3.3.2 Alkanes

Definition of Crude oil:

A mixture of hydrocarbons formed from the decomposition of organisms under high temperatures and pressure

Steps of Fractional Distillation:

1. Crude oil is vaporised in a furnace

2. Shorter hydrocarbon chains cool at the top of the fractional column, while longer hydrocarbon chains leave at the bottom

3. Gas leaves at the top of the fractional column

4. Bitumen/Residue leaves the fractional column at the bottom.

Definition of fractional distillation:

A process used to separate crude oil using its boiling points and cool it down

Definition of fractions:

A mixture of hydrocarbons of similar chain length, boiling points, and properties

Definition of viscosity:

The thickness of a liquid

Trends in the fractional column:

• Viscosity increases down

• BP increases down

• Flammability increases up

Orders of fractions from shortest to longest:

1. Gas - Gorillas

2. Petrol - Prefer

3. Naptha - Not

4. Kerosene - Killing

5. Diesel - Dirty

6. Lubricating oil - Little

7. Fuel oil - Frogs

8. Bitumen - Bro

Definition of cracking:

A process that breaks down longer hydrocarbon chains into shorter hydrocarbon chains

What is the purpose of cracking?

To make more useful products e.g. gas

Conditions for thermal cracking:

1. Temperature between 700-1200K

2. Pressure of 7atm

Products of thermal cracking:

• Alkanes

• Alkenes

Conditions for catalytic cracking:

1. Temperature between 200-400K

2. Apply pressure up to 1 atm

3. Add catalyst: Zeolite or Aluminosilicate

Products of catalytic cracking:

• Branched alkanes

• Aromatic compounds

Properties of zeolite

1. Larger surface area - More hydrocarbons can react

2. Hexagonal bonding structure - Hydrocarbons can react inside and outside the structure

Definition of complete combustion:

When combustion takes place with excess oxygen

General equation for complete combustion

Alkane + O₂(g) → CO₂(g)+ H₂O(g)

Effects of CO₂:

• Greenhouse gas

• Traps the sun’s rays, increasing the average temperature

• Increasing Global warming

Definition of incomplete combustion:

When combustion takes place with limited oxygen

General equations for incomplete combustion

Alkane + O₂(g) → CO(g)+ H₂O(g)

Alkane + O₂(g) → C(s) + H₂O

Effects of CO:

• Causes respiratory issues for humans and animals

Effects of C(s)

• Causes respiratory issues

• Leads to smog, which reduces photosynthesis as it blocks sunlight

Effects of S (impurity):

• Forms SO2 and SO3, which lead to respiratory issues

• SO2 can react with water to form Sulfuric acid, which leads to acid rain

Equations for the combustion of S:

S(s) + O₂(g) → SO₂(g)

2SO₂(g) + O₂(g) →2 SO₃(g)

SO₂(g) + H₂O(l) + 1/2O₂(g) → H₂SO₄(aq)

Effects of N (air):

• Forms NO, which causes respiratory issues and later NO2

• NO2 reacts with water, forming nitric acid, causing acid rain

Catalytic converter:

• Found in vehicles

• Used to reduce NO and CO emissions

• Made from Platinum, rhodium, palladium

• Thin sheet arranged with a honeycomb structure

Equations for the combustion of N:

N₂(g) + O₂(g) → 2NO(g)

2NO(g) + O₂(g) →2 NO₂(g)

4NO₂(g) + 2H₂O(l) + O₂(g) → 4HNO₃(aq)

Definition of flue gases:

Gases emitted from power stations

Definition of desulphurisation:

Removal of SO2 from flue gases by spraying CaO or CaCO3 (limewater)

Define a free radical:

A chemical species with an unpaired electron

3 steps of Free radical substitustion

1. Initiation - Homolytic fission takes place

2. Propagation - Free radical catalyses 2 reactions

3. Termination - 2 free radicals combine to form one product

Example of initiation:

Cl₂ →2 Cl^.

Example of propagation:

Cl^. + CH₄ → ^.CH₃ + HCl

^.CH₃ + Cl₂ → ^.CH₃Cl + Cl^.

Example of termination:

^.CH₃ + Cl^. → ^.CH₃Cl

Cl^. + Cl^. → Cl₂

^.CH₃ + ^.CH₃ → C₂H₆

Definition of Homolytic fission:

The breaking of a covalent bond so that each atom receives one electron from the shared pair, forming two radicals.

How does free radicals occur:

UV radiation catalyses homolytic fission leading to free radicals forming