A level Chemistry AQA - Alkanes

Alkanes definition

A homologous series of saturated hydrocarbons

Alkanes general formula

CnH2n+2

Physical properties - polarity

Non-polar

Why are alkanes non-polar?

The difference in electronegativities between H and C is very small so alkanes only form van der Waals forces between molecules.

Physical properties - melting/boiling points

Increase as Mr of alkanes increases

Why does mp/bp of alkanes increase as Mr increases?

As Mr increases, the number of electrons in the molecules increases and the surface area of molecules increases. Therefore the strength of van der Waals forces between molecules increases and require more energy to overcome.

Physical properties - branching

As branching increases, the melting points of alkanes decreases.

Why do melting points of alkanes decrease as branching increases?

Branching decreases the surface area of molecules which decreases the strength of van der Waals forces between molecules.

Physical properties - solubility

Insoluble in water

Why are alkanes insoluble in water?

Because they are non-polar and water is polar

Uses of fractional distilation

Separates mixtures of hydrocarbons

Crude oil definition

A mixture of hydrocarbons containing mainly alkanes

Fractional distillation definition

The continual evaporation and condensation of a mixture which causes the components to seperate because of a difference in boiling points.

Fractions definition

A group of compounds which have similar boiling points and are removed at the same level of a fractionating column

Temperature down fractionating column

Increases

Number of carbons in compounds down fractionating column

Increases

Boiling points of hydrocarbons down fractionating column

Increase

Strength of van der Waals forces down fractionating column

Increases

Order of hydrocarbon fractions in column from top to bottom

Refinery gases, gasoline, naphtha, kerosene, diesel, fuel oil, bitumen

Refinery gases no. of carbons

1-4

Gasoline no. of carbons

5-12

Naphtha no. of carbons

7-14

Kerosene no. of carbons

11-15

Diesel no. of carbons

15-19

Fuel oil no. of carbons

20-30

Bitumen no. of carbons

>30

Process of fractional distillation of crude oil

1. Crude oil is heated in a furnace and vaporises

2. The vapour is passed through the fractionating column which is cooler at the top and hotter at the bottom

3. When a substance reaches a cool enough layer it condenses and is seperated off as a liquid.

4. Shorter chain molecules with lower boiling points condense high up the column.

Cracking definition

A process where long chain hydrocarbons are broken down into shorter chain molecules which are in high demand and are more useful

Thermal cracking temperature

1000-1200K

Thermal cracking pressure

70atm

Products of thermal cracking

Short chain alkanes and alkenes

Catalytic cracking temperature

800-1000K

Catalytic cracking pressure

1-2atm

Catalytic cracking catalyst

zeolite

What is the zeolite catalyst made from?

Aluminium oxide and silicon dioxide

Products of catalytic cracking

Fuels for road vehicles and aromatic hydrocarbons

Complete combustion definition

The burning of a substance in a plentiful supply of oxygen so the carbon can be fully oxidised to carbon dioxide

Incomplete combustion definition

The burning of a substance in a limited supply of oxygen so the carbon can not be fully oxidised

Combustion in an engine process

1. Fuel mixed with a large excess of air is drawn into a combustion chamber.

2. The mixture is then compressed and ignited

3. It burns, releasing energy and causing the movement of engine parts.

Environmental consequences of burning alkanes

Global warming, acid rain, photochemical smog

pH of acid rain

<5.5

Formation of acid rain - from NO₂

• 2NO_2(aq) + H₂O_(l) + 1⁄2O_2(g) ➔ 2HNO_3(aq)

Formation of acid rain - from SO₃

H₂O + SO₃ → H₂SO₄

What is photochemical smog?

Formed when nitrogen oxides, sulfur dioxide and unburned hydrocarbons reaction with sunlight

Health issues with photochemical smog

eye irritation, lung diseases

Catalytic converter use

Reduces the amount of CO, NO_x and unburnt hydrocarbons released into the air by converting them into less toxic gases using platinum and rhodium catalysts

Flue gas desulphurisation definition

The removal of sulfur dioxide form combustion emissions

Flue gas desulphurisation process

1. The gases are passed through a mixture containing calcium oxide/calcium carbonate

2. The calcium oxide/calcium carbonate are bases so they neutralise the SO₂ and form calcium sulfate (IV)

Steps in free radical substitution

Initiation, propagation, termination

Free radical substitution condition

UV light

Ozone formula

O₃

Ozone function

Prevents a proportion of harmful UV light from the Sun from reaching the surface of the Earth

Initiation equation in FRS of ozone

Cl₂ → Cl^_* + Cl^_*

Two propagation steps in FRS of ozone

Cl*+ O₃ → ClO* + O₂

ClO* + O₃ → 2O₂ + Cl*

Overall reaction of FRS of ozone

2O₃ → 3O₂