C12- Alkanes

Question 1

What are alkanes?

Answer 1

-saturated hydrocarbons.
-main components of natural gas and crude oil.
They are very stable.
-lack of reactivity has kept crude oil in the Earth for millions of years.
-mainly used for fuels (generate heat when reacting with O2)
General formula= CnH2n+2

Question 2

Explain the bonding in alkanes.

Answer 2

• atoms are joined together by single covalent bonds.
• each carbon atom is joined to 4 other atoms by c-c bonds. These are a type of covalent bond called sigma bonds.
• therefore each carbon atom has 4 sigma bonds (either C-C or C-H)

Question 3

What is a sigma bond?

Answer 3

-a type of covalent bond.
- it is the result of the overlap of two orbitals.
Each overlapping orbital contains one electron so the sigma bond has two electrons that are equally shared between the bonding atoms.
-positioned on a line directly between bonding atoms.

Question 4

Explain the shape of alkanes.

Answer 4

• repulsion between electron pairs results in a 3D tetrahedral arrangement around each carbon atom.
• each bond angle ≈ 109.5*
• the sigma bonds act as axes which the atoms can rotate freely. Therefore these shapes are not rigid.

Question 5

What is the effect of chain length on boiling point?

Answer 5

• as the chain length increases, molecules have a greater surface area, so more surface contact is possible between molecules
• london forces will therefore be greater and so more energy is needed to overcome them= increasing BP.

Question 6

What is the effect of branching on boiling point?

Answer 6

• isomers of alkanes have the same molecular mass, but branched isomers have lower BP.
• branched isomers have fewer surface points of contact between molecules, giving fewer london forces.
• branches prevent molecules getting as close together, decreasing intermolecular forces further.

Question 7

Why do alkanes lack reactivity?

Answer 7

• C-C and C-H sigma bonds are strong.
• C-C bonds are non polar.
• electronegativity of carbon and hydrogen is so similar that the C-H bond can be considered non-polar.

Question 8

Why are alkanes used as fuels?

Answer 8

• they are readily available.
• easy to transport.
• burn in a plentiful supply of oxygen without releasing toxic products.

Question 9

Complete combustion of alkanes.

Answer 9

In a plentiful supply of oxygen, alkanes burn completely to produce carbon dioxide and water.

Question 10

Incomplete combustion of alkanes

Answer 10

• when oxygen is limited, hydrogen atoms are always oxidised to water, but the incomplete combustion of carbons produces toxic carbon monoxide (CO) or just carbon (as soot).
• CO= colourless, odourless, highly toxic. Combines irreversibly with haemoglobin to prevent it from transporting O2.

Question 11

Reactions of alkanes with halogens.

Answer 11

Alkanes and halogens react in the presence of UV radiation that provides initial energy.
(=endothermic reaction)

-it is a substitution reaction as a hydrogen atom in the alkane has been substituted by a halogen atom.

Question 12

What are the 3 stages of a reaction mechanism?

Answer 12

1. Initiation- no free radicals to free radicals.
2. Propagation- free radicals to free radicals.
3. Termination- free radicals to no free radicals.

Question 13

Explain the mechanism of bromination of alkanes.

Answer 13

1. Initiation:
   - covalent bond in bromine molecule broken by homolytic fission.
   - each bromine atom takes one electron= forms 2 radicals.
   - energy provided by UV radiation.
2. Propagation:
   - step one; bromine radical reacts with a C-H bond to form a methyl radical and hydrogen bromide.
   - step two; each methyl radical reacts with another bromine to form bromomethane and a new bromine radical. Propagation is terminated when 2 radicals collide.
3. Termination:
   - two radicals collide to form a molecule with all electrons paired.

(Look at equations!)