HI!! I would like to bring to your attention how your air conditioners are stabbing you on the back with the CFCs produced.

Giving you a rough idea before going into the details, CFC catalyses the decomposition of Ozone to diatomic Oxygen. Hence, the amount of Ozone decreases.

CFCs, are molecules in which all four atoms attached to one central carbon are Chlorines and Fluorines. The catalytic properties is due to the Chlorine atom present in the one carbon molecule. When “activated”, CFCs exerts their sinister reactions.

CFCs can be activated by UV light. When shined with UV, which the stratosphere has high exposure to, CFCs undergo a reaction to produce a Chlorine Radical, an atom with an unpaired electron.

 

This is known as the Initiation Step, which is UV-induced.

Then, the chlorine radical acts as the catalyst for the destruction of Ozone. As the radical acts as a catalyst, the radical is not consumed  and is able to catalyse more ozone destruction reactions.

 

This is known as the Propagation Step, which can go on and on untill several millions of ozone molecules are destroyed. However, two chlorine radicals can react to terminate this propagation, hence, known as the Termination Step.

Hence the overall reaction is depicted as follow.

To summarise, the chlorine radical provides an alternative pathway for the degradation of Ozone. If uncatalysed, the conversion of ozone molecules to diatomic molecules requires a high input of energy (activation energy) which makes the reaction very very very slow. Below shows the two step pathway that the chlorine radical provides to reduce the energy required to kick start the reaction.

 

 

Hope you enjoyed. Thanks 🙂

References:

H. S. Rzepa (2015) ‘On the overall reaction of Ozone decomposition catalysed by chlorine radical’ Retrieved on 24 March, 2015 from  http://www.ch.ic.ac.uk/rzepa/mim/environmental/html/cfc.htm

Lee Soo Ying (n.d.) CM1041 Kinetics Notes ‘On the catalysed and uncatalysed reaction energy diagram’ Retrieved on 24 March, 2015

 

Factors that imbalance the Chapman cycle can lead to ozone depletion such as:

1. The Release of Free Radicals

As ozone and free oxygen atoms are highly unstable, they react very easily with nitrogen, hydrogen, chlorine and bromine compounds which  are found naturally in Earth’s atmosphere.

2. The uses of Ozone-depleting substances (ODS)

ODS are very stable, nontoxic and environmentally safe in the lower atmosphere. However, their very stability allows them to float up, intact, to the stratosphere. Chlorofluorocarbon, Methyl Chloroform, Halons, Carbon Tetrachloride and hydrofluorocarbon are common OCD.

Molecules that are no longer shielded from ultraviolet radiation by the ozone layer will break up and release chlorine atoms which reacts with the ozone.

3. Natural influences on Stratospheric Ozone

Sunspots, Stratospheric winds and Volcanic Eruptions are Natural variations which influence the amount of ozone in the upper atmosphere.

The effect caused by solar activity and Atmospheric Winds does not have a large impact. Large eruptions can potentially inject significant quantities of chlorine. Majority of volcanic eruptions are too weak to reach the stratosphere remains in the troposphere where it is quickly dissolved and washed out by rain.

 

Resources:

Brian Dunbar, Shelley Canright (2009) ‘Ozone: What is it, and why do we care about it?’ Retrieved on 15 March, 2015 from  http://www.nasa.gov/audience/foreducators/postsecondary/features/F_Ozone_prt.htm

Caroline Tran, Dan Chong , Anne Keith, Jordan Shively (n.d.) ‘Depletion of the Ozone Layer’ Retrieved on 15 March, 2015 from   http://chemwiki.ucdavis.edu/Physical_Chemistry/Kinetics/Case_Studies%3A_Kinetics/Depletion_of_the_Ozone_Layer

Unknown (n.d.) ‘Ozone Depletion’ Retrieved on 15 March, 2015 from http://www.ozone-hole.org.uk/09.php

In 1930s, Sydney Chapman resolved the chemistry process of ozone continually regenerated by converting ultraviolet radiation (UV) into heat in the Earth’s stratosphere is known as Chapman Cycle. It describes the four steps by which O2 is photolyzed to produce O atoms and O3, which can then be photolyzed to recover O2.

Chapman Cycle for Ozone Production:

Figure 1: Ozone formation

 Ozone formation:

2 atomic oxygen is formed by the dissociation of O2 by high energy ultraviolet photons

O2 + ℎν → 2O

Each oxygen atom then quickly combines with an oxygen molecule to form an ozone molecule

O + O2 → O3

The ozone-oxygen cycle:

Ozone strongly absorbs appropriate wavelength of UV radiation and is dissociated by these UV photons into O and O2

O3 + ℎν(240-310 nm) → O2 + O

The atomic oxygen produced quickly reacts with another oxygen molecule to reform ozone

O + O2 → O3 + excess kinetic energy

Ozone Loss:

The ozone is in stable balanced by the reaction of ozone and free oxygen atoms

O3 + O· → 2 O2

 

Resources:

NASA. (June 2000)’Studying Earth’s Environment From Space’ Retrieved on 6 March, 2015 from  http://www.ccpo.odu.edu/SEES/index.html.

Unknown (2010) ‘ Ozone Layer’ Retrieved on 11 March, 2015 from  https://www.ec.gc.ca/ozone/default.asp?lang=En&n=DB5CBDE6-1

Unknown (n.d.) ‘Ozone-oxygen cycle’ Retrieved on 6 March, 2015 from http://en.wikipedia.org/wiki/Ozone-oxygen_cycle