Alkanes

Question 1

What are alkanes?

Answer 1

Alkanes are saturated hydrocarbons that contain carbon atoms which are sp3 hybridised and are tetrahedrally bonded to either H atom or other C atoms

Question 2

What is the general formula of alkanes?

Answer 2

CnH2n+2

Question 3

What is the general formula of cycloalkanes?

Answer 3

CnH2n

Question 4

What happens to the melting and boiling point of alkanes as the number of C atom increases and why?

Answer 4

Melting and boiling point increases as alkanes have simple covalent structures and as the number of C atoms increases, the number of electrons in the molecule increases and more energy is required to overcome the stronger id-id interactions between molecules

Question 5

Are alkanes more or less dense than water and why?

Answer 5

Less dense, the packing of alkane molecules is less as compared to water molecules due to weaker id-id interactions between alkane molecules compared to the stronger hydrogen bonds between water molecules

Question 6

What happens to the density of alkanes as the number of C atoms increases and why?

Answer 6

Density increases as when the number of carbon atom increases, molar mass increases, density increases as the increase in molar mass outweighs the increase in volume

Question 7

Are alkanes soluble in water and why?

Answer 7

All alkanes are insoluble in water
- Alkanes cannot form effective interactions with water molecules
- During the dissolution of alkanes in water, the energy needed to overcome the stronger hydrogen bonds between water molecules is not compensated by the energy released when weaker id-id are formed between alkane and water molecules

Question 8

Are alkanes generally reactive or unreactive and why?

Answer 8

Alkanes are generally unreactive towards polar and ionic reagents
- alkanes are non-polar thus unreactive towards polar reagents
- C-C and C-H bonds are very strong, a large amount of energy is required to overcome these strong bonds in order for reaction to occur

Question 9

What are free radicals?

Answer 9

Species that posses at least one unpaired electron

Question 10

What are the reagents and conditions for free radical substitution?

Answer 10

Condition: UV light
Reagents:
- for a large proportion of mono-substituted products, a large excess of hydrocarbon used
- for a large proportion of poly-substituted products, a large excess of halogen used

Question 11

What is the reactivity of halogens in free radical substitutions (most reactive to least reactive)?

Answer 11

F2 > Cl2 > Br2 > I2
- F2 gives explosive reaction even in absence of UV light
- Cl or Br reaction proceeds rapidly only in UV light
- I2 no reaction under normal lab conditions

Question 12

What is the stability of hydrocarbon radicals (most stable to least stable) and why?

Answer 12

tertiary radical > secondary radical > primary radical > methyl radical
- alkyl substituents which are electron-donating help stabilize the radical

Question 13

What other factors can stabilize hydrocarbon radicals?

Answer 13

• Radicals can be stabilised by delocalised pi electron systems like benzene ring
  -partially filled p orbital can participate in resonance with adjacent p orbitals, leading to greater stability

Question 14

What factors destabilize hydrocarbon radicals?

Answer 14

• the greater the s character of the orbital, the half-filled orbital containing the free radical is held more closely to the nucleus, electron affinity of orbital increases, destabilizing the radical (alkyl > alkenyl / aryl > alkynyl)
• Higher electronegativity, greater electron affinity of the half-filled orbital, more unstable
• Smaller atom, electron-deficient orbital spread out over a smaller volume, decrease in stability
• destabilized by electron-withdrawing groups

Question 15

What is cracking?

Answer 15

Cracking is a process where bigger hydrocarbon molecules are broken down into smaller molecules

Question 16

What are the two types of cracking?

Answer 16

• Thermal cracking: very high temperature and pressure
• Catalytic cracking: In presence of catalyst (Al2O3 or SiO2), high temperature and pressure

Question 17

What are the pollutants in car exhaust and where do they come from?

Answer 17

• CO and unburnt hydrocarbons (from incomplete combustion)
• Oxides of nitrogen, NO and NO2 (from reaction of N2 and O2 due to high temperatures in the combustion engine)

Question 18

What contributes to the formation of photochemical smog?

Answer 18

CO, unburnt hydrocarbons and oxides of nitrogen

Question 19

What pollutants contribute to the formation of acid rain?

Answer 19

nitrogen oxides and SO2

Question 20

What do catalytic convertors do?

Answer 20

Convert harmful gas in exhaust into less harmful gas via the action of catalysts like platinum, rhodium and palladium

Question 21

What reaction do Pt/Pd catalyse?

Answer 21

1. CO + 1/2O2 –> CO2
2. C + O2 –> CO2
3. CxHy + (x + y/4) O2 –> xCO2 + y/2 H2O

Question 22

What reaction do Rh catalyse?

Answer 22

2NO + 2CO –> 2CO2 + N2

Question 23

What poisons the catalyst in the catalytic converter?

Answer 23

Lead and Maganese (Pb and Mn)

Question 24

What gases cause the greenhouse effect?

Answer 24

Carbon dioxide, methane, N2O, halogenoalkanes, ozone, water vapour

Question 25

What is the enhanced greenhouse effect?

Answer 25

Greater retention of thermal energy in the earth’s atmosphere, leading to rise in global temperatures

Question 26

What does ozone do in the stratosphere and troposphere do respectively?

Answer 26

Stratosphere: useful role by absorbing harmful UV radiation from the sun
Troposphere: Harmful role as an air pollutant and a greenhouse gas

Question 27

What reactions can alkanes undergo?

Answer 27

1. Combustion
2. Free radical substitution
3. Cracking