Chapter 12 - Alkanes

Question 1

What is the general formula of alkanes ?

Answer 1

CnH2n+2

Question 2

What kind of covalent bonds are formed in alkanes ?

Answer 2

Sigma bonds

Question 3

What is a sigma bond ?

Answer 3

The overlap of two orbitals

Question 4

How are alkanes separated ?

Answer 4

Fractional distillation

Question 5

How does chain length effect boiling point ?

Answer 5

Larger chain length raises boiling point

Question 6

Why does chain length have that effect on boiling point ?

Answer 6

• Increased chain length
• Greater surface area of contact
• Stronger london forces
• More energy needed to overcome these forces
• Higher boiling points

Question 7

What is the effect of branching on boiling point ?

Answer 7

Lowers the boiling point

Question 8

Why does branching have that effect on boiling point ?

Answer 8

• More branching
• Less surface area of contact
• Weaker London forces
• Less energy required to overcome these forces
• Lower boiling point

Question 9

What are the reasons for alkanes being fairly unreactive?

Answer 9

• C-C and C-H sigma bonds are strong

- C-C bonds are non polar and C-H bonds are fairly non polar too

Question 10

What is complete combustion?

Answer 10

Alkanes burn completely to produce carbon dioxide and water.

Question 11

What is the combustion reaction of an alkane in words?

Answer 11

Alkane + oxygen —> water + carbon dioxide

Question 12

What are the two potential products from incomplete combustion of alkanes?

Answer 12

• Carbon monoxide

- Carbon or soot

Question 13

What does carbon monoxide cause ?

Answer 13

Carbon monoxide is poisonous as it binds with haemoglobin in the blood and prevents oxygen binding

Question 14

What does soot cause ?

Answer 14

Soot can cause breathing problems, make buildings dirty and clog up engines

Question 15

How can these be removed ?

Answer 15

• These can be removed by using a catalytic converter

- A catalytic converter uses a rhodium catalyst to convert harmful products into more stable ones

Question 16

What is the name of the mechanism for bromination of alkanes?

Answer 16

Free radical substitution

Question 17

What are the three stages of radical substitution?

Answer 17

• Initiation
• Propagation
• Termination

Question 18

What is initiation ?

Answer 18

• Radicals are produced normally using visible light or ultraviolet
• Called photodissociation reactions
• The bond breaks producing 2 radicals

Question 19

Describe the initiation stage of radical substitution

Answer 19

• Covalent bond in a bromine molecule is broken by homolytic fission.
• Energy provided by UV radiation.
• Br-Br -> Br* + Br*

Question 20

What is propagation ?

Answer 20

• When a radical reacts with a non radical molecule. New radicals are created which then go on to react with other non radicals.
• This is why its called a chain reaction

Question 21

Describe the propagation stage of radical substitution

Answer 21

• Two propagation steps:
• Step 1 - CH4 + Br* -> *CH3 + HBr
• Step 2 - CH3 + Br2 -> CH3Br + Br

Question 22

What is termination ?

Answer 22

• When 2 radicals react they form a non radical molecule
• This ends the chain reaction
• This is why its called termination

Question 23

Describe the termination stage of radical substitution

Answer 23

• Two radicals collide forming a molecule with all electrons paired.
• Br* + Br* -> Br2
• CH3 + *CH3 -> C2H6
• CH3 + Br* -> CH3Br

Question 24

What can occur after a radical substitution reaction?

Answer 24

Further substitution