4.1.2 Alkanes

Question 1

What is an alkane?

Answer 1

A saturated hydrocarbon containing C-H bonds only

Question 2

What is the general formula of an alkane?

Answer 2

C(n)H(2n+2)

Question 3

Are alkane bonds polar?
Why/why not?

Answer 3

Nonpolar because carbon and hydrogen have similar electronegativities

Question 4

What is the shape and angle of an alkane?

Answer 4

tetrahedral
109.5°

Question 5

Describe the σ (sigma) bond in an alkane

Answer 5

The sigma bond is a covalent bond which has a direct overlap of the electron clouds of the bonding atoms

Question 6

What type of intermolecular forces do alkanes have? Why?

Answer 6

Induced dipole-dipole interaction
because the bonds are non polar

Question 7

What happens to the boiling point as alkane chain length increases? Why?

Answer 7

The boiling point increases because there is more surface area and so more number of induced dipole-dipole interaction. Therefore more energy required to overcome the attraction

Question 8

Does a branched molecule have lower or higher boiling point compared to equivalent straight chain? Why?

Answer 8

The branched molecule has a lower boiling point because they have fewer surface area and hence less induced dipole-dipole interactions

Question 9

Are alkanes soluble in water? Why?

Answer 9

Insolube because the hydrogen bonds in water are stronger than alkanes induced dipole-dipole interaction

Question 10

How reactive are alkanes?

Answer 10

Very unreactive

Question 11

What reactions will alkanes undergo?

Answer 11

Combustion and reactions with halogens

Question 12

What type of reaction is combustion?

Answer 12

Oxidation reaction

Question 13

What is complete combustion

Answer 13

combustion that occurs with plentiful supply of air

Question 14

What are the products of complete combustion when alkanes are used?

Answer 14

Carbon dioxide and water

Question 15

What is the colour of the bunsen burner flame during complete combustion?

Answer 15

Blue flame

Question 16

What is incomplete combustion and what products are formed in the case of alkane?

Answer 16

Combustion in a limited supply of oxygen
Products: water, carbon dioxide and carbon monoxide

Question 17

What is the equation for the complete combustion of propane?

Answer 17

C₃H₈+5O₂ -> 3CO₂ + 4H₂O

Question 18

What type of hydocarbon are most likely to undergo incomplete combustion?

Answer 18

Longer chains

Question 19

What is the environmental impact of carbon monoxide?

Answer 19

It is toxic/poisonous

Question 20

What is the environmental impact of soot (carbon)?

Answer 20

Asthma, cancer, global dimming

Question 21

How are halogenalkanes formed from alkanes?

Answer 21

Radical subsitiution

Question 22

In the presence of what does alkane react with halogens?

Answer 22

UV light

Question 23

What are the three stages of free radical substitution?

Answer 23

Initiation - breaking halogen bond to form free radicals
Propagation - chain part of the reaction where products are formed but free radical remains
Termination - free radicals removed, stable products formed

Question 24

What are the equations for the reaction of CH₄ with Cl₂ to form CH₃Cl?

Answer 24

Initiation: Cl₂ -> 2Cl• (in presence of UV light)

Propagation:
Cl• + CH₄ -> HCl + •CH₃
•CH₃ + Cl₂ -> CH₃Cl + •Cl

Termination:
•CH₃ + Cl• -> CH₃Cl
2Cl• -> Cl₂
•CH₃ + •CH₃ -> C₂H₆