How CFCs thin the ozone layer
Chlorine-catalysed ozone destruction · Radical · Catalytic cycle
One chlorine atom from an old spray can could destroy thousands of ozone molecules before being caught.
Reactants approach
Drag to rotate
ΔH = -163 kJ · exothermic, releases heat
ΔH per mol O3 destroyed in this step.
Meet the molecules
- Reactantgas
Cl
chlorine atom
- Reactantgas
O3
ozone
- Productgas
ClO
chlorine monoxide
- Productgas
O2
oxygen
What you notice
About 15 to 35 km above the ground, ozone absorbs most of the Sun's UV-B and UV-C radiation, which would otherwise damage DNA and cause skin cancer. In 1985, scientists reported that each Antarctic spring the ozone over the South Pole was falling to less than half its normal level. The cause was chlorofluorocarbons (CFCs), stable gases once used in fridges and spray cans, which drift up to the stratosphere and are broken apart by UV light, freeing chlorine atoms. The 1987 Montreal Protocol phased out CFCs, and Antarctic ozone is expected to recover around 2066.
Try it
Check the UV index in a weather app on a sunny day; it measures the UV-B that reaches the ground after passing through the ozone layer.
Step by step
- 1
UV frees chlorine
High in the stratosphere, UV light breaks a C–Cl bond in a CFC such as CF2Cl2, releasing a chlorine atom with an unpaired electron: a radical.
- 2
Chlorine attacks ozone
The Cl radical pulls one oxygen atom off O3, forming chlorine monoxide, ClO, and ordinary O2. The activation energy is only about 2 kJ/mol, so the reaction happens on most collisions.
- 3
Chlorine is freed again
ClO meets a free oxygen atom: ClO + O → Cl + O2. The chlorine atom is back and can attack another ozone molecule.
- 4
One atom, many ozones
The net change is O3 + O → 2O2. Because chlorine is regenerated, one atom can destroy thousands of ozone molecules before it is locked up in a less reactive compound such as HCl.