CFC's (1 Viewer)

[onebytwo]

hello, i cant understand why CFC's are not slightly polar molecules, because it contains different halogens that have different electronegativities, so im thinking that the affinity for electrons of fluorine would be greater than that of chlorine, creating a small dipole because their isnt a pronounced disparity. if this is true then wouldnt CFC's dissolved in water and be cleared out of the atmosphere when it rains preventing it from reaching the ozone layer itself?

also because methane reacts with Cl radicals why cant we just put a whole heap of methane above the tropopause where it begins to get hotter, so that this gas can rise and react with Cl radicals and reduce the extent of damage done to ozone? (its probably expensive, but what the heck, we seem to have lots of money to waste elsewhere)

thanks for anything

 

[hyparzero]

The reason is that each carbon atom in CFCs are surrounded by single co-valent bonds with the halogens. As a result CFCs contain strong carbon-carbon single bonds and strong carbon-halogen bonds.

The carbon-halogen bonds are only very slightly polar and so there aren't any bits of the molecules which carry any significant amount of positive or negative charge which other things might be attracted to.

There isn't much electronegativity difference between carbon and chlorine, and not much between fluorine and carbon either, so there is hardly any bond polarity. Plus, a totally symmetrical CFC molecule such as 1,2-chloro-1,2-fluroethane would be completely non-polar.

Since there is a lack of strong electronegativity difference, the only forces acting between each molecule are Van der Vaal's forces.

When a molecular substance dissolves in water, you have to break the intermolecular forces within the substance. In the case of the alkanes, these are Van der Waals dispersion forces amd break the intermolecular forces in the water so that the substance can fit between the water molecules. In water the main intermolecular attractions are hydrogen bonds.

To break these forces, it costs energy, although the amount of energy to break the Van der Waals dispersion forces in something like CFC-11 is pretty negligible.

That isn't true of the hydrogen bonds in water, though.

The only new attractions between a CFC and water molecules are Van der Waals.

These don't release anything like enough energy to compensate for what you need to break the hydrogen bonds in water. Thus the CFC will not dissolve.

You are completely correct in saying that certain CFCs are slightly polar, however, these are almost neglible when attempting to dissolve them in water.



You mentioned putting methane into the troposphere, and I see some major problems with that. The production costs of methane are relatively high, and attempting to deliever the methane payload directly into the lower statosphere poses enormous problems.

Such as costs. No such method is currently available to deliver the many millions of tonnes of methane that maybe required to counter the current levers of the CFCs, not to mention the extra pollution through the use of transport fuel, and the highly flammable nature of methane.

Methane, unlike CFC's are not as stable in the upper atmospheres and readily react with UV and other organic molecules to form other compounds such as carbon dioxide. Hence, there is no guarantee that all of the methane will react with the Cl- radicals ~ more in the range of 1% to 2%.

So, yeah, i have finished ranting

 

[royy]

Have i talked about the right info for this dot point?

Analyse the information available that indicates changes in atmospheric ozone concentrations, describe the changes observed and explain how this inform

• Ozone thinning is greatest in spring (late October) over Antarctica
• During the preceding winter, HCl and ClONO2 molecules react on the surfaces of atmospheric ice crystals to form chlorine molecules (Cl2)
ClONO2 + HCl HNO3 + Cl2
• Following the long, dark Antarctic winter there is a rapid increase in UV in the stratosphere over Antarctica
• Ozone loss is worst in the world’s coldest region because of polar stratospheric clouds (PSCs) which form when temperatures fall below -80°C. It is suggested that these clouds are necessary for the chemical reactions to occur late in the Antarctic winter. Once spring appears and light shines on the PSCs, rapid ozone destruction occurs
• The increase in UV causes photodissociation of the chlorine molecules to produce additional chlorine free radicals that begin the ozone destruction process
• With a rise in temperature, the PSCs are reduced and the ozone destruction stops
• Several months later, the supply of Cl molecules is exhausted and ozone levels gradually return to normal as lower latitude air mixes with polar air
• Ozone concentration is measured in Dobson units. One unit represents one molecule of ozone to every billion molecules of air

 

[onebytwo]

thanks for your deep insights