Cracking Hydrocarbons

Large alkane molecules such as those in liquid paraffin can be broken into smaller, more useful alkanes and alkenes by heating their vapors over a hot catalyst. The products can be collected and tested to show the formation of alkenes, which are more chemically reactive and important for making fuels and other useful chemicals.

1. Set up a test tube containing a small amount of liquid paraffin connected to a delivery tube leading into an inverted test tube over water.
2. Place a catalyst such as pumice stone, broken porcelain, or aluminum oxide in the middle of the test tube.
3. Heat the catalyst strongly with a Bunsen burner until it glows hot.
4. Briefly move the flame to the paraffin to vaporize some liquid, then return the flame to the catalyst so the vapor passes over it. Alternate heating to keep the catalyst hot while producing vapor.
5. Discard the first gas collected, as it is just displaced air.
6. Collect several tubes of gas over water.
7. When finished, remove the delivery tube from the water before stopping heating to prevent suck-back.
8. Test the collected gas by (1) burning it to observe the flame color and (2) shaking it with bromine water to check for decolorization, which indicates the presence of alkenes.

Cracking a hydrocarbon - Royal Society of Chemistry:

Yr 10 Cracking paraffin demo - susanna wilkinson:

📄 Cracking long hydrocarbons to make smaller useful ones - Chemguide: https://www.chemguide.uk/14to16/organic/cracking.html

• Crack different hydrocarbon feedstocks such as kerosene or light oils.
• Perform a comparison with industrial cracking methods (catalytic cracking using zeolites vs. steam cracking).

• Wear safety goggles and a lab coat; tie back long hair.
• Carry out the experiment in a well-ventilated lab or fume hood.
• Use heatproof mats and clamps to secure apparatus.
• Never stop heating before removing the delivery tube from the water, as water could be drawn back into the hot tube and cause breakage.
• Handle bromine water with care; it is toxic and corrosive. Dispose of waste chemicals according to local regulations.

• What types of products are formed during cracking? (Smaller alkanes and alkenes.)
• Why is cracking needed in the petroleum industry? (To increase supply of shorter hydrocarbons like petrol and to produce reactive alkenes for chemical synthesis.)
• How can you test for an alkene? (Alkenes decolorize bromine water; they also burn with a more yellow, sooty flame.)
• What is the role of the catalyst in cracking? (It lowers the activation energy and allows cracking to occur at lower temperatures than thermal cracking.)
• How does industrial catalytic cracking differ from steam cracking? (Catalytic cracking favors production of fuels like petrol, while steam cracking favors production of alkenes for chemical feedstocks.)