Handout notes for UHF475 Wetland Ecology
Wetland Soils, Chemical Properties

Rob Harrison
Bloedel 218
5-7463 phone
mailto:robh@u.washington.edu

Fundamental concept #1: Atoms and molecules.

The basic unit of chemistry is the atom. Atoms are made up of, for our purposes, protons, electrons, and neutrons. Atoms combine by chemical reactions to form larger units called molecules. If we consider a certain chemical reaction, say that that occurs when hydrogen combines with oxygen to form water, it is important to understand in what proportions they combine in. We can see these proportions by looking at a chemical reaction written as follows:

2(hydrogen) + 1(oxygen) = 1(water)

2H + O = H2O reaction 1

In the following example, two hydrogen atoms combine with one oxygen atom to form one water molecule.

Fundamental concept #2: Moles-the measure of proportions.

The term "mole" is used to describe a certain number of atoms. It is not important to know the exact number of atoms in a mole (it is very large) only that one mole of hydrogen contains the same actual number of atoms as one mole of oxygen. However, one mole of oxygen does not weigh the same as one mole of hydrogen. There is one mole of hydrogen in one gram of hydrogen. Since the oxygen atom has more substance than an hydrogen atom, it takes more grams of oxygen to make up a mole. In fact, it takes 16 grams.

Thus, two moles of hydrogen atoms combine with one mole of oxygen atoms to form one mole of water molecules, as follows:

2 moles hydrogen + 1 mole oxygen = 1 mole water

2H + O = H2O

2(1 g) + 1(16 g) = 1(18g)

The chemical reaction is like a recipe; that is, we can divide the portions of hydrogen and oxygen in half and produce half as much water (1/2 moles of water), or we can double the portions to produce twice as much water (2 moles of water), or any other combination. The reaction of hydrogen and oxygen to water will be complete as long as we add twice as many moles of hydrogen to oxygen.

Now, we have yet to consider why 2 atoms of hydrogen combine with 1 atom of oxgen to form one molecule of water instead of some other proportion of hydrogen to oxygen.

Fundamental concept #3: Valence-why one mole of a given atom is "better" than the next.

It is obvious from formula 1 that one atom of oxygen combines with 2 atoms of hydrogen. Why? Because oxygen in this reaction has two points at which the hydrogen can associate with it in a chemical reaction. Hydrogen has only one. These regions of association are called chemical bonds. Hydrogen is monovalent (mono = 1) while oxygen is divalent (di = 2) in the reaction to form water.

Some atoms will tend to have fairly consistent valences when they react. For others, valences can vary with the specific reaction.

Fundamental concept #4: Ionization and covalence-the nature of chemical bonds.

Chemical bonds are formed by transfers or sharing of electrons. For instance, in the case of hydrogen and oxygen combining to form water, two hydrogen atoms each lose an electron and the oxygen atom gains it. Everything else stays the same. Electrons have negative charges (-1) while protons have positive charges (+1). Therefore, the oxygen gains a net negative charge of -2 since it took 2 electrons from the 2 hydrogens during the reaction. However, the electrons are not completely captured by the oxygen and occasionally move back in association with the hydrogen atom. The charge moves with the electron. Since the electrons are thus shared between the two atoms in this reaction, they are termed covalent bonds (co = both). In the case of the water reaction, the oxygen gets a greater share of the electron's time than the hydrogen (remember that the electron originally came from the hydrogen), so a difference of charge (called polarity) develops, its + for the hydrogen and - for the oxygen.

The following is a special case of a covalent bond. Oxygen atoms need to gain an electron because of their chemical nature. So, when oxygen atoms are in the present of other oxygen atoms, they associate by forming their own chemical bonds and follows

O + O = O2

1(oxygen atom) + 1(oxygen atom) = 1(oxygen molecule)

In this case the electrons are shared equally and no partial charge develops. Ionic bonds are formed when the electron is essentially completely transferred from one atom to another.

Summary.

1) The basic unit of chemistry is the atom.

2) Molecules are combination of atoms.

3) Chemical reactions take place on an atom by atom or molecule by molecule basis, much like a consistent recipe.

4) A mole is a unit of a given number of atoms or molecules.

5) Valence determines how many moles of one reactant will react with one mole of another (i.e. how many reaction sites they have and how many bonds will be formed in the reaction). Valence is positive when an electron is lost and negative with it is accepted.

6) Several types of chemical bonds are possible: Covalent bonds result from the sharing of an electron(s). Ionic bonds result when the electron is completely captured by one atom from another.

1) reactions of acids with Ca and Mg carbonates
2) cation exchange
3) decomposition of soil minerals
4) H+ adsorption by Al ions
5) H+ adsorption by clay minerals
6) H+ adsorption by humus
7) H+ adsorption by Fe and Al oxides

Chemical composition of granitic rock and an age sequence

of rocks deriving from granitic rock.

Wetland Soil (Hydric soil): A soil that is saturated with water long enough during the plant growing season to become anaerobic. This soil will usually be characterized by anaerobic soil zones and wetland vegetation.