Quick answer: Alkenes are hydrocarbons with a C=C double bond whose exposed π electrons attract electrophiles. This Chemistry 11-12 lab on electrophilic addition gives the aim, the equipment, the safety points, a step-by-step method, sample results and 7 analysis questions; the answers are on a separate page and the lab sheet is a free printable PDF.
Which edible oil contains the most carbon-carbon double bonds? In this lab you use dilute bromine water, the classic alkene test, as a measuring tool. You will titrate five oils, subtract a blank and convert the data into moles of Br₂ per 100 g, a value linked to the iodine value used in food analysis.
Level: Chemistry grades 11-12 (A Level · IB · Class 11-12) · Curriculum: A Level 3.3.4
Aim
To compare how much C=C unsaturation five edible oils contain by titrating each with dilute bromine water, and to convert the titres into moles of Br₂ per 100 g of oil.
Equipment
- bromine water, 0.020 mol/dm³, in a burette
- five vegetable oils: sunflower, rapeseed, olive, palm, coconut
- propanone (10 cm³ for each titration)
- top-pan balance (0.01 g), 100 cm³ conical flasks, white tile, 10 cm³ measuring cylinder
- stopwatch, gloves, goggles
Safety
Bromine water is toxic and corrosive: wear goggles and gloves, work in a fume cupboard and keep a thiosulfate solution ready for spills. Propanone is highly flammable, so no naked flames. Dispose of waste in the labelled halogen bottle.
Method
- Weigh 0.100 g of oil into a conical flask and add 10 cm³ of propanone; swirl until dissolved.
- Fill the burette with bromine water and note the reading.
- Add bromine water in 1 cm³ portions with swirling, then dropwise near the end point.
- Stop when the orange colour lasts for 30 seconds.
- Record the titre. Repeat for a second trial, then for each oil.
- Do a blank titration with 10 cm³ of propanone only.
Results
Sample measurements (blank titre = 0.3 cm³):
| Oil | Titre 1 / cm³ | Titre 2 / cm³ | Mean / cm³ |
|---|---|---|---|
| Sunflower | 24.0 | 24.2 | 24.1 |
| Rapeseed | 21.1 | 20.9 | 21.0 |
| Olive | 15.3 | 15.1 | 15.2 |
| Palm | 9.5 | 9.7 | 9.6 |
| Coconut | 1.8 | 2.0 | 1.9 |
Analysis questions
Question 1 Corrected titres
Subtract the blank titre from each mean and give the corrected titres for palm and coconut oil.Question 2 Moles of bromine
Calculate the amount of Br₂ that reacted with 0.100 g of sunflower oil.Question 3 Per 100 g of oil
Convert the result for sunflower oil to moles and to mass of Br₂ per 100 g of oil (M(Br₂) = 159.8 g/mol).Question 4 Coconut oil value
Repeat the calculation for coconut oil (mol Br₂ per 100 g).Question 5 Ranking and meaning
Rank the oils from most to least unsaturated and relate the result to their use in cooking.Question 6 Number of C=C per molecule
Olive oil gives 0.298 mol Br₂ per 100 g. The mean triglyceride has M = 880 g/mol. Calculate the average number of C=C per molecule.Question 7 Errors and improvements
Give two sources of error and one improvement.
