Lecture 36

Chemical Reactions

Fetter 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 water
	pH controls many chemical systems

pH = -log[H+]

Acid a substance that loses a proton

Base a substance that gains a proton

example:

HCO₃- + H₂O Û 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 Û H₂CO₃

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

H₂CO₃ Û H+ + HCO₃^-

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

HCO₃^- Û H+ +

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

CaCO₃ Û 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^oC.

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.

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⁺, PbCl_3-, 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)

Susceptible to hydrolysis

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

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