincNo reaction

IronNo reaction

CopperNo reaction

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Silver Nitrate and copper demonstrationWrite out your observations here:

Conclusion1. From your results, put the metals into an order of reactivity.

2. Write reaction equations for all the appropriate combinations.

3. What can you say about the reactivity of silver?

4. Draw a diagram to show what is happening to the atoms when an iron nail is dropped into a solution of copper sulphate.

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Exothermic and Endothermic Reactions

Whenever chemical reactions occur, energy is usually transferred to or from the surroundings.

In an exothermic reaction, heat is given out. This means that the temperature will rise.

1. Burning fuels – combustion releases a lot of heat!2. Neutralisation reactions – if you add acid to an alkali, you can often feel heat

being released3. Adding water to anhydrous substances (e.g. anhydrous copper sulphate)

Write down three reaction word equations that demonstrate exothermic reactions:

1.

2.

3.

In an endothermic reaction, heat is taken in. This means that temperatures will fall.

1. Photosynthesis is endothermic – energy is taken in from the Sun2. Dissolving some salts in water makes the temperature fall e.g. ammonium

nitrate3. Thermal decomposition – heat has to be supplied to make the compound

decompose e.g. calcium carbonate must be heated to 800oC

Try this:

Put two spatulas of sodium hydrogencarbonate into a test tube. Add six spatulas of citric acid. Add three or four drops of distilled water. Gently stir the mixture using a thermometer. Note what happens to the temperature of the tube.

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More on the reactivity of metals

Displacement reactions are a good way of examining how reactive the metals are. However, there are three other ways of studying them. Firstly, you can see how they react with air; secondly, with water; and thirdly, with dilute acid.

Reactions with airMost metals will lose their shiny surface over a period of time; they are said to tarnish. The dull finish that they get is due to a layer of oxide that forms.If gold is heated in air, it will not undergo any change. Metals such as lead and zinc react slowly when heated in air. Reactive metals, like magnesium and sodium, burn easily with a bright flame if heated in air.

Remember that burning in air is oxidation, so typical reaction equations are:

Sodium(s) + oxygen(g) Sodium oxide(s)

Magnesium(s) + oxygen(g) Magnesium oxide(s)

Reactions with waterIf a metal reacts with water, it will always release hydrogen. Potassium, sodium and lithium all react readily with cold water. Magnesium reacts with steam. Lead, copper, silver and gold will not react with water at all.

Potassium(s) + water(l) potassium hydroxide(aq) + hydrogen(g)

Reactions with acidWe observed the reactions of metals with acids earlier in this unit. If a metal is more reactive than the gas hydrogen, then it will displace the hydrogen from solution to form a metal salt. The hydrogen bubbles off (and can be detected using the squeaky pop test!).

The Reactivity Series

Very Potassium These metals are hard toReactive Sodium extract from their ores.

Lithium Electrolysis is used.Calcium

Fairly MagnesiumReactive Aluminium

(Carbon) Metals below carbonZinc are extracted from their ore

Not very Iron using reduction with charcoal.Reactive Lead

(Hydrogen) Metals belowCopper hydrogen do not react with

Not reactive Silver acidGoldPlatinum

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Documents

Symbols for elements - Wikispaces Web viewBe able to write simple word equations that describe chemical reactions. ... Molybdenum. 42. 98. Tc. Technetium. 43. 101. Ru