Quick answer: Crude oil is a mixture of hydrocarbons separated by fractional distillation into fractions with different chain lengths and boiling points. This Chemistry 9-10 lab on crude oil and hydrocarbons gives the aim, the equipment, the safety points, a step-by-step method, sample results and 8 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Cracking turns long, unreactive molecules into shorter, more useful ones. In this lab you crack liquid paraffin over a hot catalyst, collect the gas that forms and test it with bromine water to find out whether the product is saturated or unsaturated. You also analyse how the gas volume grows with time.
Level: Chemistry grades 9-10 (GCSE · IGCSE · O Level · Class 9-10) · Curriculum: GCSE C7
Aim
To crack liquid paraffin using hot aluminium oxide, collect the gas that forms, and show that it is an alkene by its reaction with bromine water.
Equipment
- hard-glass boiling tube with a bung and a bent delivery tube
- mineral wool, liquid paraffin (a few drops), aluminium oxide pieces
- Bunsen burner, heat-proof mat, clamp and stand
- trough of water, two test tubes, stopwatch, dropper with dilute bromine water
- safety goggles
Safety
Wear goggles. Remove the delivery tube from the water before you stop heating, or cold water will suck back into the hot tube and crack it. Bromine water is toxic and irritating: use dilute solution, keep it away from your skin and work in a ventilated room. The gas formed is flammable: keep flames away from the collecting tube.
Method
- Soak a piece of mineral wool in liquid paraffin and push it to the closed end of the boiling tube.
- Place 4 to 5 aluminium oxide pieces in the middle of the tube, clamp the tube nearly horizontal and fit the bung and delivery tube.
- Fill a test tube with water, invert it in the trough and put the end of the delivery tube under it.
- Heat the aluminium oxide strongly. Occasionally warm the mineral wool to send paraffin vapour over the hot pieces.
- Throw away the first test tube of gas (it contains air). Collect a second tube and record the total volume of gas every 2 minutes, using test tubes marked in cm³ or a gas syringe.
- Remove the delivery tube from the water, then stop heating.
- Add 2 cm³ of dilute bromine water to a tube of the gas, stopper it and shake. Repeat with 2 drops of liquid paraffin.
Results
A class recorded the total volume of gas collected and the bromine water test:
| Time (min) | 2 | 4 | 6 | 8 | 10 |
|---|---|---|---|---|---|
| Total gas volume (cm³) | 4 | 11 | 18 | 24 | 28 |
| Sample | Bromine water result |
|---|---|
| gas from cracking | orange to colourless at once |
| liquid paraffin | stays orange |
Analysis questions
Question 1 Plot the data
Plot a graph of total gas volume (y axis) against time (x axis) and describe its shape.Question 2 Average rate
Calculate the average rate of gas production over the full 10 minutes.Question 3 Rate between two times
Calculate the average rate between 2 and 6 minutes and compare it with the rate between 8 and 10 minutes.Question 4 Meaning of the bromine result
Explain what the bromine water result for the gas shows. Why was liquid paraffin tested as well?Question 5 Write an equation
Liquid paraffin contains long alkanes. Write a balanced equation in which C₁₆H₃₄ cracks into one alkane with 8 carbon atoms and two molecules of the same alkene. Name the alkene.Question 6 Safety step
Explain why the delivery tube must be taken out of the water before heating stops.Question 7 Why discard the first tube
Give two reasons why the first test tube of gas is thrown away.Question 8 Better gas measurement
Suggest two changes to the cracking lab that would make the gas volumes more trustworthy.
