Alkanes Flashcards
Polarity of alkanes
-C and H have very similar electro negativities
-bonds are non-polar
-meaning all alkane molecules are non-polar
-have simple molecular lattices with weak VDW forces between molecules
Solubility of alkanes
-forces of attraction between water molecules are hydrogen bonds
-much stronger than the VDW forces in alkanes
-alkanes are not soluble in water
Boiling points of straight chain alkanes
-as length of carbon chain increases, boiling point of alkane rises
-bc there are more e- in the molecules so the VDW forces between molecules are stronger and require more energy to break.
Boiling point of branched chain alkanes
-as number of branches increases, boiling point of the alkane decreases
-bc there are fewer points of contact between the molecules so the VDW forces between molecules are weaker and require less energy to break
Fractional distillation of crude oil
Fractional distillation is the separation of the components of a liquid into fractions which differ in boiling point.
-crude oil is vaporised and the vapour is introduced near the bottom of the column
-vapour rises up the column and creates a temperature gradient
-bc the alkanes have diff boiling points they condense at diff levels
-hydrocarbons with lowest bp do not condense and are drawn off as gases at the top of the column
-the largest hydrocarbons dont vaporise and are collected at the base of the column as a thick residue
Thermal cracking
-carried out at very high temp and very high pressure
-produces alkanes used to make petrol and a high percentage of alkenes used to make polymers
-the c-c bonds break at diff positions in the chain to give a mixture of products
Catalytic cracking
-carried out at high temp and slight pressure in the presence of a zealite catalyst
-produces branched alkanes (isomers), cyclic alkanes (+H2) and aromatic hydrocarbons (+4H2) such as benzene
Complete, incomplete and further incomplete combustion
Complete combustion forms:
-CO2 and H2O
Incomplete combustion forms:
-CO and H2O
-occurs when there is a limited supply of oxygen
Further incomplete combustion forms:
-C and H2O
-occurs when there is a very limited supply of oxygen
Pollution from combustion
-Uncombusted HC fuel passes through exhaust, which forms ozone (O3) at low levels in atmosphere.
-Combustion of HC fuel releases CO2 which is a greenhouse gas.
-Incomplete combustion of HC fuel releases CO which is toxic.
-Further incomplete combustion of HC fuel releases C which exacerbates asthma.
Pollution of Nitrogen oxides (NO/NO2)
N2 from air reacts with O2 at high temps in engine which contributes to acid rains as NO/NO2 is an acidic gas.
-N2 + O2 —> 2NO
-N2 + 2O2 —> 2NO2
Pollution of sulfur dioxide
Sulfur containing impurities in the fuel combusts to give SO2. Can be removed by reacting with H2O:
-SO2 + H2O —> H2SO3
-SO3 + H20 —> H2SO4
Catalytic converters
-fitted to most cars and remove CO, NO and unburned HC
-contain a honeycombed structure coated with a thin layer if Pt, Pd, Rh (catalysts) metals
-thin layer used to reduce amt of metal used
-honeycomb structure provides high surface area to maximise rate
Removal of NO and CO and removal of unburnt HC
2NO + 2CO —> 2CO2 + N2
Unburnt HC can be removed by
-complete combustion
-reacting with NO which forms CO2, H2O and N2
Alkanes are generally unreactive because
- C-C and C-H bonds are strong and not easy to break
- Alkanes are non-polar so do not attract electrophiles or nucleophiles
Free-radical is
A species with an unpaired electron, making it very reactive.
Free radical substitution mechanism has 3 stages
Initiation - formation of radicals from the halogen
Propagation - formation of products and there is 2 products so this stage has 2 equations
Termination - removal of radicals, any 2 radicals can combine so there is 3 possibilities
