Alkanes

Alkanes

Saturated hydrocarbons (single bonds)

Formula

CnH2n+2

Fractional distillation.

Crude oil is firstly heated then passed into the fractioning column. The fractions condense at different heights. The temperature of the column decreases upwards.

Boiling point depends on..

The size of the molecules

The larger the molecules,

The larger the van der waals forces.

Similar molecules …

Have similar size, bp and mass and therefore condense together.

Where do big molecules condense?

At the bottom at higher temperatures.

Cracking

Breaking C-C bonds to convert large hydrocarbons into smaller more useful hydrocarbons. Chemical process involving the splitting up of strong covalent bonds so requires high temperatures.

economic reasons for cracking

1. Fractions with shorter C chains are in more demand than larger fractions

2. The products of cracking are more valuable than the starting materials.

Thermal cracking

• High pressure (7000 kPa)

• High temperature (400-900*C)

• Produces mostly alkanes and sometimes hydrogen

Physical properties

Non-polar. Small difference in electronegativity between the carbon and hydrogen atom. Alkanes are simple molecules. Weak intermolecular forces between alkane molecules (van der waals forces). The strength of the van der waals increases as the relative molecular mass of the molecule increases (increasing carbon chain length). Therefore the mp and bp increases.

Branching if the alkane molecule. Effect on bp

The branched chain isomers have lower mp and bp than the straight chain molecules, as there is a smaller surface area in contact with each other. Decreases the strength of the van der waals forces between the molecules.

What increases down the fractional column?

Increasing temp/ bp/ number of carbon atoms (Mr)/ stronger van der waals forces between the molecules

Catalytic cracking

Long chain alkane is heated under pressure in the presence of a zeolite catalyst.

What is the greenhouse effect

Effect of trapping energy from the sun.

3 greenhouse gases

Water vapour, CO2 and methane

What is global warming?

Increasing average temperature of the atmosphere at the surface of the earth. Caused by the unbalance of the concentrations of greenhouse gases.

Removal of air pollutants via catalytic converters

Reduces the amount of CO, NO2 and unburmt hydrocarbons released into the air. Internal combustion engine converts them into less toxic gases.

What does a catalytic converter use

Uses a rhodium or platinum catalyst to convert harmful products into more stable products / less toxic gases like CO2, H2O or N2.

Honeycomb of ceramic material 🍯

Thin layer of rhodium or platinum catalyst. So little as possible used. Provides a large surface area for the reaction, increasing rate of conversion.

What is the equation for the reaction that removes NO (nitrogen monoxide) in a catalytic converter?

2NO +2CO » N2 +2CO2

How is sulfur dioxide removed from the combustion emissions?

By a process called flue gas desulfurisation

What do the SO2 gases do?

Passed through a wet- solid mixture containing CaO or CaCO3

What are calcium oxide and calcium carbonate?

Bases therefore neutralise the acidic sulfur dioxide to form calcium sulphite.

What does calcium sulphite give when oxidised?

Calcium sulphate

Sulphur impurities can react with water to form… so how can these impurities be removed?

H2SO4. The impurities can be removed from waste products using flue gas desulfurisation

How do alkanes react with halogens?

In the presence of UV light. Produces halogenoalkanes.

When the UV breaks down the halogen bonds, what is produced?

Free radicals