Chapter 1 - Green Chemistry

Question 1

Is the ozone sufficient to absorb the solar radiation?

Answer 1

Yes, despite when all the stratospheric ozone were compressed into a single layer at STP on Earth and the layer would be 3 mm thick, its low concentration would be sufficient to filter out the solar radiation bet. 200 & 300 nm.

Question 2

Mention the effects of UV radiation.

Answer 2

It can induce 1- skin cancer 2- genetic mutation 3- destroy crops 4- destroy other forms of vegetation

Question 3

Complete: The formation & destruction of ozone by natural processes is …

Answer 3

A dynamic equilibrium that maintains a constant concentration of ozone in the stratosphere.

Question 4

What are the trade name and common names of CFCs?

Answer 4

The trade name is Freon & some of the common ones are:

1) Freon 11 ( CFCl3 )
2) Freon 12 ( CF2Cl2 )
3) Freon 113 ( C2F3 Cl3 )
4) Freon 114 ( C2F4Cl2 )

Question 5

What are the characteristics of CFCs?

Answer 5

They are readily liquified & relatively 1) inert 2) nontoxic 3) non-combustible 4) volatile

Question 6

Mention the uses of CFCs

Answer 6

They are used

• as coolants in refrigerators and air conditioning (in place of the highly toxic SO2 & ammonia)
• in manufacture of disposable foam products ( as cups and plates)
• as solvents to clean newly soldered electronic circuit boards
• as aerosol propellants in spray cans

Question 7

What is the harmful effects of CFCs on ozone?

Answer 7

The problem is that CFCs eventually discharge into the atmosphere & because of their inertness, the CFCs diffuse unchanged to the stratosphere, where UV radiation decomposes them.
( Equation: CFCl3&raquo_space; CFCl2 + Cl , CF2Cl2&raquo_space; CF2Cl + Cl )

Thus, the reactive Cl atoms undergo other reactions that cause ozone depletion : ( Cl + O3&raquo_space; ClO + O2 , ClO + O&raquo_space; Cl + O2 )

• The Cl atom plays the role of catalyst as it is not used up in the reaction, so it can react on another reactions). Actually, it can destroy 100,000 O3 molecules before its removed by other reactions.
• The ClO is an intermediate as it is produced in the First Elementary step and consumed in the Second Step.

Question 8

Apart from CFCs, what can destroy the ozone layer?

Answer 8

Nitrogen oxides because Solar radiation decomposes them into NO, which participates in the destruction of ozone :
O3 >UV» O + O2
NO + O3 &raquo_space; NO2 + O2
NO2 + O &raquo_space; NO + O2

• NO is the catalyst because it is not used up in this reaction, so it can be used in others , and the NO2 is the intermediate because it is produced in the First Elementary Step and consumed in the Second Step.

Besides, NO2 also reacts with ClO to form Chlorine nitrate:
ClO + NO2 &raquo_space; ClONO2
Chlorine nitrate (ClONO2) is stable and serves as “ Chlorine reservoir”, which destroys the stratospheric ozone

Question 9

What are the solutions of CFCs issue?

Answer 9

1) Montreal protocol was signed by some nations, setting targets for cutbacks in CFCs production and complete elimination of these substances by year 2000. However, many nations couldn’t abide it due to the importance of CFCs to their economies.
2) recycling is a good technique as it prevents the CFCs from escaping into the atmosphere
3) hydrochlorofluorocarbon ( HCFC-123 , CF3CHCl2 ) is a good substitute of CFCs because it doesn’t have harmful effect to the ozone.
4) hydrofluorocarbon ( HFCs ) is another and better substitute due to the elimination of Cl atoms

5) sending fleet of planes to spray 50,000 tons of ethane ( C2H6 ) or propane ( C3H8 ) high over the South Pole.
Cl + C2H6&raquo_space; HCl + C2H5
Cl + C3H8&raquo_space; HCl + C3H7

6) rejuvenate the ozone layer by producing large quantities of ozone & releasing it into stratosphere from air planes. It is feasible but unrealistic as it is very very costly.

Question 10

Why is the hydrochlorofluorocarbon is a better substitute of CFCs?

Answer 10

Due to the presence of Hydrogen atom, it makes it susceptible to oxidation in the troposphere, so it never reaches the stratosphere. Actually, there it is attacked by the hydroxyl radical.

CF3CHCl2 + OH&raquo_space; CF3CCl2 + H2O

The CF3CCl2 react with O2 and decompose into CO2 water and hydrogen halides that are removed by rainwater.
However, laboratory tests have shown that HCFC-123 can cause tumor in rats but its effects on human is unclear.

Question 11

What is the difference between HFCs and HCFC-123.

Give examples of the HFCs compounds.

Answer 11

HFCs reach the stratosphere without harming the ozone as they don’t have chlorine atom that destroys the ozone.
CF3CFH2 are used in place of CFCs in refrigeration and air conditioning.

Examples of its compounds are : 
CF3CFH2,  
CF3CF2H,  
CF3CH3,  
CF2HCH3

Question 12

Describe formation of ozone.

Answer 12

• It begins with the photodissociation of oxygen molecules by solar radiation at wavelength below 240 nm
  (Equation of the oxygen decomposition: O2&raquo_space; O + O )
• The reactive oxygen atoms combine with oxygen molecules and M (some inert substance as N2), which absorbs some of the excess energy released in this exothermic reaction and prevents the spontaneous decomposition of ozone molecules.
  ( equation: O + O2 + M&raquo_space; O3 + M )
• the energy not absorbed by M is given off as heat.
• then, the ozone itself absorbs the UV radiation between 200 & 300 nm
  ( Equation: O3 > UV > O + O2 )
• the process continues when O & O 2 recombine to form O3 & causes warming the stratosphere.