29H Using Chemistry Contents Chemical reactions Burning fuels New materials – everyday reactionsNew materials – industrial processesSummary activities
3Chemical reactions everywhere! Chemical reactions take place around you and inside you all the time.How many reactions can you name?Burning, rusting, cooking and living all involve reactions.In a chemical reaction, reactants are changed into products and new materials are formed.Some chemical reactions are carried out to make new materials and some reactions are used to release energy.Are chemical reactions always useful?
79H Using Chemistry Contents Chemical reactions Burning fuels New materials – everyday reactionsNew materials – industrial processesSummary activities
8Using combustionCombustion is the chemical reaction that takes place whena substance burns.The substance reacts with oxygen and energy is released as heat and light.Combustion is an important reaction as more than 90% ofthe world’s energy comes from burning fossil fuels like coal, natural gas and petrol.Where do fossil fuels come from and what other fuels are there?
13Equations for combustion Combustion, as the reaction of substance with oxygen, is also called oxidation. The products of oxidation are oxides.Coal is mostly carbon. The product of burning coal is a gas which turns limewater cloudy. What is the equation for burning coal?carbon dioxideoxygencarbonMany fuels are hydrocarbons, which means they are made up of carbon and hydrogen. When hydrocarbons burn carbon dioxide and water (hydrogen oxide) are produced.What is the equation for burning the hydrocarbon methane?carbon dioxideoxygenmethanewater
14Incomplete combustion A good supply of oxygen is needed for a fuel to burn completely and release as much energy as possible.When carbon reacts completely with oxygen, it is all turned into carbon dioxide. This is called complete combustion:carbon dioxideoxygencarbonIf there is not enough oxygen, a fuel cannot burn completely and less energy is released. Some fuel is wasted.When carbon does not react completely with oxygen, the product is carbon monoxide, a colourless poisonous gas. This is called incomplete combustion:carbon monoxideoxygencarbon
159H Using Chemistry Contents Chemical reactions Burning fuels New materials – everyday reactionsNew materials – industrial processesSummary activities
16Rusting reactionRusting is the reaction of iron with oxygen in the presence of water. Salt helps to speed up the rate of this reaction.Why does iron rust more quickly at the seaside?
17iron oxide oxygen + iron Preventing rustingRusting is the reaction of iron with oxygen in the presence of water to form iron oxide (rust).iron oxideoxygen+ironRusting is an ‘unhelpful’ chemical reaction.How do these measures help to prevent rusting?Iron that is exposed to the weather is usually coated witha layer of paint.New products that contain iron are often boxed with a small packet of drying agent.The paint prevents oxygen and water getting to the iron.The drying agent absorbs any water that may cause rusting.
18Making bread and alcohol Yeast is a living organism which carries out chemical reactions that are used for making bread and alcohol.Yeast uses oxygen from the air for aerobic respiration:The carbon dioxide gas produced in this reaction makes bread rise by filling it with bubbles.energy)water+carbon dioxideoxygenglucose(+Yeast can also carry out respiration without oxygen. This is anaerobic respiration, which is also called fermentation:Fermentation is used to produce alcohol in beer and wine.energy)ethanol+carbon dioxideglucose(+
19Ripening fruitThe ripening of fruit is a complex collection of chemical reactions.Take apples as an example:Starch is broken down into sugars increasing sweetness.Acids are neutralized making the apples less sour.Chlorophyll (green) changes to anthocyanin (red).Pectin, a chemical which makes apples hard, is broken down and so makes the apples softer.Why do food producers and supermarkets need to know about the conditions and reactions involved in ripening fruit?
20Ripening fruitThe ripening of all fruit and vegetables involves similar chemical reactions.The speed of ripening is affected by the temperature and by the presence of a chemical called ethene.Food producers and supermarkets know the best conditions for slowing down or speeding up the ripening process so thatfruit and vegetables do not go off too soon.
21Cooking eggsCooking involves chemical reactions – changes colour, taste and texture due to atoms in food joining together in new ways.Cooking an egg changes it texture from runny to firm:Eggs contain a protein called albumen. The protein molecules are long chains of amino acids folded into a ball shape.When eggs are heated, some of the protein atoms break apart and the molecules unfold. These molecules then join to other nearby protein molecules until they are all linked in a network.
22Food spoilageChemical reactions are not always useful. The changes that take place as food “spoils” are caused by chemical reactions.Apples turn brown when exposed to the air because they react with oxygen in the air.Knowing what causes this reaction can help to slow it down:Placing sliced apples in water prevents browningbecause the apples are no longer exposed to air.The chemical ascorbic acid (vitamin C), found in lemon juice, is also known to prevent the browning reaction.
239H Using Chemistry Contents Chemical reactions Burning fuels New materials – everyday reactionsNew materials – industrial processesSummary activities
24Making plastics from oil Plastics are very useful materialsthat are made by chemical reactions in factories.The starting materials for making plastics come from crude oil, which is found in the Earth’s crust.When crude oil is pumped out of the ground (or sea bed) it is a mixture of chemicals called hydrocarbons.Crude oil is separated into different hydrocarbons at an oil refinery using a process called fractional distillation.
25Crude oil and fractional distillation Fractional distillation separates crude oil based on the different boiling points of the hydrocarbon molecules.Small molecules boil at lower temperatures than bigger ones.boils at a lower temperatureboils at a higher temperatureThe crude oil is heated and molecules of different sizes boil at different temperatures. These gases are collected separately and cooled, condensing to form different fractions.Some of the fractions obtained from crude oil, listed in orderof increasing boiling point, are:fuel gas, petrol, naphtha, kerosine, diesel and bitumen.Which of these fractions has the biggest molecules?
28Making plastics by polymerization The separated hydrocarbons obtained from the fractional distillation of crude oil have many uses.One of the main uses of the smaller hydrocarbon molecules obtained isto make plastics.Plastics consist of very long chains called polymers.The chemical reaction in which small molecules join together to make these very long polymer molecules is called polymerization.
30Making metals by smelting Metals are very useful materials but many metals are found in rocks as compounds called ores.Iron, which is used to make steel,is found in the earth as iron ore.By heating the iron ore (mainly iron oxide) with carbon, the iron can be extracted.iron oxide+carbonironcarbon dioxideThe carbon displaces the metal in a process called smelting.This process is the reverse of oxidation as the oxygen is taken away from the metal. It is called a reduction.In the reaction above, the iron oxide is reduced to iron.
32oxygen + aluminium aluminium oxide ElectrolysisElectrolysis is a chemical reaction that uses electricity to break up a compound and is another method for extracting a metal from its ore.For example, aluminium cannot be extracted by smelting and so electrolysis is used to aluminium metal from aluminium oxide.oxygen+aluminiumaluminium oxideDuring electrolysis, the metal is always formed as a coating on the electrode that is connected to the negative electrode (the cathode.)This is why electrolysis is also used for the purification of copper and for electroplating other metals.
349H Using Chemistry Contents Chemical reactions Burning fuels New materials – everyday reactionsNew materials – industrial processesSummary activities
35fuel – A substance that is burnt to provide energy. Glossarycombustion – The reaction of a substance with oxygen which releases heat and light energy.electrolysis – A reaction that uses electricity to break up a compound and can be used for extracting metal.fuel – A substance that is burnt to provide energy.fractional distillation – The process used to separate a mixture, such as crude oil, into its components which have different boiling points.hydrocarbons – Compounds that are made up of the elements carbon and hydrogen only.polymers – Materials, such as plastics, that are made up of very long molecules.smelting – Heating a metal ore with carbon to extract thepure metal.