september 16, 2014 - ghuangsir...september 16, 2014 environmental chemistry hl e9 ozone depletion...

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September 16, 2014 Environmental Chemistry HL E9 Ozone Depletion E10 Smog E11 Acid Deposition E12 Water and Soil E9 Ozone Depletion Let's do a bit of review on bonding. HL Chemistry: Ozone Depletion The natural equilibrium in ozone is entirely caused by UV light from the sun. O=O O O O Double Bond One and a half Bond Bond Enthalpy Bond Type 498 302 Wavelength of UV needed 242 nm 330 nm mechanism: detail steps of reactions

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Page 1: September 16, 2014 - ghuangsir...September 16, 2014 Environmental Chemistry HL E9 Ozone Depletion E10 Smog E11 Acid Deposition E12 Water and Soil E9 Ozone Depletion Let's do a bit

September 16, 2014

Environmental ChemistryHL

E9 Ozone DepletionE10 SmogE11 Acid DepositionE12 Water and Soil

E9 Ozone Depletion

Let's do a bit of review on bonding.

HL Chemistry:

Ozone DepletionThe natural equilibrium in ozone is entirely caused by UV light from the sun.

O=O → OO O

Double Bond One and a half Bond

Bond Enthalpy

Bond Type

498 302

Wavelength of UV needed 242 nm 330 nm

mechanism: detail steps of reactions

Page 2: September 16, 2014 - ghuangsir...September 16, 2014 Environmental Chemistry HL E9 Ozone Depletion E10 Smog E11 Acid Deposition E12 Water and Soil E9 Ozone Depletion Let's do a bit

September 16, 2014

Ozone Depletion by CFCs

C

Cl

Cl F

F →

Heterolytic fission releasing a chlorine radical induced by UV light

The chlorine radical acts as a catalyst and is not consumed in the reaction.

This becomes a hugh problem because one chlorine radical can stay in the stratosphere for years destroying huge numbers of ozone molecules.

1st step

NOx also act as catalyst in the depletion of ozone.

Net result:

Page 3: September 16, 2014 - ghuangsir...September 16, 2014 Environmental Chemistry HL E9 Ozone Depletion E10 Smog E11 Acid Deposition E12 Water and Soil E9 Ozone Depletion Let's do a bit

September 16, 2014

However, ozone depletion does not take place evenly throughout the world or even at times of the year.

Tiny ice crystals that forms in the stratospheric clouds also act as a catalyst in providing a surface upon which the reactions can take place.

But these clouds form only when the stratosphere is very cold ⇒ worse in the polar regions and during winter months.

Therefore the antarctic ozone hole is seasonal.

Smog:

Smog is a poisonous mixture of smoke, fog, air, and other chemicals.

It tends to form in large cities (heavy air pollution) and is favoured by windless conditions.

Smogs are most likely to occur when there is a temperature inversion.

Cars are one of the main sources of a primary pollutant that is responsible for smog: NOx . Recall that nitrogen monoxide, NO, forms in the high temperatures of internal combustion engines as shown:N2(g) + O2 (g) → 2NO(g)

The other main primary pollutants responsible for smog formation are the volatile organic compounds (VOCs). VOCs can also form as the result of reactions in high temperature combustion engines or can be released from other anthropogenic sources. High levels of VOCs can be produced in idling cars.

Photochemical Smog

Normal Conditions:

Sometimes weather conditions trap a layer of dense, cool air beneath a layer of less dense, warm air over a city that is located in a valley. This is a thermal inversion. This prevents the lower level air from rising and carrying pollutants with it. As a result, ground-level pollutants become more concentrated and more harmful to human health.

Normal Thermal Inversion

Page 4: September 16, 2014 - ghuangsir...September 16, 2014 Environmental Chemistry HL E9 Ozone Depletion E10 Smog E11 Acid Deposition E12 Water and Soil E9 Ozone Depletion Let's do a bit

September 16, 2014

Photochemical Smog:

Key: the formation of free radicals involving UV light from the sun. Formation of PANs (secondary pollutant)

The primary pollutant NO can react with oxygen in the atmosphere to create harmful nitrogen dioxide as shown by the following reaction mechanism:

The oxygen free radical produced in the second step is very unstable and reacts with more oxygen in the atmosphere to produce ozone. This combination requires collision with another molecule, M, to carry away the energy that would otherwise result in the ozone immediately decaying:

Ground-level ozone can be very harmful to humans as it can aggravate asthma, cause irritation to the respiratory system and cause inflammation and damage to the lungs. It can also cause material damage to substances made of rubber, for example vehicle tyres, and destroy flora (plant life).

This oxygen radical can wreak even more havoc when it reacts with moisture in the atmosphere to form hydroxyl radicals (OH·). The hydroxyl radicals can then react with available hydrocarbons (RH) in the atmosphere to produce aldehydes (RCHO), which further react with more hydroxyl radicals to form peroxyacylnitrates (PANs), RC(O)O2 NO2 , which have similar adverse health effects to ozone. The chemistry of this is shown in the reactions below:

Formation of secondary pollutant: PANs

PANs also have similar adverse health effects to ozone.

Aging coal-fired power plants burn sulfur-containing coal, which produces SO2 . SO2 is converted to sulfuric acid as shown below; its production begins with the formation of hydroxyl radicals:

Acid DepositionOne of the other important anthropogenic contributors of acid deposition is the primary pollutant NOx (NO and NO2 ). The mechanism for the formation of nitric acid and nitrous acid, which are both found in acid deposition, is shown:

Page 5: September 16, 2014 - ghuangsir...September 16, 2014 Environmental Chemistry HL E9 Ozone Depletion E10 Smog E11 Acid Deposition E12 Water and Soil E9 Ozone Depletion Let's do a bit

September 16, 2014

The textbook is a bit more complicated, it's better to use the syllabus instead!

The atmosphere contains a natural defence mechanism against acid deposition—ammonia,which neutralizes the acids to form ammonium salts to some extent. Ammonia (NH 3) is a weak base, in atmosphere it can neutralize the acids to form ammonium sulfate and ammonium nitrate.

As they sink to the ground or are washed out of the atmosphere in rain, the ammonium salts then migrate into the soil, where nitrification and acidification can occur:

E 12 Water and Soil

Humus and smectites (black swelling clays) have the greatest CEC, as they are both negatively charged, and so can attract positively charged cations. The cations that are exchanged can be acidic—H+, Fe2+, Mn2+ and Al3+—or basic—Ca2+, Mg2+ and K+—in nature. These cations are not removed from the soil with water; instead other cations have to be added in order for an exchange to take place. The CEC of soil is therefore defined as its capacity to exchange cations with the soil solution. It is often used as a measure of potential soil fertility, with the higher CEC indicating a more fertile soil. A soil that has a low percentage of clay can have its CEC increased by the addition of manure, which works to increase the CEC.

Cation exchange (CEC)

The larger the CEC the more cations the soil can hold

These cations are exchanged with cations such as H ions on the root hairs of plants and thus can be absorbed into the plant

The higher the CEC, the higher the soil pH, and the more nutrients are available for exchange.

Page 6: September 16, 2014 - ghuangsir...September 16, 2014 Environmental Chemistry HL E9 Ozone Depletion E10 Smog E11 Acid Deposition E12 Water and Soil E9 Ozone Depletion Let's do a bit

September 16, 2014