C1.32a Cracking paraffin You are going to crack (thermally decompose) the longer-chain alkanes in liquid paraffin to produce shorterchain alkanes and alkenes. You will need to work with a partner.
Apparatus delivery tube and bung
Bunsen burner
porous pot
eye protection
dilute bromine water
liquid paraffin
clamp and stand
washing-up bowl
heat resistant mat
3 × test tubes and bungs
test-tube rack
teat pipette
boiling tube containing mineral wool
Health and safety ●
Wear eye protection
●
When you put the delivery tube into the boiling tube, hold the boiling tube near the open end in one hand, and the bung of the delivery tube in the other – make sure the bung is pushed firmly in
●
Paraffin and the product gases are harmful and highly flammable – keep them away from flames
●
Only heat the boiling tube at the bottom and in the middle – do not heat the clamp
●
Take the delivery tube out of the water before you stop heating it
●
Bromine water is harmful
Method A Put some mineral wool in the sealed end of a boiling tube. B Soak the mineral wool in paraffin using a teat pipette, but do not add more than the wool can soak up. C Clamp the boiling tube horizontally, placing the clamp at the mouth of the tube. D Put some porous pot in the middle of the tube. This provides a larger hot surface for the decomposition to take place on. E Fit a delivery tube into the boiling tube leading to a bowl of water. The end of the delivery tube must be under water. F
Fill a test tube with water and hold it upside down in the water. You should not have any air in the tube.
G Heat the porous pot and the paraffin, but hold the Bunsen burner flame mainly under the porous pot. Do not heat the clamp.
porous pot
clamp gas collected
paraffin and mineral wool strong heat
water
H As gas is produced, collect it using test tubes of water as shown, one after another. Do not collect the first few bubbles. When a tube is nearly full of gas, put a bung in it before you remove it from the water. I
Collect two or three tubes of gas in total. Keep the bunged tubes in a test tube rack.
J
Lift the delivery tube out of the water before you stop heating – this is to stop ‘suck back’.
K Test some of the original liquid paraffin and some of the gas you collected to see if either contains unsaturated molecules. This can be done by adding a few drops of bromine water to each and shaking the stoppered tube.
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Sheet 1 of 2
C1.32a Cracking paraffin (cont.)
Recording your results 1
2
a
What happened when bromine water was added to liquid paraffin and shaken?
b
What does this tell you about liquid paraffin ?
a
What happened when bromine water was shaken with the gas produced?
b
What does this tell you about the gas produced?
3
Why must the paraffin be heated strongly?
4
Suggest why the first few bubbles of gas were not collected.
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Sheet 2 of 2
C1.32b Industrial cracking Read the text and answer the questions about cracking methods. You do not need to remember the details of these methods.
Why is cracking needed? Before cars became common, there was not a great demand for fuels. Most of the fuels available from crude oil were used in lamps and for steam engines. However, when the internal combustion engine was invented, it needed a particular fraction of oil – the one we call petrol. As cars became more common, the oil producers could not provide enough petrol without producing more crude oil overall. This meant some of the other fractions that had no use as fuels could not be sold. Cracking was used to solve this problem. Today, cracking is still used to produce fuels for cars, lorries and aeroplanes, but the alkenes produced during cracking are also very important for the chemical industry. Alkenes are used to make plastics and many other things.
How is cracking done? The method for cracking that you can use in a school laboratory is similar to a method called catalytic cracking, first introduced in 1942. This involves passing the hydrocarbon mixture to be cracked over a heated catalyst, aluminium oxide. The aluminium oxide speeds up the reaction. The modern version uses temperatures up to 600 °C and is used to produce petrol and LPG from longer hydrocarbon molecules, together with alkenes. Hydrocracking was first introduced in 1920. It produces mainly alkanes used for fuel such as petrol and kerosene. In this method, the hydrocarbon and hydrogen are passed over a heated catalyst at about 400 °C. Steam cracking uses high temperatures and pressures – up to 850 °C. The products depend on the temperature of the reaction, how much steam is used and the kinds of hydrocarbons used. It can produce a lot of alkenes. 1
2 3
4
a
Why was cracking originally introduced?
b
Why is this process needed today?
Use the text to help you to describe the three different methods used for cracking. Which of these three methods is used to: a
produce fuel for cars and jet engines
b
produce petrol
c
produce the chemicals used to make plastics?
a
Use your answer to question 1 and the table on the right to help you to explain in more detail the economic and environmental benefits of cracking crude oil.
b
Describe any drawbacks to using this process.
© Pearson Education 2010. Edexcel GCSE Science Activity Pack This document may have been altered from the original.
Fraction
gases petrol kerosene diesel fuel oil and bitumen
Supply from crude oil (%)
Demand (%)
2 10 14 19
5 28 9 28
40
20
C1.32c Cracking questions Name 1
Class
Date
The box names the different fractions found in crude oil. bitumen
diesel oil
fuel oil
gases
kerosene
petrol
Write the fractions in order, starting with the fraction with the most atoms in the molecules.
2
3
What are the following fractions used for: a
petrol
b
kerosene
c
diesel
d
fuel oil?
a
What is ‘cracking’? _______________________________________________________________
b
Which of the fractions in the box are usually cracked?
c
Explain your answer to part b. _______________________________________________________________
4
The diagram shows a molecule of petrol.
H
H
H
H
H
H
H
H
H
C
C
C
C
C
C
C
C
H
H
H
H
H
H
H
H
H
When it is cracked, this molecule splits into one alkane with 6 carbon atoms, and one alkene with 2 carbon atoms. Draw the structural formulae of the molecules that are formed.
© Pearson Education 2010. Edexcel GCSE Science Activity Pack. This document may have been altered from the original.
C1.32d Refinery cracking Imagine you work at an oil refinery. The table shows the different fractions in the crude oil you buy, and the amount of each fraction you can sell once they have been separated.
Fraction
Crude oil (barrels)
Quantity you can sell (barrels)
gas
1 200
2 000
petrol
7 200
11 200
kerosene & diesel
64 000
148 000
fuel oil & bitumen
15 600
8 000
1
Why are the numbers in the two columns different? Explain in as much detail as you can.
2
What is cracking? Explain in as much detail as you can.
3
How can you use cracking to improve the profits of the refinery?
4
A new oil well is discovered. The crude oil from this new well has more small molecules and fewer long ones. Should you switch to buying crude oil from this new well? Explain your answer.
H5 Write balanced equations to show what happens in the following reactions. Draw diagrams to help you to work out the formulae if you need to.
6
a
A molecule of petrol with 7 carbon atoms produces ethene when it is cracked.
b
Hydrocarbon molecules with 20 carbon atoms produce propene when they are cracked.
a
Which of the molecules in your answers to question 5 are unsaturated?
b
Explain what 'unsaturated' means. Use a diagram to illustrate your answer.
Extra challenge 7
The oil shown in the table would be called a 'light crude'. A 'heavy crude' contains 4 000 barrels of petrol, 14 400 barrels of kerosene and diesel, and 48 000 barrels of fuel oil and bitumen. Explain why an oil well producing light crude is likely to be more profitable.
Š Pearson Education 2010. Edexcel GCSE Science Activity Pack This document may have been altered from the original.