Alkanes Final

Uses of chlorofluorocarbons (CFCs)

• refrigerators

• Propellants for aerosols

CFC properties

• Inert

• Non-flammable

• Non-toxic

How does the chlorine radical as a catalyst in ozone depletion

• the chlorine radical is reformed chemically unchanged from the first step

Benefits of the ozone layer

• Absorbs uv light

• Reduces rates of skin cancer

Formula for chlorine radicals catalysing the decomposition of ozone

Why is the ozone layer important?

It absorbs harmful high energy uv light to prevent it reaching earth

What can uv light cause?

Skin cancer

How is crude oil separated?

Fractional distillation

How is petroleum separated?

Fractional distillation

Process: Fractional distillation

• crude oil heated to form a vapour

• Gas passes into fractionating column with negative temperature gradient

• Fractions condense at a height where the temperature is lower than their boiling point

Order of compounds in the fractionating column

Cracking

• breaking c-c bonds in log chain hydrocarbons into short chain hydrocarbons

Economic reasons for cracking

• most useful fractions contain shorter chain hydrocarbons

• Most in demand “””””

Thermal Cracking Condition

Temperature: 700-1200K

Pressure: 7000kPa

Thermal cracking mostly produces

• high percentage of alkanes

Catalytic Cracking Conditions

Temperature: 720K

Pressure: 5atm (mild)

Catalyst: Zeolite catalyst

Catalytic cracking mostly produces

• Motor fuels

• Aromatic hydrocarbons

Features of zeolites

• honeycomb structure with large SA

Benefits of catalytic cracking

• catalytic cracking is cheaper

• Lower pressure used

Complete Combustion of alkanes

• sufficient oxygen present

• → carbon dioxide + water

Incomplete Combustion

• insufficient oxygen

• → carbon monoxide/carbon (soot(solid)) + water

Pollutants of internal engine combustion

• carbon monoxide

• Nitrogen oxides

• Sulphur dioxide

Carbon Monoxide as a pollutant

• a result of incomplete combustion of petrol vapour

Sulphur dioxide as a pollutant

• sulphur converted to sulphur dioxide

• (Unless removed from atmosphere) forms sulphuric (IV) acid

• Forms acid rain

Flue gases

Mixture of waste gases

Removal of sulphur dioxide from flue gases

• using calcium oxide/ calcium carbonate in acid-base reaction

Nitrogen Oxides as pollutants

• nitrogen has a triple bond

• requires a lot of energy to break

• High temperatures of engine provide enough energy

Equation for nitrogen oxides

Nitric acid formation

• nitrogen monoxide oxidised to nitrogen dioxide

• Nitrogen dioxide reacts with atmosphere to form nitric acid

4NO₂(g) + 2H₂O(l) + O₂ → 4HNO₃(aq)

Catalytic converter - how does it work?

• Nitrogen oxides oxidise carbon monoxide to carbon dioxide and are themselves reduced

What is used on catalytic converters?

• platinum catalyst is used in catalyst converters

What happens to unburnt hydrocarbons in the catalytic converter?

• they are oxidised to carbon dioxide and water

Nitrogen monoxide removed from catalytic converters

• when reacted with octane

Chlorination of Alkanes Steps

FREE RADICAL SUBSTITUTION

1. Initiation

2. Propagation

3. Termination

Initiation

• UV radiation provides energy to break chlorine molecule into 2 chlorine free radicals

Why does initiation reaction takes place?

The halogen-halogen (name the halogen) bond is weaker than the C-H bond in alkane (name the alkane).

First propogation

• chlorine radical reacts with alkane

• Halogen halide + alkyl radical

Second Propogation

• Alkyl radical reacts with halogen molecule

• → Halogenoalkane + chlorine radical

Termination

• two radicals react together

How do you prevent further substitution in the chlorination of alkanes?

Excess methane

Requirements for free radicals substitution

• UV Radiation