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Reactions of Alkanes

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A series of free IGCSE Chemistry Activities and Experiments (Cambridge IGCSE Chemistry).

Reactions of Alkanes
1. Combustion
Alkane + O2 → CO2 + H2O
2. Substitution Reaction
Reaction by a halogen in presence of UV light. The halogens are F2, Cl2, Br2, I2.
Example:
Methane + chlorine → chloromethane + hydrogen chloride
3. Cracking
Cracking breakes a big molecule into smaller molecules. (at least one alkene is formed).
Example:
hexane → butane + ethene
Investigate how hydrocarbons burn in air, with propane



Substitution Reaction: Chlorination of Methane
The following three videos will show the initiation, propagation and termination phases when chlorine reacts with methane.
Cracking a hydrocarbon
The demand for petrol is greater than the gasoline fraction obtained by distilling crude oil. Cracking larger hydrocarbons produces smaller alkanes that can be converted into petrol. It also produces small alkenes, which are used make many other useful organic chemicals (petrochemicals), especially plastics. This experiment models the industrial cracking process.
Liquid paraffin (a mixture of alkanes of chain length C20 and greater) is vaporised and passed over a hot pumice stone catalyst. A gaseous product is obtained which is flammable and which will decolorise bromine water and acidified permanganate ions.
Procedure
a. Place about a 2 cm3 depth of mineral wool in the bottom of the boiling tube and gently press it in place with a glass rod. Drop about 2 cm3 of liquid paraffin on to the wool, using a dropping pipette. Use enough paraffin to completely soak the mineral wool.
b. Clamp the boiling tube near the mouth so that it is tilted slightly upwards. Place a heap of catalyst (pumice stone or porous pot fragments) in centre of the tube and fit the delivery tube.
c. Fill the trough about two-thirds full with water and position the apparatus so that the end of the delivery tube is well immersed in the water. Fill four test-tubes with water and stand them inverted in the trough.
d. Strongly heat the catalyst in the middle of the tube for a few minutes, until the glass is up to a dull red heat. Avoid heating the tube too close to the rubber bung.
e. While keeping the catalyst hot, flick the flame from time to time to the end of the tube for a few seconds to vaporise some of the liquid paraffin. Try to produce a steady stream of bubbles from the delivery tube. Be careful not to heat the liquid paraffin too strongly or let the catalyst cool down. To avoid suck-back do not remove the flame from heating the tube while gas is being collected. If suck-back looks as if it is about to occur, lift the whole apparatus by lifting the clamp stand.
f. When a steady stream of gas bubbles is established, collect four tubes full of gas by holding them over the Bunsen valve.
g. When gas collection is complete, first remove the delivery tube from the water by tilting or lifting the clamp stand. Only then stop heating.
h. Test the tubes of gas as follows:
(i) Use a lighted splint to see if the gas is flammable.
(ii) add 2 - 3 drops of bromine water, stopper and shake well.
(iii) add 2 - 3 drops of acidified potassium manganate(VII) solution, stopper and shake well.

The gas mixture which collects has a characteristic smell (camping gas), burns with a yellow flame, and decolourises bromine water and acidified potassium manganate(VII) solution. This shows the presence of unsaturated molecules.

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