Chemistry 2nd Paper · Classes 11-12 · Chapter 1 · 1.12

CFCs and Ozone Depletion.

Inside a fridge or a spray can, CFC is a harmless, inert gas. Yet once it climbs into the sky, it punches a hole in the ozone layer. How? The answer is the story of one chlorine atom that keeps coming back.

1. How the ozone layer forms and stays

High in the stratosphere, UV rays split O₂ molecules into two O atoms. An O atom then joins another O₂ molecule and makes O₃, which is ozone. O₂(g) → 2O(g) (driven by UV), then M + O(g) + O₂(g) → O₃ + M. M is any particle that soaks up the leftover energy of the reaction.

For hundreds of thousands of years this layer has shielded living things from the harmful effect of UV rays. O₂ and O₃ sit in dynamic equilibrium, one being made while the other breaks down, so the density of the layer stayed steady: 3O₂ ⇌ 2O₃.

2. The ozone hole and the Dobson unit

In 1982 the British scientist Jo Foreman noticed that the ozone layer in the stratosphere above Antarctica had grown thinner. This thinning is what we call ozone depletion, the ozone hole, or the ozone cavity.

The size of the hole is measured in Dobson units (DU). A 0.01 mm thick layer of ozone gas at one atmosphere is one Dobson unit. The normal value is 350 DU in temperate regions, 250 DU in the tropics and 450 DU in sub-polar regions.

Why is ozone denser in sub-polar regions than in the tropics? Because stratospheric winds carry ozone gas from the tropics to the sub-polar regions.

3. CFC: who did it, and where it was used

In 1986 Dr Susan Solomon blamed chlorine pollution. In 1995 the Nobel winners Crutzen, Molina and Rowland established that chlorine atoms released from CFC act as a catalyst in ozone depletion. In the 1980s one million tonnes of CFC entered the air every year.

Table 1.8 gives three CFCs and their uses. CFC-11 (CFCl₃): soft foam, refrigerators, freezers, spray cans. CFC-12 (CF₂Cl₂): rigid plastic foam, refrigerators, air conditioners, spray cans. CFC-113 (CF₂Cl.CFCl₂): grease, glue and washing the solder of electric circuits. Refrigerators and spray cans belong to two of them, so take care.

4. The chlorine atom's cycle

CFC is inert, non-flammable and a gas. So it spreads through the troposphere, rain and other processes do not clear it from the air, and slowly it reaches the stratosphere. The very qualities that make CFC good in a fridge carry it up into the sky.

There, UV rays break the C-Cl bond of the CFC molecule. The result is a Cl atom with an unpaired electron, a free radical: CF₂Cl₂ → CF₂Cl• + Cl• (driven by UV).

This Cl• reacts with ozone and gives the chlorine monoxide radical and O₂: O₃ + Cl• → ClO• + O₂. Then ClO• reacts with an oxygen atom and hands back Cl•: ClO• + O → Cl• + O₂. Put the four steps in order.

The Cl atom is a homogeneous catalyst here, since it and ozone are in the same phase. Its half-life in the stratosphere is about two years, and in that time a single Cl atom can break down one lakh (100,000) ozone molecules.

A Br atom is a stronger catalyst than Cl but stays in the stratosphere for less time. The sources of bromine are the halons (BCF, CBr₂ClF) used in fire extinguishers and the pesticide CH₃Br.

Among the ozone-depleting substances, CFC is the main one. The others are N₂O, NO, CH₄, halons, BCF, CH₃Br and CCl₄. Data up to September 2003 say that 80% of the ozone layer over the south polar region has been destroyed.

5. The Montreal Protocol and the substitutes

To protect the ozone layer the Montreal Protocol was signed in Canada in 1987. Its first two terms: stop making CFCs and halons by 1995, and phase out CCl₄, CH₃Br and methyl chloroform (CH₃CCl₃) step by step.

Substitutes were named too. In aerosols a hydrocarbon such as isobutane (CH₃)₃CH replaces CFC. In refrigerators an HCFC such as CHF₂Cl replaces CFC.

An HCFC lasts less time in the troposphere than CFC, because the •OH radical attacks its H atom and makes water and a carbon free radical: CHF₂Cl + •OH → H₂O + •CF₂Cl. So much of it breaks down before it can reach the stratosphere.

By 2040 HCFCs are to be phased out and HFCs used in motor-car air conditioners. An HFC has no Cl atom, and an F atom is a weak catalyst for breaking ozone. The damaged ozone layer will take a hundred years to fill again.

6. Where the CFC number comes from

The number in a CFC's name comes from the abc rule: a = number of C − 1, b = number of H + 1, c = number of F, and the number is abc. For CFCl₃, a = 0, b = 1, c = 1, so 011, and dropping the leading zero gives 11. Hence CFC-11. When H is present the name says HCFC, so CHF₂Cl is HCFC-22.

By the same rule, CF₂Cl₂: a = 1 − 1 = 0, b = 0 + 1 = 1, c = 2, so 012 = 12, which is CFC-12 (Freon-12). And CF₂Cl.CFCl₂ (C₂F₃Cl₃): a = 1, b = 1, c = 3, so 113, which is CFC-113. Put in numbers yourself.

7. Check yourself

Three questions, starting with the first.

For the second, recall the cycle.

Finish with the last one.

The whole note on eight cards.

Source: Chemistry 2nd Paper, Classes 11-12 (NCTB), Chapter 1 “Environmental Chemistry”, section 1.12.