Alkanes

What are alkanes?

Alkanes are saturated hydrocarbons, containing C-H and C-C bonds.

Alkanes are saturated compounds. Explain what is meant by the term “saturated”.

• Alkanes contain only single bonds.

• They have the maximum number of hydrogens for a given number of carbons.

What is the general formula for alkanes?

CnH2n+2

What type of covalent bonds are found in alkanes?

Sigma (σ) bonds.

What is a sigma bond?

A bond formed when electron orbitals from adjacent atoms directly overlap.

A sigma bond contains a pair of electrons, one from each atom from either side of the bond.

Are sigma bonds fully rotational?

Yes.

Explain why alkanes are non-polar molecules.

• Alkanes only contain C-H and C-C bonds.

• Because carbon and hydrogen have similar electronegativity values, the C-H are essential non-polar.

Explain why alkanes are insoluble in water.

• Because alkanes are non-polar, there are no permanent dipoles.

• This means they are unable to form hydrogen bonds with water molecules ∴ they cannot dissolve.

What happens to the melting and boiling points as the length of the carbon chain in alkanes increase?

As the length of the carbon chain increases, the melting / boiling points increases.

Explain why the melting / boiling point increase as carbon chains in alkanes gets longer.

• Alkane molecules have weak Van der Waals forces between them (since they are non-polar).

• The stronger the Van der Waals forces between its molecules, the higher the melting and boiling points of an alkane. This is because greater amounts of energy are required to break them apart.

• The longer the carbon chain, the stronger the Van der Waals forces present ∴ greater melting and boiling points.

Why are the Van der Waals forces present in longer chain alkanes stronger than in shorter chain alkanes?

1. There are more electrons in longer chain molecules → the greater the number of electrons, the stronger the Van der Waals forces.

2. Longer carbon chains have a larger surface area → there are more points of contact where Van der Waals forces can form.

Explain why branched alkanes have lower melting / boiling points that straight chain alkanes.

• The branches prevent carbon chains from packing closely together, and hence less contact points can form between them.

• As a result, there are less points of contact where Van der Waals forces can form ∴ melting / boiling points are lower.

What 2 important reactions do alkanes undergo?

• Combustion

• Halogenation (photochlorination of alkanes)

Write a word equation for the complete combustion of an alkane.

Alkane + oxygen → carbon dioxide + water.

Write a word equation for the incomplete combustion of an alkane.

Alkane + (insufficient) oxygen → carbon monoxide + water.

What are fossil fuels?

Fuels that were derived from organisms that lived several million years ago.

What are the advantages of using fossil fuels?

They are available in many forms so can be used for a variety of uses.

For example, coal and gas are used in power stations. Crude oil is refined into many different fractions and these are used for varying purposes.

What are the disadvantages of using fossil fuels?

• Fossil fuels are non-renewable resources because they take millions of years to form and are being used up very quickly.

• Carbon dioxide, a greenhouse gas is formed during the combustion of hydrocarbons. These gases are responsible for raising the Earth’s temperature and the consequential effects of this – rising sea levels and changes to crop suitability.

• Acid rain is rain with a lower than expected pH. It is formed from the oxides of sulfur and nitrogen that are by-products of burning fossil fuels that contain sulfur and nitrogen. Acid rain causes damage to aquatic life, plant life and buildings and also initiates health issues for people with breathing difficulties.

• Carbon monoxide is formed when incomplete combustion of fossil fuels happens. It is extremely poisonous. Incomplete combustion occurs when there is insufficient oxygen available.