Module 4 - Alkanes

Question 1

What is the general formula for alkanes?

Answer 1

CnH2n+2

Question 2

What type of hydrocarbon is an alkane?

Answer 2

They are saturated. All the carbon carbon bonds are single bonds

Question 3

What is the shape around the carbon atoms in alkane molecules?

Answer 3

In an alkane molecule, each carbon atom has four pairs of bonding electrons and no lone pairs, so the molecule has a tetrahedral shape as each bonding pair repels each other equally. The bond angle is 109.5 degrees.

Question 4

What types of bond is in alkanes?

Answer 4

Sigma bonds

Question 5

What is a sigma bond?

Answer 5

The head on overlap of orbitals.

Question 6

How is a sigma bond formed?

Answer 6

Draw it out.

One p orbital, another o orbital. Arrow between the two. Form an oval above and below the c -c bond

Question 7

How does chain length affect boiling point of alkanes?

Answer 7

As chain length increases, the surface area of contact also increases. This means there are stronger London forces between molecules which require more energy to overcome. This means boiling point increases.

Question 8

How does branching affect boiling point of alkanes?

Answer 8

The more branching a molecule has, this lowers the boiling point as the surface area of contact decreases, so London forces are weaker so less energy required to overcome them.

Question 9

Why are alkanes not reactive?

Answer 9

The C-C and C-H sigma bonds are strong
C-C bonds are non-polar
The electronegativity of carbon and hydrogen is so similar that the C-H bins is considered non-polar

Question 10

What are the products of complete combustion?

Answer 10

Carbon dioxide and water.

Question 11

What are the products of incomplete combustion?

Answer 11

Carbon monoxide and water.

Question 12

What is the main use of alkanes?

Answer 12

They are mainly used as fuels, due to their reaction with oxygen to produce heat

Question 13

Why is incomplete combustion an issue?

Answer 13

Carbon Monoxide is an invisible gas. It can bind to Haemoglobin in the blood before oxygen can meaning less oxygen can be carried around the body, leading to oxygen deprivation

Question 14

What is the reaction mechanism for the reaction for alkanes and halogens?

Answer 14

Radical substitution

Question 15

What are thee three steps in radical substitution?

Answer 15

Initiation
Propagation
Termination

Question 16

What happens during initiation?

Answer 16

Free radicals are produced. UV provides energy to break halogen eg Cl2. This is photo dissociation.

The bond splits equally so this is homolytic fission. Cl2 -> 2Cl*

Question 17

What happens during propagation?

Answer 17

The radical binds with the molecule to produce another radical and HCl or HBr.

This new radical bonds with the halogen eg Cl2 to form the haloalkane and another Cl* radical

Question 18

What is a termination reaction?

Answer 18

This is when two radicals bond together. For example Cl* + Cl* -> Cl2. There are three termination steps per reaction

Question 19

What are the two limitations of radical substitution

Answer 19

1) further substitution

2) substitution at different points in the carbon chain