Chapter 12 - Alkanes

Question 1

Are Alkanes reactive and why?

Answer 1

Alkanes lack reactivity.
• Alkanes contain only C–H and C–C bonds, which are relatively strong and difficult to break.
• C-C bonds are non polar
• The electronegativity of carbon and hydrogen is so similar they can be considered non polar.

Question 2

How does chain length effect the boiling point?

Answer 2

Boiling point increases as chain length increases because there is now a larger surface area therefore more points of contact are available. There are now stronger London forces and more energy is needed to break the forces.

Question 3

What is a sigma bond?

Answer 3

A sigma bond is the overlap of 2 orbitals.

Question 4

What type of bonds do alkanes have?

Answer 4

Each carbon is bonded to 4 other atoms by a single covalent bond called a sigma bond

Question 5

What are the shape of alkanes?

Answer 5

• Each carbons is surrounded by 4 bonding electron pairs
• therefore it’s is a tetrahedral structure around each carbon atom ( 109.5’)
• Free rotation around single bonds

Question 6

Does boiling point increase or decrease on alkanes as more branches are added.

Answer 6

• Boiling point decrease
• There is less surface area therefore less points of contact per molecule
• There are weaker London forces therefore less energy is needed to break them

Question 7

Describe how Fractional distillation works.

Answer 7

• Different fractions in crude oil have different boiling points.
• Increase the the temperature of the system so each fraction vaporises separately from the other fractions and can be collected.

Question 8

What is complete combustion and what does it form?

Answer 8

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

Question 9

What is Incomplete combustion and what does it form?

Answer 9

When there is not enough oxygen present for complete combustion to happen. The hydrogen atoms in the alkane will be oxidised to water but the carbon atoms are not fully oxidised.

• This produces carbon monoxide or carbon instead of carbon dioxide.

Question 10

What are the steps in free radical substitution?

Answer 10

Step 1 - Initiation
Step 2 - Propagation
Step 3 - Termination

Question 11

What are the limitations of radical substitution in organic synthesis?

Answer 11

• Multiple substitution- in the second step CH3Cl is formed. Another Cl• can collide with the CH3Cl and further substitute a hydrogen. This can happen until all the hydrogen are substituted.
• Substitutions at different points on the carbon chain

Question 12

Explain why the straight chain isomer of C10H22 is converted by the petroleum industry into its branched chain isomers.

Answer 12

Branched chains have more efficient combustion

Question 13

What is meant by the term fractional distillation?

Answer 13

Seperation by differences in boiling point