Dynamic Reactions

Energy Analysis
Earth’s atmosphere contains 3 billion metric tons of ozone (O3).
Earth’s stratospheric ozone plays a critical role in shielding us from
the high-energy ultraviolet radiation (UV-C and most of UV-B) emitted by
the sun. Life on Earth would be much harsher for most living organisms
without this protection. Several decades ago, scientists identified a
hole in the ozone. It was determined that specific synthetic chemicals,
chlorofluorocarbons (CFCs), were degrading our ozone layer. Once in the
atmosphere, these CFCs react to release chlorine free radicals, which
are very reactive. You can see the current status of the ozone hole at NASA’s Ozone Watch. (Links to an external site.)
The natural level of ozone in the stratosphere is maintained through a dynamic equilibrium, referred to as the Chapman Cycle. Ozone forms when sunlight splits molecular oxygen (O2) into two oxygen radicals. Oxygen radicals are very reactive. Among other reactions, they can either join together to reform O2 or one can join O2 to form O3.
Ozone can then react with an additional oxygen radical to reform
molecular oxygen, or it can react with sunlight to form molecular oxygen
and an oxygen radical. Ozone is constantly being created and destroyed
through these reactions within the stratosphere. This reaction with
sunlight is how we are protected from the harmful UV-B and UV-C!
Step 1: hv + O2 –> 2O∙
Step 2: O2 + O∙ –> O3
Step 3: O3 + O∙ –> 2O2
or
O3 + hv –> O2 + O∙
However, ozone destroyed by catalyzed decomposition from chlorine
radicals (and other chemicals) shifts the natural dynamic equilibrium,
resulting in more high-energy light reaching Earth’s surface.
Consider: Choose one (1) of the following to respond to with your initial post:
Evaluate the enthalpy diagram for the catalyzed and uncatalyzed
decomposition of ozone. Identify eight accurate and significant pieces
of information, explained in 1–3 paragraphs (in your own words).
The rate of formation and destruction of ozone depends upon the
intensity of sunlight. Comment on how ozone production varies 1) over a
day and 2) over Earth’s seasons. Specifically, identify and explain the
rate equations for the Chapman Cycle.
Using key ideas from kinetics (e.g.catalysts), and equilibrium (e.g.
LeChatelier’s principle), explain why CFC usage has been banned in many
countries. Specifically, consider the balance between the Chapman Cycle
and CFC destruction of ozone. Specifically state the equilibrium
expression that relates to the concentration of ozone and molecular
oxygen.

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