Lecture 6

1. Lecture 6 Protecting the Ozone Layer (cont.) April 15, 2005

2. Homework Assignment #2 Chapter 2 end of book problems: (don’t do Blue 13.b and 13.c) #8, 13, 20, 24, 43, 51

3. More Lewis Dot Structure Examples HCl hydrochloric acid CH4 methane CO2 carbon dioxide CO carbon monoxide N2 nitrogen CH3CH2OH ethanol CCl2F2 dichlorodifluoromethane

4. Patterns in molecules • Halogens (Group 7A) and hydrogen are drop and replace analogs of each other. Everywhere you see a hydrogen in a molecule, it can usually be replaced by a halogen Cl H Cl C Cl C H H H Cl

5. Bimolecular Oxygen: O2 1. Each oxygen has 6: 12 total 2. With two atoms, there’s only a linear structure possible 3. Place electrons: start with single bond, one oxygen; run out before both have octets O O O O O O

6. O O O Ozone: O3 O O O O O O Resonant structures O O O

7. Ozone: a form of oxygen (O3) • Formed from molecular oxygen by absorbing energy (light, electricity) O2 + energy O + O O + O2 O3 • Reactive and toxic, problem in the troposphere (lowest layer of atmosphere where we live)

8. Stratospheric (good) ozone • Absorbs UV light, desirable feature in higher layer of atmosphere where we don’t live O3 + UV light O2 + O O + O2 O3 UV light (harmful) dissipated as heat in O2 molecules (harmless)

9. The Ozone Hole • Lowered concentrations of ozone over South Pole first discovered in early 80’s • Less ozone = more UV light at surface of earth, higher skin cancer due to increased DNA damage • If there were no ozone, life as we know it could probably not survive

10. Why is there a hole in the ozonelayer? • Chlorofluorocarbons: compounds of carbon, fluorine, and chlorine • Very unreactive • Spray can propellants, air conditioner • Are not broken down in troposphere • Drift upward into stratosphere

11. Why is there a hole in the ozonelayer? CCl2F2 + UV light CClF2 + Cl Cl + O3 ClO + O2 Cl + O3 ClO + O2 ClO + ClO Cl2O2 Cl2O2 + light O2 + Cl + Cl Net: 2O3 3O2 Cl , et al. is a free radical. An unstable chemical species with an unpaired electron.

12. Why is there a hole in the ozonelayer? • Cl atoms are regenerated, free to react with other O3 molecules • Catalyst, can break down hundreds or thousands of O3 molecules before being used up in another process

13. What can be done? • Montreal Protocol (signed 1987, took effect 1996): banned use of CFC’s worldwide • Other molecules (hydrofluorocarbons) have been developed: slightly more reactive than CFC’s, but do not produce chlorine atoms in UV light • Since then, ozone levels have slowly been coming back up • Black market CFC’s

14. Why a hole over Antarctica? • Assuming the chemistry is correct, why is the hole so pronounced at the South Pole? • South Pole’s lower stratosphere is the coldest spot on Earth Wind patterns cause the polar vortex and so warmer (ozone richer) air can’t move in. Polar stratospheric clouds form. Reactions that convert safer Cl molecules to more reactive ones, can occur on the surface of the clouds. Rapid increases in sunlight (spring) then break down the Cl containing molecules, releasing Cl . Seasonal variation of ozone hole

15. Polar Stratospheric Clouds Antarctic PSCs Some facts: 1. The Antarctic Ozone inventory has been reduced by 60-70% over the last 50 year. It continues to deepen and widen every year, although not monotonically. 2. The size of the ozone hole over Antarctica has increased from "zero" in the mid-1970's to an area slightly larger than North America. 3. An Arctic ozone "dent" is appearing, reductions of ~15% over northern Europe. Over the US, ozone reductions are about a few percent. No significant reductions in tropical ozone have been detected Arctic PSCs