Hydrocarbons Flashcards
What are alkanes
Alkanes are saturated hydrocarbons which are member s of a homologous series
↳ all the carbon-carbon bonding is single
↳ bonds are spaced tetrahedrally around carbon atoms
What are the physical properties of alkenes?
- Boiling point increases as they get more carbon atoms in their formula
- More atoms = greater intermolecular van der waalsforces
- Greater intermolecular force= more energy to separate the molecules
- Greater energy required, higher boiling point
- the greater the branching, the lowerthe boiling point
- The straight chain molecules have greater interaction than branched
Why are alkanes unreactive?
- Absence of a functional group
- non polar therefore un reactive toward water or polar reagent like HBr
- Alkanes react with non-polar reagents like cl2 or br 2 only in the presence of uv light or sunlight
- c-c and ch bonds have high bond enthalpy and therefore require a lot of energy to overcome
Why do alkanes make good fuels?
- Release huge amounts of energy when burnt
- readily available and easy to transport
When does complete combustion occur
In excess oxygen
Product of complete combustion
Water and carbon dioxide
When does incomplete combustion occur
In insufficient oxygen
Products of incomplete combustion
Water and various other product and pollutants including carbon particulates, carbon monoxide and carbon dioxide
What are the problems with carbon monoxide?
- carbon monoxide is poisonous
- It irreversibly binds to haemoglobin, reducing the amount of oxygen that can be carried in the blood
- Cells may become de oxygenated, leading to cell death
- if too much carbon monoxide is inhaled, this could lead to death
What are, carbon particulates?
- Small fragments of unburned hydrocarbon
- unless removed from the waste products in industry, these can cause serious respiratory problems as they pollute the air
What is crude oil a source of ?
Aliphatic and aromatic alkanes
What is cracking?
The process in which complex organic molecules are broken down into smaller organic molecules such as simple alhanes
Why is cracking used?
To obtain more useful and smaller alkanes and alkenes of which there is a greater demand for
What’s markovnikov’s rule?
In the electrophillic addition of a hydrogen halide to an asymmetric alkene, the hydrogen atom will bond to the carbon that is already bonded to the most hydrogens.
this leads to the formation of the major product
What are the classifications of carbocation
Primary , secondary and tertiary
What is a primary carbocation
Carbocation is bonded to one alkyl group
What is secondary carbocation
Carbocation is bonded to two alkyl groups
What is tertiary carbocation
Carbocation is bonded to three alkyl groups
What is the trend in carbocation stability ?
- The more alkyl groups attached to the carbocation the more the positive change is spread out the increasing stability
- stability increases from primary→ secondary→tertiary
What is additional polymerisation
The joining together of unsaturated alkene monomers to form a long chain polymer
- no other products are formed so the atom economy is 100%
How do alkenes undergo addition polymerisation?
The pi bond breaks and each electron goes towards forming a sigma bond with an adjacent monomer unit
Why are polymers difficult to dispose of ?
- They are non-biodegradable
- the combustion of polymers can often form harmful compounds such as Hcl for chlorinated polymers
Why are alkanes suitable for use as fuels?
Alkanes have high standard enthalpy changes of combustion so they release a lot of energy upon combustion
Why are atmospheric oxides of nitrogen pollutants?
- Nitrogen monoxide can contribute to the production of acid rain by reacting with chemicals in the clouds
- nitrogen dioxide can contribute to photochemical smog
What are the environmental consequences of unburnt hydrocarbons?
Hydrocarbons in the atmosphere can absorb infraredradiation, contributing to the greenhouse effect
What are the environmental consequences of the emission of greenhouse gases?
The greenhouse effect:
- electromagnetic radiation from the sun passes through the atmosphere. Some radiation is aborted by earth sotemperature increases
- heat is radiated from the earth as infrared radiation which is absorbed by greenhouse gases in the atmosphere. This causes the bunds in these molecules to vibrate before this energy is re-emitted
- as a result, the atmosphere warms up even further contributing to global warming
Chemical test for alkene
Add bromine water
If a c=c bond is present, the orange solution will decolorise to form a colorless solution
what reactions produce alkanes
hydrogenation and cracking
what is the hydrogenation reaction
addition reaction
equation: alkene + hydrogen gas -> alkane
catalyst: heat + platinum/nickel catalyst
(see notes for diagram)
what is the cracking reaction
large hydrocarbon fed into steel chamber and heated strongly then passed over Al2O3 catalyst
equation: large alkane -> smaller alkane + alkene
