cer miscellaneous Flashcards
What is a homologous series, and what is the general formula of the alkanes?
A series of compounds with similar chemical properties and the same general formula where the formula of each member differs by CH2.
Alkanes have the general formula CnH2n+2
How are alkanes separated from crude oil?
By fractional distillation - different alkanes have different boiling points
Why are alkanes good fuels?
They are unreactive and have a highly exothermic combustion reaction.
What are some disadvantages of crude oil as a fuel source? alternatives?
Fossil fuel - combustion will produce CO2, a greenhouse gas on combustion and CO, which is toxic, during incomplete combustion. Also, crude oil is a finite resource so could become very pricey.
Alternatives: biofuel produced from plants/animal waste - e.g. ethanol from fermented sugar cane. These use the energy of the sun and are renewable.
Why might straight-chain alkanes by made to undergo isomerisation reactions?
Branched molecules break down into more stable radicals, reducing knocking (higher octane rating)
What is reforming and why might straight-chain alkanes be made to undergo reforming reactions?
Reforming converts straight chain alkanes into cyclic alkanes and arenes, which aid the combustion process - combust more efficiently
Why do alkanes undergo cracking following fractional distillation?
To break long-chain alkanes into shorter chain hydrocarbons - alkenes - (using heat and catalysts) which have lower boiling points and are more easily vaporised when used as a fuel
what are the conditions required for the reaction of alkanes with halogens?
the presence of ultraviolet light OR a temperature of about 300degrees C
how are radicals formed? e.g in a halogen molecule?
by the breaking of a covalent bond - eg the Cl-Cl bond in a halogen molecule. Bond breaking is known as bond fission - homolytic fission produces radicals
what is the first stage in the free radical substitution mechanism known as?
initiation
Describe the initiation stage of free radical substitution, using the reaction of Cl2 and methane as an example
Cl-Cl bond is broken by homolytic fission, 2 chlorine radicals formed. UV radiation provides energy for this.
0 radicals –> 2 radicals
describe the first and second stages of propagation using equations (Cl2 + CH4 reaction as an example)
- CH4 + Cl. –> .CH3 + HCl
- .CH3 + Cl2 –> CH3Cl + Cl.
1 radical –> 1 radical
Possible equations for termination stage of free radical substitution
.CH3 + .CH3 —> C2H6
.Cl + .CH3 –> CH3Cl
.Cl + .Cl –> Cl2
Why is free radical substitution not a reliable way of making halogenoalkanes?
Hard to control which product is formed as there are many possible termination steps.
Can produce a large variety of different halogenoalkanes as more than one hydrogen atom can be substituted
outline the uses of chloroethene and tetrafluoroethene
to produce the plastics PVC and PTFE
PVC used for electrical insulation, vinyl window frames, drainpipes..
PTFE used to make non-stick frying pans
