C15

Question 1

Ozone layer

Answer 1

• formed at outer edge of stratosphere
• 10km - 40km above Earths surface
• absorbs most of biologically damaging UV radiation (UV-B) from Suns rays

Question 2

UV-B rad

dangers of uv-b

Answer 2

most linked radiation to sunburns

fears that increased UV-B can lead to increased genetic damage + skin cancer in humans

Question 3

reaction of ozone in stratosphere

Answer 3

O2 -> 2O
steady state set up involving O2 and O radicals
O2 + O (equilibrium sign) O3

Human activity, especially use of CFCs has disrupted this equilibrium

Question 4

CFCs main uses

Answer 4

refrigerants e.g. AC units and as aerosol propellants

Question 5

Why are CFCs very stable

Answer 5

due to the high strength of C-F and C-Cl bonds in their molecules

Question 6

What happens to CFCs at stratosphere?

Answer 6

UV radiation provides sufficient energy to break C-X bond in CFCs by homolytic fission forming radicals

C-Cl bond: lower bond enthalpy - so this bond breaks

Question 7

What is the breakdown of CFCs called?

Answer 7

Photodissociation, because its breakdown is initiated by radiation

Question 8

1987 Montreal Protocol

Answer 8

• introduced steps for complete removal of CFCs
• except for few products where no alternative could be found
• e.g. refrigeration / AC –> coolants using hydrocarbons, ammonia, carbon dioxide

Question 9

Properties of CFCs

Answer 9

• Unreactive
• Inert - chemically stable in troposphere
• Non-Flammable
• Non-toxic
• Volatile

As a gas: easily compressed, low bp, liquefied under pressure

Question 10

Nucleophile

Answer 10

Electron pair donor

Question 11

Hydrolysis

Answer 11

Chemical reaction invoking water / aq solution of hydroxide - causes breaking of bond in a molecule
-results in molecule being split into 2 parts

Question 12

How can haloalkanes be converted into alcohols?

Answer 12

using aqueous NaOH

reaction mixture heated under reflux

Question 13

Hydrolysis and carbon halogen bond strength

Answer 13

Going down the group carbon halogen bond strength decreases

Weaker the carbon-halogen bond, the less energy needed to break this bond, so less time taken for reaction to reach completion

Question 14

Measure rate of reaction of primary haloalkanes

Answer 14

Carry out reaction using silver nitrate:
Ag+ + X- –> precipitate of AgX
Water is present in aq silver nitrate

As haloalkanes are insoluble - carry out reaction in presence of ethanol solvent
Ethanol allows water and haloalkane to mix