3.3.2 Alkanes - Chemistry Alevel

Refinery gas - use & number of carbons

C1-C4, Bottled gas for camping and stoves.

Gasoline - use & number of carbons

C5-C10, Car fuel (petrol)

Naphtha - use & number of carbons

C8-C12, Making plastics, drugs, medicines and fabrics.

Kerosene - use & number of carbons

C10-C16, Jet engine fuel.

Diesel - use & number of carbons

C15-C30, Truck, bus and diesel car fuel.

Fuel oil- use & number of carbons

C25-C50, Fuel in ships and power stations.

Lubricating oil - use & number of carbons

C45-C70, Machine lubricants and wax for candles and polishes.

Bitumen - use & number of carbons

C70 and above, Surfacing roads.

Fractional distillation process

The crude oil is heated up and turns into a gas (which enters the fractionating column). The fractionating column is hot at the bottom and becomes progressively cooler towards the top (temperature gradient). This means that larger molecules (with the high boiling points) turn back into to liquids nearer the bottom. At the high temperatures there the smaller molecules stay as gases and rise up the column and condense as different fractions.

Fractional distillation order

Refinery gas, gasoline, naphtha, kerosene, diesel, fuel oil, lubricating oil, bitumen.

Fractional distillation trends (top to bottom) size

small molecules -> big

Fractional distillation trends (top to bottom) bp

lower boiling points (volatile) -> high (in-volatile)

Fractional distillation trends (top to bottom) Van Der Waals forces

weaker Van Der Waals forces -> strong

Fractional distillation trends (top to bottom) colour

Lighter colour -> darker

Fractional distillation trends (top to bottom) flammability

Burns easily -> flame retardant

Fractional distillation trends (top to bottom) viscosity

Thinner (low viscosity) -> thicker (higher viscosity)

Fractional distillation trends (top to bottom) combustion type

Clean flame (complete combustion) -> dirty flame (incomplete combustion)

Hydrocarbons as fuels

Shorter chain alkanes (with weaker Van Der Waals forces) are valuble as clean fuels.

Complete combustion - equation

Alkane + oxygen (plentiful supply) -> carbon dioxide + water (g).

Incomplete combustion - equation

Alkane + oxygen (limited supply) -> carbon monoxide + water (g).

Incomplete combustion problems

Carbon monoxide is poisonous (combines with haemoglobin in blood, leading to oxygen starvation), less energy is released, soot (carbon) can be produced if supply of oxygen is limited further and causes dirty deposits.

Combustion in an internal combustion engine

A small amount of petrol is mixed with oxygen that is drawn into the combustion chamber. The mixture reacts explosively forcing the movement of the engine parts. The very high temperature of the combustion chamber causes unwanted side reactions as the normally unreactive N reacts in the heat making NO and NO2.

Why is cracking needed

Short chained alkanes are more useful/volatile and make better fuels and short chained alkenes are more reactive and can be used to make polymers. However more long chained hydrocarbons are made in fractional distillation.

What does cracking involve?

Cracking involves breaking a single carbon-carbon bond in the alkane. The products of cracking are in greater demand and, here, more valuable than the starting materials.

Cracking word equation

Long-chaine alkane -> shorter-chained alkane + shorter-chained alkene (or hydrogen)

Thermal cracking conditions (temp, pressure, catalyst)

High temperature (800-900C),high pressure, no catalyst

Thermal cracking products

High proportion of small chain alkenes (ethene forms poly(ethene)) - turns bromine water brown -> colourless

Catalytic cracking conditions (temperature, pressure, catalyst)

Lower temperature (450C), lower pressure, zeolite catalyst (Al2O3, SiO2)

Catalytic cracking products

Aromatic hydrocarbons cycloalkanes. (Branched alkanes -> burn more uniformly - good as motor fuels).

Main green house gas

Water - main natural, carbon dioxide - made naturally & artificially, methane

Photochemical smog - formation

Smog is formed when nitrogen oxides (NOx), sulphur dioxide and unburnt hydrocarbon fuels react with sunlight.

Smog problems

Causes lung diseases like emphysema.

Carbon dioxide formation

Complete combustion of hydrocarbons

Carbon dioxide effect

Green house gas which contributes to global warming

Carbon dioxide - reducing release

Use alternative fuels which are carbon neutral.

Carbon monoxide formation

Incomplete combustion of hydrocarbons

Carbon monoxide effect

A toxic gas

Carbon monoxide removal

Catalytic converter, exhaust gases from cars are passed through the catalytic converter, consists of a ceramic honeycomb coated in a thin layer of catalyst metals (Pt, Pd, Rh)

Unburnt hydrocarbons formation

Not all of the fuel in the internal combustion engine combusts.

Unburnt hydrocarbons effect

Green house gases and also react with NOx to form a photochemical smog

Unburnt hydrocarbons removal

Catalytic converter, exhaust gases from cars are passed through the catalytic converter, consists of a ceramic honeycomb coated in a thin layer of catalyst metals (Pt, Pd, Rh)

Oxides of nitrogen formation

High temperature of internal combustion engine, when a spark ignites the fuel, causing nitrogen and oxygen from the air to react: N2 + O2 -> 2NO

Oxides of nitrogen effect

React with water and oxygen in the atmosphere to form acid rain.

Oxides of nitrogen removal

Catalytic converter, exhaust gases from cars are passed through the catalytic converter, consists of a ceramic honeycomb coated in a thin layer of catalyst metals (Pt, Pd, Rh)

Sulfur dioxide formation

Crude oil contains traces of sulfur compounds which combust to form sulfur dioxide.

Sulfur dioxide effect

React with water and oxygen in the atmosphere to form acid rain.

Sulfur dioxide removal

Already mostly removed from petrols in motor cars but in coal-burning factories the waste gas is passed through scrubbers containing CaO (basic)which reacts with SO2 gas (acidic) to produce a salt (calcium sulfate). This is called 'Flue-gas desulferisation'

Why honeycomb shape

As it maximises the surface area and minimises the use of the expensive metals.

Catalytic converter equations

CO + NO → CO₂ + ½N₂

C₈H₁₈ + 25NO → 8CO₂ + 9H₂O + 12.5 N₂

Reactivity of alkanes

The alkanes and cycloalkanes, except for cyclopropane, are the least chemically reactive class of organic compounds.

Why alkanes are not reactive

Alkanes contain strong carbon-carbon single bonds and strong carbon-hydrogen bonds. The carbon and hydrogen have very similar polar/electron negativity values and s alkane molecules are non-polar. This means they cannot attract other molecules or ions (what makes alkanes so unreactive)

Halogenoalkane how forms from alkane (visual description)

As alkanes are unreactive, if we mix methane and bromine together in the dark there is no reaction as the non-polar alkane is not attracted to the bromine. If however you mix it under a UV light the red-brown bromine colour will fade and discolourises and misty fumes of hydrogen bromide will form.