Cracking liquid paraffin

Transcription

1 Cracking liquid paraffin Teachers and Technicians notes Specification references: C1.5.1 Obtaining useful substances from crude oil (a) (b) (d) Hydrocarbons can be cracked to produce smaller more useful molecules The products of cracking include alkanes and unsaturated hydrocarbons called alkenes Alkenes react with bromine water, turning it from orange to colourless Learning objectives To crack liquid paraffin To test for unsaturation using bromine water Description of activity This investigation into cracking liquid paraffin can either be carried out as a demonstration or for selected groups of more competent students as a practical activity. A suggested method for the practical is provided on the accompanying worksheet. How Science Works The following aspects can be addressed: Managing risks (4.2.1b) Making observations from first hand evidence of an object or an event (4.3.1a) Choosing the most appropriate equipment or technique for the task (4.3.2a) Choosing the most appropriate form of presentation (4.4.2c) Resources Safety screen (for demonstration) Pyrex boiling tube Delivery tube and bung Bunsen valve to fit delivery tube 4 test tubes and bungs Test tube rack Trough or plastic bowl Bunsen burner and heat proof mat Clamp and stand Crucible Small pieces of porous pot/pumice stone Dropping pipettes Wooden splints Liquid paraffin (medicinal quality) Mineral wool 0.01 mol dm -3 bromine water Page 1 of 5

2 Practical tips The set-up of the apparatus is illustrated on the students worksheet. Expected results Paraffin Gaseous Product Appearance Colourless liquid Colourless gas Smell None Slightly sweet smell (may resemble petrol) Lit splint No effect (difficult to set alight unless a wick is used or the spill is held over the paraffin for a period of time) Burns with a yellow flame (sometimes smoky) Bromine water No reaction Decolourised The properties of medicinal paraffin surprise students, as it does not have a smell, nor does it burn easily. However paraffin is a general name given to alkanes, with medicinal paraffin containing heavier alkanes above C 20. The fuel that is commonly known as paraffin is actually kerosene (C 11 C 15 ). This is a difficult practical for students to carry out safely and correctly. As such it should be done as a demonstration with groups who are not practically very competent and/or aware of and able to deal with safety issues. Small pieces of broken crucibles and evaporating basins can be used as porous pot. It needs to be heated very strongly in order for the reaction to take place. Bunsen valves help prevent suck back' occurring. A thin 1cm long slit is cut in a length of clean rubber tubing. One end of the tubing is placed at the end of the delivery tube and a small length of glass rod or wood is used as a stopper at the other end of the rubber tubing. It needs to be emphasised to students that they must continually heat the porous pot until the delivery tube has been removed from the water to prevent suck back occurring. Suck back of water into the hot boiling contents of the boiling tube may cause it to crack. When demonstrating the practical it is therefore advisable to use a safety screen in front of the apparatus. Health and safety Wear eye protection Safety screen for demonstration Liquid paraffin harmful (CLEAPSS Hazcard 45B) Bromine water harmful (CLEAPSS Hazcard 15) Mineral wool (CLEAPSS Hazcard 86) Timing Approximately 15 minutes if demonstrating the practical and 30 minutes if students are carrying out the experimental procedure Different approaches Support students by carrying out this experiment as a demonstration. Extend students by getting them to research how cracking is carried out at a refinery. Page 2 of 5

3 Answers to evaluation questions on the students' worksheet 1 It contains air which was in the apparatus at the start of the practical, not the gas formed 2 To prevent sucking back 3 As a catalyst 4 A gas was formed. 5 Yes, because there were no other reactants the product(s) must also be hydrocarbons. 6 The product is more reactive. It decolourised bromine and also burnt in air. Paraffin did neither. 7 The gaseous product is unsaturated/an alkene. It decolourised bromine water which is the test for unsaturation/alkenes Answer to extension question Cracking is used at an oil refinery to break down larger hydrocarbons into smaller more useful hydrocarbons. There is more demand for smaller hydrocarbons such as octane and ethene than there is supply, and so larger hydrocarbons, which are less in demand but have a larger supply than needed, are broken down to form smaller hydrocarbons. Page 3 of 5

4 Cracking liquid paraffin Students' worksheet Aims To crack paraffin to form smaller hydrocarbons To test for unsaturation using bromine water Method 1 Wear eye protection. 2 Push mineral wool to the bottom of a boiling tube (approximately 5 cm depth). 3 Use a dropping pipette to measure 2 cm 3 of liquid paraffin. Hold the pipette just above the mineral wool and add the paraffin so that it soaks into the mineral wool. 4 Clamp the boiling tube at the open end, so that it is pointing slightly upwards. 5 Fill the centre of the boiling tube with porous pot. 6 Fit the delivery tube and bung into the boiling tube. 7 Place the end of the delivery tube with the Bunsen valve into a plastic bowl or trough which has sufficient water to easily cover the end of the Bunsen valve. 8 Strongly heat the porous pot until it glows red. 9 Heat the paraffin and mineral wool for a few seconds. 10 Heat the porous pot again. Occasionally heat the paraffin for a few seconds but mainly continue to heat the porous pot. 11 Fill a test tube full of water and place over the Bunsen valve to collect the gas by displacement of water. Discard the first test tube once it is filled. 12 When the next test tube is full of gas, keep the end of the test tube under water and put a bung in it. 13 Remove the test tube from the water and place in the test tube rack. 14 Continue heating the porous pot until three test tubes of gas have been collected. Remove the delivery tube from the water. Once this has been done, the porous pot no longer needs to be heated. Page 4 of 5

5 Testing paraffin and the product of the reaction 1 Continue to wear eye protection. All tests should be carried out on both paraffin and the gaseous product. 2 Carefully smell the gas produced. Hold the test tube away from yourself, take the bung out and gently waft the gas towards you. Repeat for paraffin. 3 Take a second test tube of gas, remove the bung and hold a lit spill over the top of the test tube. 4 Place a few drops of paraffin in a crucible and hold a lit spill over it. 5 In a fume cupboard add a few drops of bromine water to a third tube of gas. Replace the bung and shake well. Repeat for paraffin. Health and safety Wear eye protection Liquid paraffin harmful Bromine water harmful Mineral wool - care Results Design a suitable results table to show your observations. Remember to include the appearance of both paraffin oil and the product of the reaction. Evaluation 1 Why was the first test tube of gas not used? 2 Why is the delivery tube removed from the water at the end of the practical? 3 What is the porous pot used as? 4 What evidence is there that the product molecules are smaller than the reactant molecules? 5 Paraffin is a hydrocarbon. Is the gaseous product a hydrocarbon? Explain your answer. 6 Which is more reactive, the gaseous product or the paraffin? Use evidence from your investigation to explain your answer. 7 Bromine water tests for alkenes/unsaturation. Is the gaseous product or paraffin an alkene? Explain your answer. Extension work Explain how a similar reaction is used at an oil refinery. Why is the reaction important? Page 5 of 5