cracking

Why is cracking necessary?

There is a high demand for shorter chain alkanes but a higher supply for longer chain alkanes, but the fractional distillation of crude oil produces more long chain alkanes therefore we must break it down.

What is cracking?

The thermal decomposition of longer alkanes into smaller alkanes and alkenes

Cracking formula?

Long chain alkanes goes to short chain alkanes + alkenes

Why are smaller alkanes in a higher demand as fuels than longer alkanes?

• Petrol (small chains) is more useful than bitumen (long chains)

• Smaller alkanes are more volatile (easier to ignite) and are less viscous

How can we tell if we have successfully cracked kerosene?

• New product formed that decoloured orange bromine water.

• Smelt like petrol

• Product was more flammable

Why don’t we collect the first bubbles of gas in catalytic cracking?

The first few bubbles of gas are from the air inside the test tube and therefore don’t contribute to our results.

What are the two types of cracking?

• Steam cracking (high temperature + steam) producing more alkenes

• Catalytic cracking (high temperature + catalyst) more alkanes

What two types of reactions is cracking?

• Thermal decomposition

• Endothermic reaction

What are the two products of cracking?

• Saturated hydrcarbons (alkenes)

• Unsaturated hydrocarbons (alkenes)

How can we test for the unsaturated products of cracking?

Test unsaturated hydrocarbons with bromine water or any other halogen.

Unsaturated hydrocarbon (colourless) + bromine water (orange / yellow) goes to products (colourless), if turns colourless then unsaturated hydrocarbons are present

How can we test for the saturated products of cracking?

Test saturated hydrocarbons with bromine water or any other halogen

Saturated hydrocarbon (colourless) + bromine water (orange / yellow) doesn’t react and instead stays orange / yellow

Why can we test for products of cracking using bromine water or any other halogen?

Because unsaturated hydrocarbons have a C-C double bond in alkenes making them much more reactive than alkanes