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CHAPTER 2. CHEMICAL COMPOUNDS, REACTIONS AND EQUATIONS

I. | NOMENCLATURE |
Chemical nomenclature is the term given to the naming of compounds, based on a set of standard rules. Since most chemical reactions envolve electron transfer to some extent, one can get a good idea of how atoms will react with each other by considering the oxidation/reduction reaction. Oxidation/Reduction reactions are the processes of losing and gaining electrons respectively. An easy way of remembering which term is which is by thinking of the process in terms of their mathematical meaning,

For the reaction
I2O5(s) + 5 CO2(g) --> I2(s) + 5 CO2(g)
2 I5+ + 5 C2+ --> 2 I0 + C4+
I is reduced: 5+ --> 0, C is oxidized: 2+ --> 4+

For the reaction
2 Hg2+(aq) + N2H4(aq) --> 2 Hg(l) + N2(g) + 4 H+(aq)
2 Hg2+ + N2- --> 2 Hg0 + N0
Hg is reduced: 2+ --> 0, N is oxidized: 2- --> 0

For the reaction
3 H2S(aq) + 2 H+(aq) + 2 NO3-(aq) --> 3 S(s) + 2 NO(g) + 4 H2O(l)
3 S2- + 2 N5+ --> 3 S0 + 2 N2+
N is reduced: 5+ --> 2+, S is oxidized: 2- --> 0

Thus if we imagine the process as motion in a one-dimensional coordinate system the first example indicates that Iodine moved from 5+ to 0, this is a decrease in value so we call it reduction. With the reduction of Iodine comes the oxidation of Carbon, which goes from 2+ to 4+, this is an increase in value. Notice that all other reactions follow the same pattern: an oxidation is accompained by a reduction. Oxidation numbers or charges are assigned to indicate where the electrons are in a reaction. The oxidation number is assigned according to a standard set of rules. They are as follows:

- An atom of a pure element has an oxidation number of zero.
- For single atoms in an ion, their oxidation number is equal to their charge.
- Fluorine is always -1 in compounds.
- Cl, Br, and I are alway -1 in compounds except when the are combined with O or F.
- H is normally +1 and O is normally -2.
- The oxidation number of a compound is equal to the sum of the oxidation numbers for each atom in the compound.

In terms of electron transfer there are two kinds of chemical compounds: ionic and covalent. True ionic compounds are formed by complete electron transfer, the compounds with the most ionic character are the ones formed by alkalis (1st column elements) and halogens (7th column elements), for example NaCl is formed by Na+ and Cl-. True covalent compounds are formed by like elements, the compounds with the most covalent character are the diatomic molecules such as O2 and H2. The ionic/covalent character is simply a question of electronegativity; if a high electronegativity element such as Cl (Chlorine) is paired with a low electronegativity element such as Na (Sodium), it is the Cl that pulls the electrons more strongly thus becomes negatively charged. When two O (oxygen) atoms are paired they both pull electrons with the same strenght thus the electrons end up in between the two atoms (ideally 50% of the total distance).
It is clear then that in order to make a neutral molecule, the positive charge of the cation (positively-charged ion) must equal the negative charge of the anion (negatively-charged ion). In order to create a molecule with 0 (zero) charge or a charged polyatomic ion you must combine the atoms in certain proportions. Some common ions are,

CATIONS ANIONS
1+ 2+ 3+ 1- 2- 3-
ammonium (NH4+) cadmium (Cd2+) aluminum (Al3+) acetate (C2H3O2)- carbonate (CO3)2- nitride (N3-)
copper(I) (Cu+) cobalt(II) (Co2+) chromium(III) (Cr3+) clorateClO3)- dichromate (Cr2O72-) phosphate (PO43-)
hydrogen (H+) copper(II) (Cu2+) iron (III) (Fe3+) cyanide CN- sulfite (SO32-) phosphide (P3-)
silver (Ag+) iron (Fe2+) indium (In3+) thiocyanate SCN- sulfate (SO42-) phosphate (PO43-)

...

II. | THE MOLE |
The amount of substances in the laboratory are measured in terms of concentration, mass and other quantities measurable by the laboratory equipment. However, chemical reactions occur between atoms and molecules, for example, when copper is heated in air,

Cu + 1/2 O2 --> CuO

We find that 16.00 grams of Oxygen are required to react with 63.55 grams of Copper. Since these atoms react in a 1:1 ratio we can conclude that 16.00 grams of oxygen contain as many atoms as 63.55 grams of copper. These two amounts are equal to one mole of atoms (abbreviated mol). The mol is the counting unit for atoms just like the dozen is the counting unit for eggs. The only difference is that one mol is a very large number, namely 6.022 X 1023


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