Part 2: Haloalkanes, radicals and ozone

Connect CFC stability to stratospheric radical chemistry, and show how a catalyst can remove many ozone molecules.

Ozone absorbs harmful ultraviolet radiation

Stratospheric ozone absorbs ultraviolet radiation that would otherwise reach the surface. Ozone is continually formed and broken down in photochemical processes. UV can dissociate O₂ into oxygen atoms; an oxygen atom can combine with O₂ to form O₃ with energy transferred to another molecule.

Ozone’s role depends on location: stratospheric ozone is protective, whereas ozone close to the ground is an air pollutant. Ozone depletion and greenhouse warming are distinct processes, though some chemicals contribute to both.

Stable near the surface, reactive under intense UV

CFCs were used as refrigerants, aerosol propellants and blowing agents because useful examples were volatile and relatively unreactive under ordinary conditions. Their persistence allows transport to the stratosphere, where UV breaks C–Cl bonds homolytically and releases Cl•.

In CF₂Cl₂ photodissociation, show a radical on the carbon-containing fragment as well as on chlorine; atoms and unpaired electrons must be accounted for.

Chlorine and nitrogen monoxide catalytic cycles

A chlorine radical consumes ozone to form ClO•, then ClO• reacts with an oxygen atom and regenerates Cl•. Add the steps and cancel intermediates and the regenerated catalyst to find the net reaction. A single radical can repeat the cycle many times before removal.

Nitrogen monoxide radicals can also catalyse ozone breakdown. Sources include atmospheric electrical discharges and high-altitude combustion contexts. The simplified cycle below again regenerates the radical and gives the same overall reaction.

Evaluate replacements by more than one property

Evidence linking CFC-derived radicals to ozone depletion prompted international restrictions and development of alternatives. Chlorine-free replacements avoid this chlorine-radical route, but some are powerful greenhouse gases; “no chlorine” does not mean “no environmental impact”.

Compare ozone-depleting potential, climate effect, toxicity, flammability, efficiency and containment. Long atmospheric lifetimes explain why stopping a source does not instantly remove chemicals already present.

Cancel the cycle and identify each role

In Cl• + O₃ → ClO• + O₂ followed by ClO• + O → Cl• + O₂, Cl• is consumed and then regenerated: it is the catalyst. ClO• is formed and then consumed: it is an intermediate. Both cancel when the two equations are added, leaving O₃ + O → 2O₂.

The catalyst need not appear in the net equation, and “unchanged overall” does not mean it never reacts. It participates in the steps and can repeat them. A termination or other removal process can eventually stop a particular radical’s participation, so catalytic cycling does not imply an infinite lifetime.

Check oxygen atoms explicitly: the net left side contains 3 + 1 = 4 and the right side 2 × 2 = 4. Replacing the atomic O in a supplied step by O₂ without changing the products would break atom conservation.

Transfer the model to a supplied radical cycle

If a question supplies Br• + O₃ → BrO• + O₂ and asks for a second step using an O atom that regenerates Br•, work backwards from the required catalyst. The balanced second step is BrO• + O → Br• + O₂. Add the steps to obtain the same net ozone loss as the chlorine model.

This is an application of supplied chemistry, not a reason to assert that all atmospheric cycles have identical rates or importance. The key tasks are atom conservation, radical regeneration and distinguishing initiation from the catalytic propagation steps.

Photodissociation of the original halogen-containing molecule generates radicals and begins the process. The two ozone-consuming cycle steps then propagate radical chemistry. The overall reaction hides these species, so it cannot by itself identify which particular catalyst was involved.

Connect a useful property to its environmental consequence

Low ordinary reactivity made some CFCs useful because they did not readily attack materials in service. The same persistence allowed them to survive long enough to reach regions with more energetic UV, where bond cleavage could generate chlorine radicals. A property can therefore be useful in one setting and problematic in another.

Replacing a CFC requires more than removing its chlorine atoms on paper. A proposed substitute must work at the operating temperature and pressure, be containable and have acceptable flammability, toxicity and environmental behaviour. A supplied comparison table should be used to weigh these factors instead of assuming every chlorine-free substance is harmless.

Keep the radiation mechanisms separate: ozone depletion concerns reduced absorption of incoming harmful UV by stratospheric ozone; greenhouse effects concern absorption and emission of infrared radiation. A substance can affect both without the processes being the same.

Quick checks

Original Finesse questions. Reveal the indicative worked solutions after attempting each question; these are not official OCR A mark allocations.

Sources