Sunday, June 12, 2011

3.4 I can recall the products of complete and incomplete combustion of alkanes


Description: Machine generated alternative text: Combustion As you know, the fuels we get from crude oil are called hydrocarbons. Petrol is a mixture of hydrocarbons. Can you remember which 2 elements make up a hydrocarbon? Petrol contains octane. Can you guess how many carbon atoms there are in octane? When we burn a fuel, the reaction is called combustion. The fuel reacts with oxygen gas from the air and heat is given out. But what is made in the reaction? You can find out in the next experiment: Experiment 11.3 Products of combustion Set up the apparatus as shown Let it run until you see some changes in the U-tube and in the limewater. • What do you see inside the U-tube? • WhQt happens to the limewater? • Which gas makes limewater go milky? • Why should you repeat the experiment with just air being sucked through? • What is your conclusion? When hydrocarbons burn, they are oxidised. We get carbon dioxide and water formed (never hydrogen!): hydrocarbon + oxygen carbon dioxide + water Incomplete combustion There is only a small amount of oxygen inside a car engine. There is not enough to turn all the carbon in the hydrocarbons into carbon dioxide. Some carbon monoxide, a toxic gas, is also made. Catalytic converters on exhausts, once warmed up, can turn carbon monoxide into carbon dioxide. Sometimes fuels burn with a smoky flame. Incomplete combustion means that carbon itself is given off as soot. Particles from diesel are also polluting the air in our cities. Unburnt hydrocarbons in particulates can cause cancer. Combustion is a useful reaction - to water pump (to suck the gases through the apparatus) ti tlammable liqukis liquid fuel (such as hexane) crushed ice blue colbalt chloride and water paper (which turns pink in water) Carbon monoxiae is given on irom car exhausts. This gas reacts with haemoglobin. It stops your blood carrying oxygen around your body. This toxic gas can kill unsuspecting people when given off by faulty gas heaters. 147
Description: Machine generated alternative text: Don’t try to learn these equations! Practise working them Out for a wide range of different hydrocarbons. You will find guidance on how to work out the ethane equation in Chapter 5. page 38. - 19.4 As weÐI as all the other poisonous or cancer-causing compounds, cigarette smoke contains carbon monoxide due to incomplete combustion.
·         Saturated hydrocarbons are called ALKANES
·         Small chain ALKANES are used as fuels
3.       The reaction is called COMBUSTION REACTION
4.       The word equation for  this chemical reaction is :
fuel (E.G. PROPANE) + OXYGEN → WATER + CARBON DIOXIDE + ENERGY (heat, light, sound)
5.       If there is insufficient oxygen then the word equation is:
fuel (PROPANE) + OXYGEN → CARBON DIOXIDE + CARBON MONOXIDE + WATER + ENERGY (heat, light, sound)
6.       The problem with this reaction is that CARBON MONOXIDE is produced this gas is a POISONOUS/HARMFUL GAS to the human body (see objective 5.11)
7.       Where can you find incomplete combustion?
8.       What is used to reduce the harmful products from incomplete combustion?
9.       During incomplete combustion a smoky flame is often seen, explain this observation
10.   Describe the two chemical tests for water
11.   Describe the chemical test for carbon dioxide gas
12.   Describe a physical test for water
Extn: write a balanced equation (with state symbols) for the combustion of propane gas
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