Alkanes

Question 1

Explain general unreactivity of alkanes

Answer 1

Alkanes are saturated and generally unreactive

1. Non-polar bond
   C-H bond is non-polar, thus do not attract polar or charged reagents like H+, OH-, or MnO4-
2. Bond strength
   C-C and C-H bonds are relatively strong.
   C-C : 350kJmol-1
   C-H : 410kJmol-1

However, alkanes do react with oxygen (combustion) and halogen (free-radical substitution) under appropriate conditions

Question 2

Explain alkanes’ unreactivity towards polar reagents

Answer 2

Alkanes are insoluble in polar solvents like water.

• they can only interact with polar solvents through id-id interaction
• the energy released during the formation of weak id-id interaction between alkane molecule and polar solvent molecules such as water is insufficient to overcome the relatively stronger hydrogen bonds between water molecules and id-id interaction between alkane molecules

Question 3

Complete Combustion

Answer 3

• Alkanes burn in excess oxygen to form carbon dioxide and water. The reaction is highly exothermic, hence they are used as fuels
• Combustion of alkanes burns with a non-luminous flame with little to no soot produced and a higher temperature flame is obtained. Burning of longer chain alkanes give rise to sooty flames

CxHy (l) + (x+y/4) O2 (g) -> xCO2 (g) +y/2 H2O (l)

Question 4

Incomplete combustion

Answer 4

• Where supply of oxygen is limited, alkanes burn to form carbon monoxide, water and soot (carbon)

Question 5

Free radical substitution

Answer 5

CxHy + X2 -> CxHy-1X + HX

X2 = Cl2 or Br2

Conditions: UV light, room temperature,
heat may be supplied to initiate the reaction because reaction proceeds very slowly at room temp

Products: mixture of chloroalkanes/ bromoalkanes

Observations:
- Greenish-yellow colour of Cl2 gas decolourises. White fumes of HCl produced
- Reddish-brown colour of Br2 gas decolourises. White fumes of Hbr produced

Question 6

Free radical substitution mechanism

Answer 6

1. Initiation
2. Propagation
3. Termination

Question 7

1. Initiation
2. Propagation
3. Termination

Answer 7

DRAW (lecture notes pg 11)

Question 8

Pollutant: Carbon monoxide

Answer 8

• formed by incomplete combustion of fuel in car engine
• Combines strongly and irreversibly with haemoglobin and makes it ineffective as oxygen carrier in human body -> suffocation or blood poisoning
• Catalytic removal: Conversion of CO to CO2
  2CO (g) + O2 (g) -> 2CO2 (g)
  platinum as catalyst

Question 9

Pollutant: Oxides of nitrogen

Answer 9

• formed by reaction of N2 with O2 at high temperatures
• catalyses formation of acid rain and forms photochemical smog
• Catalytic removal: Conversion of NO to N2
  2NO (g) -> N2 (g) + O2 (g)
  platinum as catalyst
  2NO (g) + 2CO (g) -> N2(g) +2CO2 (g)
  no catalyst

Question 10

Pollutant: Unburnt hydrocarbon

Answer 10

• formed by incomplete combustion of fuel
• forms photochemical smog
• Catalytic removal: Oxidation of unburnt hydrocarbon to CO2 and H2O
  CxHy (l) + (x+y/4)O2 (g) -> xCO2 (g) + (y/2)H2O (l)

Question 11

Catalytic converter

Answer 11

• the catalyst in the converters would be poisoned by the lead present in petrol. Therefore, cars fitted with catalytic converters must use unleaded petrol
• A honeycomb structure is used to maximise the surface area on which catalysed reactions can take place

Question 12

Gases that are involved in the enhanced greenhouse effect

Answer 12

• carbon dioxide and methane are greenhouse gases responsible for global warming and climate changes
• one of the major sources of carbon dioxide comes from burning of fossil fuels for energy production, hence there is a need to find alternative fuels for mankind