3.3.2 - Alkanes Flashcards
What is an alkane?
A saturated hydrocarbon containing only C-H and C-C bonds
What is the general formula of an alkane?
C(n)H(2n+2)
Are alkanes bonds polar or nonpolar and why?
nonpolar:
the electronegativities of hydrogen and carbon are so similar.
Which intermolecular forces do alkanes have and why?
The only intermolecular forces between the molecules are weak Van der Waals forces as the bonds are nonpolar, these forces get stronger the larger the molecule.
Are alkanes soluble or insoluble in water and why?
insoluble as the hydrogen bonds that hold the water molecules together are much stronger than van der Waals forces.
Are alkanes reactive?
alkanes are relatively unreactive because of their strong C-C and C-H bonds. They will burn, and react with halogens, under suitable conditions.
Which reactions will alkanes undergo?
Combustion and reaction with halogens
What is crude oil?
a mixture of hydrocarbons with similar boiling points and properties. Crude oil is a mixture of branched and unbranched alkanes of varying sizes.
How is crude oil formed?
formed by the breakdown of animal and plant remains at high pressures and temperatures over millions of years
Is crude oil renewable or non renewable?
non-renewable as the process to form it takes a very long time.
Name the fractions of crude oil from high to low boiling point
HIGH BOILING POINT
tar/bitumen
fuel oil
lubricating oil and waxes
diesel oil
kerosene/paraffin
gasoline/petrol (naphtha)
gases
LOW BOILING POINT
tar/bitumen
c70-upwards
roads and roofing
fuel oil
c35-c70
ships, power stations
lubricating oil and waxes
c19-c35
candles and engine oil
diesel oil
c15-c18
lorries and taxis
kerosene/paraffin
c12-c16
jet fuel, lighting
gasoline/petrol (naphtha)
c4-c12
cars
gases
c1-c3
fuel on site
how does fractional distillation work?
- crude oil heated until mostly vaporised
- passed into a fractionating column that is cooler at the top than at the bottom
- vapours rise up the column and via trays and bubble caps, condense when temperature < their boiling point
- shortest chain hydrocarbons condense at the top as they have the lowest boiling points
Trends in fractionating column
- viscosity decreases as you go up
- chain length decreases as you go up
- flammability increases as you go up
- boiling point range decreases as you go up
What is cracking?
process of splitting the long chain hydrocarbons from crude oil into shorter chain hydrocarbons (by breaking C-C bonds) that are more in demand, and so more valuable.
What does cracking also create?
alkenes, which are useful in industry as feedstock for making substances like plastics.
what are the two types of cracking?
thermal cracking
catalytic cracking
Thermal cracking conditions
- temperature of 700-1200K
- up to 7000 kPa pressure
what are the intermediate products from thermal cracking?
free radicals
What does thermal cracking mainly produce?
-alkenes, can be used to make many valuable products e.g. polymers
disadvantage of thermal cracking
The carbon chain could break at any point so the products are difficult to predict, and lots of alkenes are produced during thermal cracking.
what are the conditions for catalytic cracking?
- slight pressure (but above atmospheric)
- temperature around 500 degrees C
- zeolite catalyst (lowers temperature and pressure needed for cracking reaction to take place) with a honeycomb structure to give a large surface area
What are the main products of catalytic cracking?
mostly branched alkanes, cycloalkanes, and aromatic compounds that can be used in motor fuels. The products can be separated by fractional distillation.
what is a fuel?
something which releases heat energy when combusted
what is incomplete combustion?
combustion in a limited supply of oxygen
general incomplete combustion word equation
fuel + oxygen = carbon monoxide + water
products of incomplete combustion and their dangers
-water (greenhouse gas = global warming)
- carbon monoxide (poisonous)
- carbon (soot) - global dimming/breathing problems/cancer
what types of hydrocarbons are most likely to undergo incomplete combustion?
longer chains
what is complete combustion?
combustion in a plentiful supply of oxygen
general complete combustion word equation
fuel + oxygen = carbon dioxide + water vapour
what are nitrogen oxides?
- have general formula NO(x)
how is nitrogen monoxide formed and the equation
- nitrogen monoxide formed when the high pressure/temperature in a car engine cause the nitrogen and oxygen atoms from the air to react together
- N2 (g) + O2 (g) = 2NO (g)
how is nitrogen dioxide formed?
nitrogen monoxide can react further to from nitrogen dioxide
2NO (g) + O2 (g) = 2NO2 (g)
what is the environmental impact of nitrogen oxides?
- forms nitric acid, causes acid rain
- photochemical smog
how is sulphur dioxide formed?
burnt e.g. in car engines/power station, sulphur reacts to form sulphur dioxide gas
how is acid rain formed?
sulphur/nitrogen dioxide gets into the atmosphere and
dissolves in the moisture forming sulfuric/nitric acid
negatives of acid rain
-destroys trees/vegetation
-corrodes buildings/statues
-pollutes rivers/lakes which kills aquatic life
what is the environmental impact of unburnt hydrocarbons?
photochemical smog
what is the environmental impact of carbon dioxide?
global warming:
greenhouse gases in the atmosphere e.g. CO2 absorb infrared energy and emit some of the energy they absorb back to Earth, keeping it warm (greenhouse effect)
too many greenhouse gases= enhanced greenhouse effect causing global warming
what are flue gases?
gases given out by power stations
flue gas desulphurisation
powdered calcium carbonate or calcium oxide is mixed with water to make an alkaline slurry. the flue gases mix with this so the acidic sulphur dioxide reacts with the calcium compounds to form a harmless salt (calcium sulphite)
CaO (s) + SO2 (g) = CaSO3 (s)
Catalytic converter structure
ceramic honeycomb coated with platinum, palladium and rhodium metals
what do catalytic converters catalyse (equations)
they catalyse these reactions of products from car exhausts
2CO (g) + 2NO (g) = N2 (g) + 2CO2 (g)
hydrocarbons + NO = N2 + CO2 + H2O
define carbon neutral activities
activities that produce no net/overall carbon dioxide emissions
how are halogenoalkanes formed from alkanes?
free radical substitution reaction (chain reaction)
what is a free radical?
A free radical is an atom with a lone unpaired electron. Free radicals are very reactive.
what is the 1st stage of free radical substitution?
initiation: breaking halogen bond to form free radicals (needs presence of UV light)
what is the 2nd stage of free radical substitution?
propagation: chain part of the reaction where products are formed but free radical remains
what is the 3rd stage of free radical substitution?
termination: free radicals removed, stable products formed
