12 Alkanes

Question 1

What is the general formula for an alkane?

Answer 1

CnH2n+2

Question 2

What uses do these alkanes have?:
a) C-C4
b) C5-C9
c) C10-C16
d) C12-C20

Answer 2

a) gas
b) petrol
c) kerosene
d) diesel

Question 3

What bonds are present in alkanes?
Explain what these bonds are.

Answer 3

Sigma bonds (covalent)
Forward overlap of two orbitals (can freely rotate)

Question 4

What is the shape and bond angle of alkanes?

Answer 4

Tetrahedral shape
109.5 degrees

Question 5

What is used to separate alkanes from crude oil?

Describe this process.

Answer 5

Fraction mall distillation.
Liquid + vapour pass into a tower that is cooler at the top, Ross until they reach a tray which is cool enough to condense.

Question 6

What is catalytic cracking?

Answer 6

Low temperature + pressure using zeolite

Question 7

What is thermal cracking?

Answer 7

Beta alkanes at a high temperate and pressure to procure branches alkanes etc.

Question 8

Why are alkanes not very reactive?

Answer 8

C—C and C—H bonds are strong
C—C bonds are non-polar
Electronegativity of carbon and hydrogen are so similar that the C—H bonds are considered non-polar

Question 9

What does complete combustion of alkanes produce?

Answer 9

CO2 + H2O

Question 10

What does incomplete combustion of alkanes produce?

Answer 10

CO + H2O

Question 11

Explain how complete combustion of alkanes can contribute to environmental problems.

Answer 11

Produces CO2, this contributes to global warming.

Question 12

Explain how incomplete combustion of alkanes can be dangerous.

Answer 12

Produces CO
A toxic, odourless, clear gas

Question 13

What are the three stages for the mechanism for the reaction of a halogen with an alkanes, and what is it called?

Describe these three stages.

Answer 13

Initiation- covalent bond of diatomic halogen molecules break by homolytic fission

Propagation- radicals react with alkane twice (cycle)

Termination- two radicals collide, it stops the reaction

Question 14

What is the trend in melting points in alkanes?

Answer 14

Increases as the chain length increase because there is a greater surface area of contact between molecules, therefore there are stronger induced dipole-dipole interactions.

Question 15

What is the trend in volatility in alkanes?

Answer 15

Decrease as chain length increases because the longer the chain, the more opportunity for chain-chain interaction.

Question 16

What is the trend in viscosity in alkanes?

Answer 16

Increases as chain length increases, because the intermolecular attractive fleeces increase, thus means nearby molecules get tangled more.

Question 17

Describe and explain the effect of branching on the boiling points of alkanes.

Answer 17

The more branches the chain, the lower the boiling point.
This is because the molecules cannot pack together as tightly
So they have less surface area of contact
This the induced dipole-dipole forces between molecules get weaker
And the boiling points are lower

Question 18

What are the conditions for the reaction of a halogen with an alkane?

Answer 18

Room temperature
UV light