2. Alkanes

Question 1

What reactions can alkanes undergo?

Answer 1

Combustion
Cracking
Free radical substitution

Question 2

Reactivity of X2 in FRS reactions

Answer 2

F2 > Cl2 > Br2 > I2
If X is F, there is an explosive reaction even in the dark.
If X is Cl or Br, reaction proceeds rapidly only in the presence of UV light.
If X is I, there is no reaction under normal laboratory conditions.

Question 3

What are the steps in a FRS reaction?

Answer 3

Initiation
Propagation
Termination

Question 4

What is the general formula of alkanes?

Answer 4

CnH2n+2

Question 5

State and explain the trend in melting and boiling points for alkanes.

Answer 5

MP & BP increase as the number of carbon atoms increases –> alkanes have simple covalent structures and exist as discrete molecules held tgt by weak id-id forces
Strength of id-id forces between the molecules increases as the number of electrons in the molecule increases, requiring more energy to overcome

Question 6

Describe and explain the trend in density for alkanes.

Answer 6

Alkanes are less dense than water.
As the no. of carbon stoke increases (Mr increases), density increases as the increase in Mr outweighs the increase in volume. (Density = mass per unit volume)

Question 7

Describe and explain solubility of alkanes

Answer 7

All alkanes are insoluble in aqueous solutions.
Water is a polar solvent. However, alkane molecules are non-polar and cannot form effective interactions with the water molecules –> Does not mix with water and liquid alkane will form a layer above water.

Question 8

Describe and explain the chemical properties of alkanes.

Answer 8

Alkanes are generally unreactive towards polar and ionic reagents