Environmental Chemistry includes:

• Aquatic Chemistry
  pH, solubility, redox, equilibrium, complexation, kinetics
• Atmospheric Chemistry
  Atmospheric heat balance
  Chemical reactions in the atmosphere
  Movement of air masses
• Chemistry of the Geosphere
  Soil formation and erosion processes
  Air and water interactions with soil
  pH and ion exchange properties
• Chemistry of the Biosphere

Basic cell processes

Nutrients needed and wastes produced

Basic toxicology

• Technology and energy
  Effect of human technology on natural cycles
  Production and use of fuels and other energy sources and their effect on the environment

• How are environmentally important compounds produced?
• Where do they go?
• What cycles are they involved in or do they disrupt?

Water Chemistry

• Hydrology
• Limnology
• Oceanography

Cycle of water from land and surface water (evaporation and transpiration) to atmosphere and back by precipitation.

In US:

• over 200 inches of precipitation annually in Pacific NW
• < 25 inches southwestern desert areas)
• NJ gets 100-150 inches per year.
• Consumption patterns are often inverse to precipitation

Properties of water

• Look at position of oxygen in periodic table: note other hydrides of elements in oxygen family (H₂O, H₂S, H₂Se, H₂Te)
• Solvent properties -- High dielectric constant makes it excellent solvent for ionic or polar organic substances -- basis of an aqueous biology
• Transparent in visible and near UV regions of spectrum
• Expands on freezing -- a very unusual property shown by few liquids. What are consequences of this property? Ice cubes float in our drinks!
• Unusually high heat of vaporization, heat of fusion, and heat capacity. Tends to stabilize temperatures around it. Also slows evaporation to keep water in liquid form.

Arises from density changes with temperature. Water’s density increases as temperature goes down -- until 4^oC. Then it becomes less dense, and freezes at 0 ^oC. How does this affect a deep body of water?

The epilimnion: Upper layer--warmed by sun--well oxygenated--lighter because it is warmer.

The hypolimnion: Deeper water layer--cooler--low in oxygen--may be nutrient rich from exchange with sediments.

The thermocline is the border between these regions.

Acid-base reactions:

In water we can define acids as substances which generate H⁺ in solution.

Bases generate OH^-.

Water is both: H₂O ® H⁺ + OH^-

The equilibrium constant for this reaction is:

but [H₂O] is considered to be a constant and is combined into the K_w.

Calculate the pH of pure water:

CO₂ and H₂O interact in important ways to control the pH and buffer changes in pH of water:

Examples:

CO₂ + H₂O Û H₂CO₃

H₂CO₃ Û H⁺ + HCO₃^-

HCO₃^- Û H⁺ + CO₃^2-

Identify: Carbonate ion

Carbonic acid

Hydrogen carbonate ion (bicarbonate ion)

• Remember Le Chatelier’s Principle.

Other phenomena which happen in water:

• Hydration (binding with water molecules)
• Complexation (binding with other ligands)
• Precipitation (ions forming insoluble compounds which become solid particles)
• Oxidation/reduction (redox) processes

Oxidation is the loss of electrons:

Fe ® Fe³⁺ + 3 e^-

Oxidation can only take place if a reduction (gain of electrons) also takes place.

O₂ + 2H⁺ + 4 e^-® 2 OH^-

Then the ions formed may react:

Fe³⁺ + 3 OH^- ® Fe(OH)₃ (insoluble iron oxide)

• Chelation-- Complexation with a molecule which can bind to a metal in more than one site (e.g. EDTA with Ca²⁺ and many other metal ions

Air contains:

78.1% N_2, 21.0% O₂, 0.9% Ar, 0.03% CO₂

Variable amounts of water vapor (1- 3%, usually)

Trace gases (Ne, CH₄, Kr, No_x, So_x, O₃, CO, NH₃, etc.)

Absorption of light of the proper wavelength (i.e. energy) can break chemical bonds

O₂ + hn ® O + O

The O radicals are very reactive and immediately do something--like

O + O₂ + M ® O₃ + M forming ozone.

(M is a particle surface used to hold the atoms together long enough to react. Some reactions will not happen at all without such an assistance--others will happen much less frequently )

• Redox reactions

NO or N₂O may be oxidized to N₂O₅ which reacts with water droplets to form nitric acid.

N₂O₅ + H₂O ® 2 HNO₃

Organic gases in the air, (methane, propane etc.) can be oxidized, especially by the O radicals formed in photolysis of O₂, making reactive organic species which can polymerize into larger molecules and eventually form visible particles -- smog.

• Benefits--small particles serve as nucleation sites for precipitation formation (condensation nuclei)
• Aerosols--Colloidal sized particles which are not readily settled out (subject to Brownian Motion)

dispersion aerosols

condensation aerosols

• Effects of particulates: on health, on atmospheric reactions, on visibility, on precipitation patterns

Most of the effects of particulates in the atmosphere are surface effects and so smallest particles have largest surface / mass ratio and greater effects than larger particles. These are least likely to settle out and are also hardest to contain in scrubbers.

• Basis for agriculture, mining, fuel production
• Soil is important part of geosphere

• composed of particles of mineral matter and some organic material,
• proportions of these vary widely

• Soil layers are called horizons. These are continually exchanging material with atmosphere and with hydrosphere.
• Environmental problems in geosphere include depletion and erosion of topsoil, contamination of soil at industrial and waste disposal landfill sites.

• Most living organisms inhabit a thin layer at the interface between the atmosphere and the geosphere or the hydrosphere.
• Importance of photosynthesis for oxygen balance of earth.

CO₂ + H₂O ® {CH₂O} + O₂ (g)

Also organisms contribute to the breakdown of organic matter either with O₂ or without:
{CH₂O} + O₂ (g) ® CO₂ + H₂O (aerobic)
or
2 {CH₂O} ® CO₂ (g)+ CH₄ (anaerobic)

• Photosynthetic origin of fossil fuels
• Changes in biosphere -- Humans have changed living organisms for their own use through selective breeding, and now can change them even more rapidly and drastically by genetic manipulation.

Matter is continually cycled among all the areas of the environment:

For the general reaction:

aA + bB ® cC the reaction

cC ® aA + bB also takes place

the quantity is a constant when the reaction is at equilibrium.

At equilibrium the rate of the forward reaction is equal to that of the reverse reaction.

Equilibrium position related to magnitude of K_eq.

Relation of rate of reaction (Kinetics) to reaction constant (equilibrium)

Oxygen in Water

• Solubility (Equilibrium Concentration )
• At 25 ^oC in equilibrium with air solubility is 8.32 mg/l. (How much gas is this??)
  1 mole of O₂ occupies 22.4 liters at 0 ^oC and 1 atmosphere pressure, and weighs 32 g.
  
• Actual concentration depends on kinetics as well as equilibrium.
  (What is rate of dissolution as compared with rate of consumption in the water?)

** Rate of dissolution depends strongly on contact and amount of surface between air and water.
** Rate of consumption depends on reactions in water which use O₂.
{CH₂O} + O₂ ® CO₂ + H₂O
(degradation of organic material)

• How many grams of O₂ are used in consuming 1 gram of organic matter? (1 "mol" CH₂O = 30 g)

What is the qualitative effect of raising temperature on O₂ concentration in water?

Strong acids (free mineral acids)

Hydrochloric HCl

Nitric HNO₃

Sulfuric H₂SO₄ All totally ionize in water--

Acetic and other carboxylic acids ---COOH group

Carbonic acid H₂CO₃

Acidity of some hydrated metals:

Al(H₂O)₆³⁺ Û Al(H₂O)₅OH²⁺ + H⁺

Buffers

• Definition of a buffer
• Composed usually of weak acid and its salt or weak base and its salt
• Most common buffer in natural waters involves carbonate/bicarbonate/carbonic acid/carbon dioxide system

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