Lecture 36

Chemical ReactionsFetter 10

 

There are many reactions that operate to transfer mass among fluids, gases and solids.

 

1) Acid-Base reactions

Influence the pH and ion chemistry of waterpH controls many chemical systems

 

pH = -log[H+]

 

Acid a substance that loses a proton

Base a substance that gains a proton

 

example:

HCO3- + H2O H3O+

acid + base acid + base

 

Important natural acid-base systems include CO2-water and alkalinity

 

The following reactions are important in carbonate equilibria systems

1) Dissolution of carbon dioxide in water to form carbonic acid.

H20 + CO2 H2CO3

2) The dissolution of carbonic acid in water to form the bicarbonate ion.

H2CO3 H+ + HCO3-

3) The bicarbonate ion then dissolves in water to form carbonate.

HCO3- H+ +

4) And finally, the dissolution of calcite/aragonite in water to form calcium and carbonate.

CaCO3 Ca2+ +

 

This information can be graphed to show the distribution of the different

ionic species with a specific total carbonate activity at different pH.

 

at pH = 6.38, 50 % H2CO3 and 50 % HCO3-

 

at pH = 10.38, 50 % HCO3- and 50% CO3

 

at pH = 8.3, ~ 100 % HCO3-

 

 Figure 10.1 Distribution of major species of dissolved inorganic carbon at 20 o C.

2) Solution, Volatilization, and Precipitation

Water is capable of dissolving gases, liquids and solids

These reactions are responsible for much of the natural loading of

chemical species to aquifers.

 

APL's - aqueous phase liquids

organic compounds capable of dissolving in water

 

Rule of thumb: the smaller the organic compound, the more soluble it is in water.

 

Compound Molecular weight

(g/mol)

Solubility

(g/m3)

Benzene (C6H6) 78.0 1,780

Biphenyl (C12H10) 154.0 7.48

 

Smaller compound find space in the structure of water more easily than do larger

compounds.

 

3) Complexation reactions

 

Complexation facilitates the transport of many potentially toxic

metals such as copper, cadmium and lead.

 

A complex is formed by combining simple cations, anions

and sometimes organic molecules.

 

For example: Cr(OH)2+, PbCl3-, MEDTA

(ethylenediaminetetraacetic acid, a detergent)

 

The formation of complexes may allow certain solutes to travel much

further and faster than otherwise predicted.

 

4) Surface reactions

 

Reactions between solutes and the surfaces of solids that are

capable of removing solutes from solution.

 

Sorption is the lumped term for processes that include adsorption,

chemisorption, absorption, and ion exchange.

 

 

5) Oxidation-reduction reactions

 

Reactions that are generally mediated by micro-organisms. The bugs speed up the reactions.

 

Oxidation - removal of an electron to increase the oxidation number

Fe2+ = Fe3+ + e-

 

Reduction - addition of an electron to lower oxidation number

Cr4+ + e- = Cr3+

 

Redox reactions influence the mobility of metals in solution. Some metals

may have different mobilities in different pH and Eh solutions. (Eh is the redox potential)

 

Unfortunately, Eh is a very tenuous parameter to measure in the field.

 

Biodegradation reactions are redox reactions that involve the oxidation of

organic compounds into simpler forms.

 

Eh-pH diagrams can be useful to determine what species are potentially stable.

 

 

6) Hydrolysis

 

Reactions between an organic molecule and water or a component ion of water.

 

For instance, a hydroxyl group (OH) may be added to an organic

compound making it more soluble and potentially more susceptible to biodegradation.

CH3-CH2-CH2Br + H2O = CH3-CH2-CH2OH + HBr

 

Resistant to hydrolysis

(Domenico and Schwartz, 1990)

 

Alkanes Alkenes Benzene PAHs

Alcohols DDT Phenols

 

Susceptible to hydrolysis

 

Amides Carbamates Sulfonic acid esters

Trazines

 

 

 

7) Isotopic processes

 

Radioactive decay of isotopes of certain chemical species.

 

Some elements have isotopes with the same atomic number, but

varying numbers of neutrons in the nucleus.

 

For instance, Oxygen has three isotopes, all stable

Isotopes Average abundance

99.76

0.037

0.10

 

Radioactive decay occurs by emission of

particle -

particle - electron

 

Radioactive decay is irreversible.

Decay follows a first-order rate law.

 

Half life is the time required to reduce the number of parent atoms by one-half.

Radon - 3.8 days

238Uranium - 4.5 x 109 years

14Carbon - 5,000 years

The isotopes of hydrogen, oxygen, carbon, sulfur, CFC's, krypton, and nitrogen

are useful for many environmental studies.

 

The environmental isotopes are used for such applications as age dating water

and determining rates/degrees of mixing.

 

ENV 302 - Lectures