Alkanes Flashcards
Crude Oil
- A FOSSIL FUEL that arose from the decomposition of dead sea plants and animals which were put under het and pressure millions of years ago.
- PETROLEUM is an alternative name for crude oil.
- It is a mixture of about 150 HYDROCARBONS , mainly unbranched alkanes but also cycloalkanes and arenes.
- It exists as a THICK BLACK LIQUID and is difficult to IGNITE, is of limited use in its native state.
- The refining of crude oil takes place via FRACTIONAL DISTILLATION.
- The individual hydrocarbons obtained through fractional distillation are used as FUELS and to make PETROCHEMICALS such as polymers and synthetic fibres.
Fractional distillation of crude oil STEPS
- Crude oil , a mixture of different length hydrocarbons, is fed in.
- The crude oil is VAPOURISED in the furnace.
- GASEOUS crude oil enters the fractional distillation column.
- The fractionating column is HOTTER at the BOTTOM and COOLER at the TOP.
- SHORT-chained hydrocarbons have LOWER boiling points and condense near the TOP where the temperature is LOWER than their boiling point.
- LONG-chain hydrocarbons have HIGHER boiling points and condense near the BOTTOM.
- The RESIDUE contains hydrocarbons which are TOO LONG , boil and are removed at the bottom.
- GASEOUS (petroleum gas) do not condense because the temperature at the top is NOT LOW ENOUGH.
Fractional Distillation
The SEPARATION of the components in a LIQUID mixture into FRACTIONS.
FRACTIONS~ mixtures of components with similar boiling points.
SEPARATION is achieved because individual components across the fractions all have DIFFERENT boiling points.
The products from fractional distillation and the temperatures that they condense at.
20 degrees:
- Liquefied petroleum gas
- 1 to 4 carbons
Use ~ camping gas
150 degrees:
- Gasoline
- 4 to 12 carbons
Use ~ petrol
200 degrees:
- Kerosene
- 11 to 15 carbons
Use ~ jet fuel, petrochemicals and heating.
300 degrees:
- Diesel
- 15 to 19 carbons
Uses ~ fuel and petrochemicals.
370 degrees:
- Lubricating oil
- 20 to 30 carbons
Uses ~ lubricating oil and petrochemicals
RESIDUE:
Fuel oil ~ 30 to 40 carbons, power station fuel.
Paraffin wax ~ 40 to 50 carbons ,candles.
Bitumen ~ 50+ carbons, road surfacing.
Effect of CHAIN LENGTH on boiling points of alkanes
An increased chain length means:
- More points of CONTACT between neighbouring molecules.
- STRONGER LONDON FORCES between the molecules.
- MORE ENERGY or higher temperatures are required to separate LONGER hydrocarbons.
This is how different alkanes in crude oil are separated as their BP vary due to their different CHAIN LENGTHS.
Effect of BRANCHING on boiling points of alkanes
In comparison with UNBRANCHED isomers, BRANCHED isomers:
- Have FEWER points of CONTACT due to the branches preventing the molecules from coming close together.
- Have WEAKER LONDON FORCES.
- Require LESS ENERGY or lower temperature are required for them to be separated.
What is the trend in BOILING POINT going along the homologous series ALKANES.
- The number of CONTACT POINTS INCREASES going along the homologous series.
- The strength of LONDON FORCES INCREASES.
- This means MORE ENERGY is required to separate the molecules.
- Therefore , the boiling point INCREASES.
Reactions of alkanes
- Alkanes are relatively INERT (unreactive)
- his is because their C-C and C-H bonds are STRONG and a chemical reaction that involves the breaking of these bonds require a LOT OF ENERGY .
C-C 347 KJ mol^-1
C-H 413 KJ mol^-1
- They are NON-POLAR and do not attract H+ , OH- or polar molecules.
This means they DO NOT REACT with:
- Acids
- Bases
- Oxidising or reducing agents
- Nucleophiles
- Electrophiles
- Polar reagents
They DO REACT with:
- Oxygen
- Halogens
Complete combustion of alkanes
- Occurs when an alkane is burned in EXCESS OXYGEN.
- Products are CARBON DIOXIDE & WATER.
- OXIDATION REACTION as each element present is combining FULLY with oxygen.
- SHORT alkanes usually burn CLEANLY which means they undergo COMPLETE combustion.
- NATURAL GAS is used for HEATING and COOKING in homes.
- The LARGEST component in natural gas is METHANE:
CH4(g) + 2O2(g) ——— CO2(g) + 2H2O(l)
INCOMPLETE combustion of alkanes
- Occurs when burning the alkane in LIMITED OXYGEN.
- Products are CARBON MONOXIDE & WATER.
- The shortage of oxygen in car engines means some incomplete combustion is inevitable , i.e when burning OCTANE:
C8H18(l) + 8 1/2 02(g) —– 8CO(g) + 9H2O(l)
Properties of CARBON MONOXIDE
- Produced when any fossil fuel is burnt in LIMITED OXYGEN.
- Odourless
- Colourless
- Poisonous
- Irreversibly reduces the amount of oxygen that HAEMOGLOBIN can carry in red blood cells.
Incomplete & complete combustion with a BUNSEN BURNER
HOLE OPEN:
- Flame is blue
- Complete combustion of the gas is taking place.
HOLE CLOSED:
- Incomplete combustion of the gas is taking place.
- Produces carbon monoxide and solid carbon.
- The flame is yellow and luminous because of the carbon particles being produced.
- Heating apparatus with a luminous flame is AVOIDED as it gets covered in BLACK SOOT , solid carbon.
CH4(g) + O2(g) ——– C(s) + 2H2O(l)
How can our HIGH DEMAND for short chained hydrocarbons be produced with our low demand , SURPLUS long chained hydrocarbons?
CRACKING:
- Long chain alkanes are broken down into a mixture of :
- SHORTER chain alkanes and BRANCHED
used to make fuels. - ALKENES used to make polymers.
- This is due to the RANDOM process of BOND BREAKING.
- Both C-H and C-C bonds are broken in the process.
What speeds up the reaction in the CRACKING process?
ZEOLITE CATALYST:
- Speeds up the reaction WITHOUT being used up.
- It LOWERS the temperature requirement and therefore the ENERGY COST of the cracking process.
- Zeolites are crystalline alumino -silicates with PORES of EXACT SIZE.
- Long chain alkanes fit into the pores and the catalyst -alkane mixture is subjected to 450 degrees which results in cracking.
Halogenation of Alkanes
- A halogen atom replaces a HYDROGEN atom in the alkane.
- SUBSTITUTION reaction~ two moles of reactant produces two moles of product.
- The first step involves HOMOLYTIC FISSION of the halogen bond to produce TWO RADICALS.
- This step requires the presence of UV LIGHT.
- Type of reaction: RADICAL SUBSTITUTION
Example:
C6H12 + Br2 ————— C6H11Br + HBr
alkane + halogen ————–haloalkane + hydrogen halide
