Chapter 12 - Alkanes

Question 1

What is the general formula of an alkane?

Answer 1

CnH2n+2

Question 2

Are alkanes unsaturated or saturated hydrocarbons?

Answer 2

Saturated

Contain only C-C and C-H bonds

Question 3

What type of covalent bond features in alkanes?

Answer 3

Sigma bond

Question 4

What is a sigma bond?

Answer 4

Result of the overlap of two orbitals. One from each bonding atom

Question 5

What is the shape of an alkane?

Answer 5

Each carbon has 4 sigma bonds and this results in a tetrahedral shape around each carbon atom, with a bond angle of 109.5 degrees.

Question 6

How is crude oil split into alkanes?

Answer 6

Split into fractions by fractional distillation in a distillation tower. Separation like this is possible because the boiling points of the alkanes are different .

Question 7

Why does boiling point increase with chain length?

Answer 7

• As chain length increases, molecules have larger surface area, so more contact is possible between molecules. The london forces between the molecules will be greater so more energy required to overcome the forces.

Question 8

What is the effect of branching on boiling point? Why’s this?

Answer 8

• As branching increases, the boiling point decreases
• Fewer surface points of contact between the molecules of the branched alkanes, giving fewer London forces.
  Also branches prevent molecules getting as close together - reducing intermolecular forces further

Question 9

Are alkanes reactive?

Answer 9

No, alkanes have low reactivity

Question 10

Why do alkanes have low reactivity?

Answer 10

• C-C and C-H bonds are strong (high bond enthalpy)
• C-C bonds are non-polar.
• Carbon and hydrogen electronegativity is so similar that the C-H bonds can be considered to be non polar

Question 11

What is combustion?

Answer 11

Process of burning something. Gives out heat

Question 12

Why are alkanes used as fuels?

Answer 12

• Readily available
• Easy to transport
• Burn in plentiful supply of oxygen without releasing toxic products

Question 13

What is complete combustion?

Answer 13

Reactant burning completely in plentiful supply of oxygen to produce carbon dioxide and water

Question 14

Write the equation of complete combustion of methane (CH4)

Answer 14

CH4(g) + 2O2(g) -> CO2(g) +2H20(l)

Question 15

What is incomplete combustion?

Answer 15

When reactant is burnt in a limited supply of oxygen. Forming carbon molecules (CO) (or just carbon) and water.

Question 16

Give incomplete combustion equation of heptane (C7H16)?

Answer 16

C7H16(l) + 71/2O2(g) -> 7CO(g) +8H2O(l)

Question 17

What is carbon monoxide and what are the risks of it?

Answer 17

Carbon monoxide is a colourless, odourless, and highly toxic gas. The risk is that it combines with haemoglobin in red blood cells, preventing it from transporting oxygen around the body.

Question 18

What is cracking?

Answer 18

Break down of larger hydrocarbon into smaller hydrocarbons

Question 19

What are the conditions needed for cracking?

Answer 19

Zeolite catalyst and 450 degrees

Question 20

What is free radical substitution?

Answer 20

Photochemical reaction involving substitution of hydrogen in alkane - breaking a C-H bond

Question 21

What are the conditions for free radical substitution?

Answer 21

Presence of ultraviolet (UV) light

Question 22

What are the 3 steps involved in free radical substitution?

Answer 22

Initiation - halogen molecule split by homolytic fission to form 2 radicals in the presence of UV light.

Propagation - Chain reaction begins where products (radicals) of one reaction supply reactants of the next. Occurs until all the reactants have been used up.

Termination - Where 2 radicals combine to form 1 molecule.

Question 23

Write a reaction mechanism for free radical substitution of Ch4 and Br2

Answer 23

Initiation: Br-Br -> Br. +Br.
propagation:
step1: CH4 +Br. -> CH3. +HBr
step2: CH3 +Br2 -> CH3Br + Br.
Termination:
Br. +Br. -> Br2
CH3. + CH3. -> C2H6
CH3. + Br. -> CH3Br

Question 24

What are the limitations of radical substitution in organic synthesis?

Answer 24

Further substitution - substitution can happen until all hydrogen atoms have been substituted
Substitution at different positions in a carbon chain - mixture of monosubstituted isomers by substitution.