17.3 Cracking hydrocarbons

Question 1

After fractional distillation, fractions are cracked. Why?

Answer 1

1. Fractions contain impurities, (mainly sulfur compounds) if they are left in fuels they will be burned to form harmful SO2 gas.
2. Some fractions are separated into single compounds smaller groups of compounds.
   Ex. Methane, ethane etc.

Question 2

What is cracking?

Answer 2

Breaking down molecules to smaller ones.

Question 3

State the steps of cracking a hydrocarbon in a lab.

Answer 3

1. Soak mineral wool into the hydrocarbon oil and place it int a test tube.
2. Place and tube inverted into a beaker of water and put a delivery tube from the test tube into the inverted test tube on the water.
3. Put Aluminium oxide (catalyst) in the test tube with the hydrocarbon and heat it.
4. Once you stop heating lift the delivery tube out of he water so the water doesn’t get sucked up the hot test tube.

Question 4

Thermal decomposition takes place when you crack. List 3 difference between the reactant and the product.

Answer 4

1. The product is less thick than the reactant, since it has smaller molecules.
2. The product burns easier than the reactant, since it now has smaller molecules and short carbon chains.
3. The product is also a hydrocarbon since nothing new was added.

Question 5

How is cracking carried out in a refinery?

Answer 5

1. Long chain hydrocarbons is heated to vaporize it.
2. The vapour is usually passed over a hot catalyst. To speed up the reaction.
3. Thermal decomposition takes place.

Question 6

Why is cracking important?

Answer 6

1. Helps you crack large moleculed fractions to obtain small moleculed ones, which are usually more useful.
2. Cracking ALWAYS produces short chain compounds with DOUBLE C-C bonds. Which means the compound is REACTIVE.

(They can be used to make plastics and other substances)

Question 7

How do you crack a naphtha fraction?

Answer 7

Many naphtha fractions are cracked since it is the fees took for making many useful chemicals.

Decade C10H22 + 540 degrees Celsius + catalyst
Will give you pentane C5H12 , PROPENE C3H6 + ETHENE C2H4
The - ene family is very reactive because of the double carbon bonds, which makes them useful

Question 8

How do you crack ethane?

Answer 8

Ethane is cracked into ETHENE and hydrogen gas.

Steam and a temperature of 800+ degress Celsius is used