Alkenes, Cracking, Addition Polymers (TOPIC 7) Flashcards
What is the general formula for alkenes?
CnH2n
Why are alkenes classed as unsaturated hydrocarbons?
As they can make more bonds when the double bonded carbon can open up to bond with other atoms.
How do we test for an alkene?
Mix an alkene and bromine (orange) together. If an alkene is present they will react as the alkene has a double bond in an addition reaction and the dibromoethane produced will be colurless.
On the other hand the alkane is saturated so cannot react with bromine water and will stay orange
Describe the reactions of alkanes with halogens in the presence of UV light.
Chlorine and bromine react with alkanes in the presence of UV light.
In these reactions a hydrogen atom from the alkane is replaced by a halogen atom. Substitution reaction.
this is how bromine and methane react together to from bromomothane.
methane + bromine -> (uv light). Bromomethane + hydrogen bromide
What is cracking?
Thermal decomposition method used to to break down long chained hydrocarbons into shorter more useful alkanes and alkenes. This takes place at a temperature of 600 - 700 degrees and in the presence of a catalyst of either alumina or silica.
Why is cracking useful?
The long chained hydrocarbons are very viscous and are not in as high demand as the shorter ones with low boiling points like gasoline. Catalytic cracking also produces an alkene which are used to make polymers.
Why are polymers hard to get rid off?
They do not biodegrade
And they release toxic gases when they are burnt
What are the coniditions for addition polymerisation
Catalyst and high pressure
Ethene + bromine →
dibromoethane
