Chapter 10 - Alkanes

Question 1

Why are alkanes least reactive?

Answer 1

The only 2 types of bonds it contains, C-C and C-H are strong and non-polar (due to similarity in electronegativity). Thus, they are unreactive towards polar reagents.

Question 2

What are alkanes held together by?

Answer 2

Weak dispersion forces

Question 3

How does the physical state of alkanes change down the group? (3)

Answer 3

1) Methane to butane: gas
2) C13 to C17 straight chain alkanes: liquid
3) C18 or more: solids

Question 4

How does the physical properties of alkanes change down the group?

Answer 4

1) boiling point: increases regularly with increasing number of carbon atoms; melting point not as regular
2) density: increase slightly with increasing number of carbon atoms
3) viscosity: increases with increasing number of carbon atoms.

Question 5

Why does the boiling point of alkanes increase with increasing number of carbon atoms in the chain?

Answer 5

As the number of carbon atoms increase, the number of electrons in the molecule increases, hence size of electron cloud increases, leading to stronger dispersion forces between the molecules. Thus more energy is required to overcome the stronger dispersion forces between the molecules.

Question 6

Why is the increase in melting point of alkanes not as regular as boiling points?

Answer 6

The dispersion forces between the molecules in the crystal depend on both the size of molecule and the arrangement of molecules in a crystal lattice.

Question 7

What is the solubility of alkanes in water?

Answer 7

They are insoluble in water. Alkanes can only form pd-pd intermolecular forces with water, which are much weaker than the hydrogen bonding between water molecules themselves and dispersion forces between alkane molecules.

Question 8

What is the solubility of alkanes in non-polar solvents?

Answer 8

They are soluble. Alkanes can form favourable dispersion forces with non-polar solvents.

Question 9

Why does the density of alkanes increase with the increase in number of carbon atoms?

Answer 9

As the number of carbon atoms increase, the number of electrons in the molecule increases, hence size of electron cloud increases, leading to stronger dispersion forces between the molecules. The increase in the strength of dispersion forces cause the alkane molecules to attract more closely together, resulting in a slightly smaller volume of the liquid.

Question 10

Why does the viscosity (resistance to flow) of alkanes increase with the increase in number of carbon atoms? (2)

Answer 10

1) As the number of carbon atoms increase, the number of electrons in the molecule increases, hence size of electron cloud increases, leading to stronger dispersion forces between the molecules. Thus, dispersion forces increase.
2) longer-chain alkanes have higher viscosity as the long molecules tend to “tangle-up” with each other.

Question 11

What are 2 chemical reactions alkanes can partake in?

Answer 11

1) Free radical substitution (with halogens)

2) Combustion

Question 12

What is free radical substitution?

Answer 12

It is when alkanes react with halogens in the presence of ultraviolet light or heat to form halogenoalkanes. In this reaction, one or more of the hydrogen atoms in the alkane can be substituted by the halogen atom(s).

Question 13

What is observed when free radical substitution occurs? (2)

Answer 13

1) greenish-yellow chlorine gas decolourises slowly or

2) reddish brown bromine liquid decolourises slowly

Question 14

Describe the mechanism of the free radical substitution. (3)

Answer 14

1) Initiation: Cl-Cl bond is homolytically broken to give chlorine free radicals. (Cl•) The energy required comes from the light absorbed/heat supplied.
2) Propagation: The highly reactive chlorine radical collides with an alkane molecule and abstracts a hydrogen atom to form HCl and an alkyl radical. This alkyl radical then reacts further with another molecule of Cl2 to form chloroalkane (eg CH3Cl) and REGENERATE a new chlorine radical. The chlorine radical formed can then react with another alkane molecule.
3) Termination: Two radicals collide and combine to form a stable product. In this stage, radicals needed for the propagation steps are consumed, terminating the chain reaction.

Question 15

Explain why the reaction of alkanes with halogens require only a flash of UV light rather than prolonged radiation.

Answer 15

It is a chain reaction that is self sustaining as the chlorine radical is regenerated in the propagation step. Hence, only a flash of UV light is required to produce some Cl• to initiate the reaction. Prolonged radiation might cause uncontrolled explosion as too many reactive radicals will be formed.

Question 16

Why is free radical substitution difficult?

Answer 16

The extent and the position of substitution is difficult to control. In the presence of excess halogen, further substitutions of the product can occur, resulting in a mixture of multi-substituted halogenoalkanes.

Question 17

How is the stability of alkyl radicals determined?

Answer 17

The more alkyl groups attached to it, the more stable the radical is and hence it forms more easily.

Question 18

What are 2 factors that affect the proportion of products formed during free radical substitution?

Answer 18

1) ratio of different types of H atoms present in the alkane. (NEEDS CLARIFICATION)
2) Stability of each type of carbon radical intermediate present.

Question 19

What is the reactivity of alkanes towards halogenisation?

Answer 19

Decreases in the following order:

F2 > Cl2 > Br2 > I2

Question 20

Describe the combustion of alkanes.

Answer 20

Gaseous alkanes (must vaporise) react with excess oxygen in complete combustion to give carbon dioxide and water. It is a highly exothermic process but has a high activation energy. 
When burnt in limited supply of oxygen (incomplete combustion), carbon (soot) and carbon monoxide are formed.

Question 21

Why is carbon monoxide harmful?

Answer 21

It binds irreversibly with haemoglobin in blood, forming a stable complex and reducing the capacity of haemoglobin to transport oxygen. It causes drowsiness and headaches, and can be fatal.

Question 22

How does unburnt hydrocarbons harm the environment?

Answer 22

In the presence of sunlight, they react with oxygen, ozone and nitrogen oxides to form photochemical smog.

Question 23

How are oxides of nitrogens formed?

Answer 23

In combustion chambers of motor vehicles. At high temperatures of combustion, atmospheric nitrogen and oxygen combine to form NO, which further oxidises to form NO2.

Question 24

How does oxides of nitrogen harm the environment?

Answer 24

1) they irritate lungs, causing bronchitis and pneumonia and lower resistance to respiratory infections.
2) they form photochemical smog: choking mixture of ozone, oxides of nitrogen and other gases. Harming plants and causing chest pains and breathing difficulties in children.
3) NO2 dissolves in water to give a mixture of nitrous acids (HNO2) and nitric acids (HNO3) & catalyses the formation of SO2 to SO3, which dissolves in water to form sulfuric acid. These acids produce acid rain: rendering farm land unsuitable for cultivation, corrode buildings and affect marine lives by acidifying water.

Question 25

How does methane impact the environment?

Answer 25

It is a highly potent greenhouse gas. As concentration of greenhouse gases increases, there will be a greater proportion of radiation trapped in the atmosphere, resulting in an increase in global temperatures.

Question 26

What are catalytic converters?

Answer 26

It removes pollutant gases from the exhaust of a vehicle by oxidising/reducing them. The exhaust gases pass through a converter containing an alloy of platinum or rhodium, that act as catalysts.

Question 27

What are the 3 redox reactions that occur in catalytic converters?

Answer 27

1) carbon monoxide is oxidised to carbon dioxide
2) oxides of nitrogen are reduced to nitrogen
3) unburnt hydrocarbons oxidise to carbon dioxide and water.