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3 of 55© Boardworks Ltd 2007 What is a chemical reaction? A chemical reaction is a change that takes place when one or more substances (called reactants) form one or more new substances (called products). reactantsproducts For example: carboncarbon dioxideoxygen + +
4 of 55© Boardworks Ltd 2007 Reactions all around us How many chemical reactions go on around us everyday?
5 of 55© Boardworks Ltd 2007 How can you spot a chemical reaction? Chemical reactions can appear very different. As you observe a chemical reaction, you may detect: a colour change precipitate (solid) forming energy being produced (fizzing, burning) an odour being produced.
6 of 55© Boardworks Ltd 2007 Why doesn’t the mass change? In a chemical reaction, no atoms are made or destroyed. The reaction just changes how the atoms are bonded together.
7 of 55© Boardworks Ltd 2007 Does mass change during a reaction?
8 of 55© Boardworks Ltd 2007 What is a word equation? For example, when a piece of sulfur is burned in oxygen gas it produces a white solid called sulfur dioxide. A word equation uses the names of the reactants and products to show what happens in a chemical reaction. + sulfuroxygen sulfur dioxide The word equation for this reaction is:
9 of 55© Boardworks Ltd 2007 What is a symbol equation? A symbol equation uses the formulae of the reactants and products to show what happens in a chemical reaction. This equation shows that one atom of sulfur (S) reacts with one molecule of oxygen (O 2 ) to make one molecule of sulfur dioxide (SO 2 ). A symbol equation must be balanced to give the correct ratio of reactants and products. + SO2O2 SO 2 +
10 of 55© Boardworks Ltd 2007 What do state symbols show? State symbols are added to a symbol equation to show whether the reactants and products are: With state symbols in place, this symbol equation now shows that the sulfur is a solid, the oxygen is a gas and the sulfur dioxide is also a gas. solid – symbol is (s) liquid – symbol is (l) gas – symbol is (g) dissolved in water – symbol is (aq). + S (s) O 2 (g) SO 2 (g)
11 of 55© Boardworks Ltd 2007 Chemical reactions – true or false?
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13 of 55© Boardworks Ltd 2007 What is thermal decomposition?
14 of 55© Boardworks Ltd 2007 Thermal decomposition – easy or hard? The more reactive a metal, the harder it is to decompose its carbonate by heating. increase in reactivity Predict how easy it is to decompose these carbonates: copper carbonate calcium carbonate sodium carbonate manganese carbonate zinc carbonate iron carbonate. potassium sodium calcium magnesium aluminium manganese zinc iron copper silver gold
15 of 55© Boardworks Ltd 2007 Thermal decomposition – activity
16 of 55© Boardworks Ltd 2007 Thermal decomposition of carbonates FeOFeCO 3 + CO 2 iron oxide iron carbonate + carbon dioxide heat What are the word and symbol equations for the thermal decomposition of these carbonates? When the metal carbonate is heated, it decomposes to produce a metal oxide and carbon dioxide. copper carbonate manganese carbonate zinc carbonate.
17 of 55© Boardworks Ltd 2007 When calcium carbonate is heated, it decomposes to produce calcium oxide and carbon dioxide. This reaction is carried out in industry to make calcium oxide (quicklime) from calcium carbonate (limestone): Calcium oxide is used to make concrete and calcium hydroxide (slaked lime). Heating calcium carbonate CaOCaCO 3 + CO 2 calcium oxide calcium carbonate + carbon dioxide heat
18 of 55© Boardworks Ltd 2007 Baking powder and self-raising flour contain sodium hydrogencarbonate (also known as sodium bicarbonate). What effect does this reaction have on dough as it is being baked? When sodium hydrogencarbonate is heated, it decomposes to make sodium oxide, carbon dioxide and water. The carbon dioxide released during the reaction creates tiny bubbles, which help the dough to rise. Heating sodium hydrogencarbonate
19 of 55© Boardworks Ltd 2007 What is the decomposition equation?
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21 of 55© Boardworks Ltd 2007 What are indicators? Indicators are chemicals that change colour in the presence of an acid or an alkali. There are many different indicators. Lots of them come from plants, like red cabbage. Different indicators turn different colours. Universal indicator is a mixture of indicators. It is useful because it shows a range of colours from pH 1–14. Geranium plants grown in acidic soil have red flowers, while geraniums grown in alkali soil have blue flowers.
22 of 55© Boardworks Ltd 2007 What does the pH scale show? The pH scale runs from 1 to 14. The numbers 1 to 6 represent acidic conditions and 8 to 14 represent alkali conditions. A pH value 7 is neutral What are the pH values of some everyday items?
23 of 55© Boardworks Ltd 2007 What is the order of pH?
24 of 55© Boardworks Ltd 2007 What are acids? Acids are substances that: Hydrochloric acid is a strong acid. Ethanoic acid (vinegar) is a weak acid. Have a pH below 7 and turn universal indicator yellow, orange or red. Turn litmus red. Form solutions containing hydrogen ions (H + ).
25 of 55© Boardworks Ltd 2007 What are alkalis? Have a pH above 7 and turn universal indicator blue or purple. Alkalis are substances that: Sodium hydroxide is a strong alkali. Ammonia is a weak alkali. When ammonia is dissolved in water, it forms ammonium hydroxide (NH 4 OH), a fertilizer. Turn litmus blue. Can neutralize acids Form solutions containing hydrogen ions (H + ).
26 of 55© Boardworks Ltd 2007 What are bases? All alkalis are bases. Bases are substances that react with acids by absorbing hydrogen ions (H + ). Some bases are soluble in water – these are called alkalis. All alkalis contain hydroxide ions (OH – ). The more OH – ions in the solution, the stronger the alkali. The oxides, hydroxides and carbonates of metals, such as sodium hydroxide, are bases. Ammonia is a base that does not contain a metal. alkalis (soluble bases) bases (react with acids)
27 of 55© Boardworks Ltd 2007 Acids and bases – true or false?
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29 of 55© Boardworks Ltd 2007 + acidbasea salt How are salts made and named? When an acid reacts with a base, a neutralization reaction occurs and produces a chemical called a salt. The name of the salt depends on the names of the reactants. The first part of the salt’s name comes from the base: e.g. sodium hydroxide sodium… The second part of the salt’s name comes from the acid: e.g. sulfuric acid sulfate… For example, if sodium hydroxide neutralizes sulfuric acid, the product is a salt called sodium sulfate.
30 of 55© Boardworks Ltd 2007 What is the name of the salt?
31 of 55© Boardworks Ltd 2007 What are salts used for? Table salt is sodium chloride. This is the salt used to flavour and preserve food. Indigestion remedies often contain magnesium salts. Salts can also be used as coloured pigments in paints, and to help fuels burn better. The colours of fireworks are formed when certain salts burn. Calcium chloride, for example, burns a bright red colour.
32 of 55© Boardworks Ltd 2007 Water is formed because OH – ions from the alkali react with H + ions from the acid to produce molecules of water (H 2 O). Making salts 1: acid + alkali When an acid reacts with an alkali, the products are a salt and water. acidalkalisaltwater + + For example: ++ ++ sodium hydroxide NaOH (aq) sodium chloride NaCl (aq) water H 2 O (aq) hydrochloric acid HCl (aq)
33 of 55© Boardworks Ltd 2007 When is the reaction complete? There is no obvious sign when this reaction is complete, so an indicator is used to show when the solution is neutral. This process is called titration. The reaction between sodium hydroxide and hydrochloric acid produces sodium chloride, which is soluble in water. ++ ++ sodium hydroxide NaOH (aq) sodium chloride NaCl (aq) water H 2 O (aq) hydrochloric acid HCl (aq)
34 of 55© Boardworks Ltd 2007 What is a titration?
35 of 55© Boardworks Ltd 2007 Making salts 2: acid + metal When a metal is added to an acid, the products are a salt and hydrogen gas. For example: This method of making salts only works with some metals. hydrochloric acid 2HCl (aq) + zinc Zn (aq) zinc chloride ZnCl 2 (aq) hydrogen H 2 (aq) acidmetala salthydrogen + + What would happen if potassium was used? Or copper?
36 of 55© Boardworks Ltd 2007 Which metal will react the most?
37 of 55© Boardworks Ltd 2007 Making salts 3: acid + metal oxide When a metal oxide is mixed with an acid, the products are a salt and water. calcium chloride + water calcium oxide CaO (aq) hydrochloric acid 2HCl (aq) CaCl 2 (aq) H 2 O (aq) For example: acidmetal oxidesaltwater + + Water forms because oxygen ions from the oxide join up with H + ions from the acid to produce molecules of water (H 2 O).
38 of 55© Boardworks Ltd 2007 Obtaining salts from metal oxides When a metal oxide is added to an acid, it dissolves as it reacts. How could the solid copper sulfate salt be separated from the water? You know when you have added enough of the metal oxide because it stops dissolving. ++ copper oxide CuO (s) sulfuric acid H 2 SO 4 (aq) copper sulfate CuSO 4 (aq) water H 2 O (aq)
39 of 55© Boardworks Ltd 2007 Obtaining salts from copper oxide
40 of 55© Boardworks Ltd 2007 Making salts 4: acid + carbonate When a carbonate is mixed with an acid, the products are a salt, carbon dioxide and water. copper nitrate + water + carbon dioxide copper carbonate CuCO 3 (s) nitric acid 2HNO 3 (aq) Cu(NO 3 ) 2 (aq) H 2 O (aq) + CO 2 (g) For example: What would you expect observe in this reaction? acidcarbonatesaltwater + + carbon dioxide + Water and carbon dioxide are formed because the carbonate ions (CO 3 2- ) react with H + ions from the acid.
41 of 55© Boardworks Ltd 2007 Making salts – summary Salts can be made by reacting acids with bases. acid + metala salt + hydrogen acid + basea salt + water acid + carbonatea salt + carbon dioxide + water There are four ways of making salts from acids:
42 of 55© Boardworks Ltd 2007 Matching reactants and salts
43 of 55© Boardworks Ltd 2007 Complete the neutralization reaction
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45 of 55© Boardworks Ltd 2007 Sometimes when two solutions are mixed, they react to form an insoluble solid product. For example, mixing solutions of lead nitrate and sodium chloride produces a yellow precipitate of lead chloride. What is a precipitation reaction? The insoluble solid product is called a precipitate. You can spot a precipitate because the mixture goes cloudy. lead nitrate Pb(NO 3 ) 2 (aq) sodium chloride 2NaCl (aq) lead chloride PbCl 2 (s) sodium nitrate 2NaNO 3 (aq)
46 of 55© Boardworks Ltd 2007 How can hard water be softened? Hard water contains magnesium and calcium ions, which can clog up pipes and heating elements. These ions can be removed using precipitation reactions. The magnesium ions are removed by this reaction: What are the word and symbol equations for the removal of the calcium ions? Sodium carbonate is added to the hard water to form precipitates, which can then be removed by filtration. sodium carbonate Na 2 CO 3 (aq) magnesium chloride MgCl 2 (aq) sodium chloride 2NaCl (aq) magnesium carbonate MgCO 3 (s) + + + +
47 of 55© Boardworks Ltd 2007 Many metals form hydroxide precipitates, which have characteristic colours. Precipitates of hydoxides For example, when sodium hydroxide solution is added to a solution of iron(III) chloride, the reaction produces a brown precipitate of iron(III) hydroxide. ++ FeCl 3 (aq)3NaOH (aq)Fe(OH) 3 (s) ++ 3NaCl (aq) iron(III) chloride sodium hydroxide iron(III) hydroxide sodium chloride
48 of 55© Boardworks Ltd 2007 Identifying precipitates
49 of 55© Boardworks Ltd 2007 Identifying metals
50 of 55© Boardworks Ltd 2007 Precipitation – true or false?
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52 of 55© Boardworks Ltd 2007 Glossary acid – A substance that releases H + ions in solution. alkali – A substance that releases OH - ions in solution. base – Any substance that reacts with an acid. decomposition – A type of reaction in which a compound is broken down into two or more substances. neutralization – A type of reaction in which an acid reacts with a base to produce a salt. precipitate – A solid, insoluble product of a reaction. precipitation – A type of reaction in which two aqueous solutions react to form an insoluble product. salt – A substance formed when an acid reacts with a base. titration – A method of indicating when an undetectable reaction, such as neutralization, is complete.
53 of 55© Boardworks Ltd 2007 Anagrams
54 of 55© Boardworks Ltd 2007 Completing equations
55 of 55© Boardworks Ltd 2007 Multiple-choice quiz
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