17
7.
As electrons are placed into atoms, they successively oc-These concepts permit the chemist to examine chemical reactions cupy higher energy levels, or shells. Electrons in filled and determine the mass relationships that are involved. For ex-levels are unimportant as 'far as chemical reactivity is con-ample, consider the simple pyrotechnic reaction cerned. It is the outer, partially-filled level that determines chemical behavior.
Hence, elements with the same
KC1O,,
+
4 Mg ; KC1 + 4 MgO
outer-shell configuration display markedly similar chemi-1 mole
4 moles
1 mole
4 moles
cal reactivity.
This phenomenon is called periodicity,
161.2 g
and an arrangement of the elements placing similar ele-138.6 g
97.2 g
74.6 g
ments in a vertical column has been developed - the pe-In a balanced chemical equation, the number of atoms of each riodic table. The alkali metals (lithium, sodium, potas-element on the left-hand, or reactant, side will equal the num-sium, rubidium, and cesium) are one family of the pe-ber of atoms of each element on the right-hand, or product, riodic table - they all have one reactive electron in their side.
The above equation states that one mole of potassium per-outer shell.
The halogens (fluorine, chlorine, bromine,
chlorate (KC10 4 , a reactant) will react with 4 moles of magnesium and iodine) are another common family - all have seven metal to produce one mole of potassium chloride (KCI) and 4 moles electrons in their outer shell and readily accept an eighth of magnesium oxide (MgO).
electron to form a filled level.
In mass terms, 138.6 grams (or pounds, tons, etc.) of potassium perchlorate will react with 97.2 grams (or any other mass The mass of one atom of any element is infinitessimal and is im-unit) of magnesium to produce 74.6 grams of KC1 and 161.2 grams possible to measure on any existing balance. A more convenient of MgO. This mass ratio will always be maintained regardless of mass unit was needed for laboratory work, and the concept of the quantities of starting material involved. If 138.6 grams (1.00
the mole emerged, where one mole of an element is a quantity mole) of KC10 4 and 48.6 grams (2.00 moles) of magnesium are equal to the atomic weight in grams. One mole of carbon, for mixed and ignited, only 69.3 grams (0.50 mole) of the KC1O 4 will example, is 12.01 grams, and one mole of iron is 55.85 grams.
react, completely depleting the magnesium. Remaining as "excess"
The actual number of atoms in one mole of an element has been starting material will be 0.50 mole (69.3 grams) of KC10 4 - there determined by several elegant experimental procedures to be is no magnesium left for it to react with! The products formed 6.02 X 10 23 !
This quantity is known as Avogadro's number, in in this example would be 37.3 grams (0.50 mole) of KC1 and 80.6
honor of one of the pioneers of the atomic theory. One can then grams (2.00 moles) of MgO, plus the 69.3 grams of excess KC10 4 .
see that one mole of carbon atoms (12.01 grams) will contain ex-The preceding example also illustrates the law o f conservation actly the same number of atoms as one mole (55.85 grams) of o f mass. In any normal chemical reaction (excluding nuclear re-iron.
Using the mole concept, the chemist can now go into the actions) the mass of the starting materials will always equal the laboratory and weigh out equal quantities of atoms of the vari-mass of the products (including the mass of any excess reactant).
ous elements.
200 grams of a KC1O 4 /Mg mixture will produce 200 grams of prod-The same concept holds for molecules. One mole of water ucts (which includes any excess starting material).
(H
The "formula" for the preceding illustration involved KC10 4
20) consists of 6.02 X 10 23 molecules and has a mass of 18.0
grams. It contains one mole of oxygen atoms and two moles of and Mg in a 138.6 to 97.2 mass ratio. The balanced mixture -
hydrogen atoms covalently bonded to make water molecules. The with neither material present in excess - should then be 58.8%
molecular weight of a compound is the sum of the respective KC10 4 and 41. 2% Mg by weight. The study of chemical weight atomic weights, taking into account the number of atoms of each relationships of this type is referred to as stoichiometry. A element that comprise the molecule. For ionic compounds, a simi-mixture containing exactly the quantities of each starting ma-lar concept termed formula weight is used.
The formula weight of
terial corresponding to the balanced chemical equation is re-sodium nitrate, NaNO
ferred to as a stoichiometric mixture. Such balanced composi-3 ,
is therefore:
tions are frequently associated with maximum performance in Na + N + 3 O's = 23.0 + 14.0 + 3(16.0) = 85.0 g/mole high-energy chemistry and will be referred to in future chapters.
18
Chemistry of Pyrotechnics
Basic Chemical Principles
19
ELECTRON TRANSFER REACTIONS
For the reaction
Oxidation-Reduction Theory
KC1Oy + ? Mg -> KC1 + ? MgO
A major class of chemical reactions involves the transfer of one the oxidation numbers on the various atoms are: or more electrons from one species to another. This process is referred to as an electron-transfer or oxidation-reduction reac-KC10
tion, where the species undergoing electron loss is said to be 4 :
This is an ionic compound, consisting of the potassium ion, K+, and the perchlorate ion, C10,, - . The oxidation num-oxidized while the species acquiring electrons is reduced. Pyro-ber of potassium in K+ will be +1 by rule 2. In technics, propellants, and explosives belong to this chemical re-C104-1 the
4 oxygen atoms are all -2, making the chlorine atom +7, by action category.
rule 4.
The determination of whether or not a species has undergone Mg: Magnesium is present in elemental form as a reactant, a loss or gain of electrons during a chemical reaction can be making its oxidation number 0 by rule 2.
made by assigning "oxidation numbers" to the atoms of the vari-KC1: This is an ionic compound made up of K + and C1- ions, ous reacting species and products, according to the following with respective oxidation numbers of +1 and -1 by rule 2.
simple rules
MgO: This is another ionic compound. Oxygen will be -2 by rule 1, leaving the magnesium ion as +2.
1. Except in a few rare cases, hydrogen is always +1 and oxygen is always -2. Metal hydrides and peroxides are Examining the various changes in oxidation number that occur the most common exceptions. (This rule is applied first -
as the reaction proceeds, one can see that potassium and oxygen it has highest priority, and the rest are applied in de-are unchanged going from reactants to products. Magnesium, creasing priority. )
however, undergoes a change from 0 to +2, corresponding to a 2. Simple ions have their charge as their oxidation number.
loss of two electrons per atom - it has lost electrons, or been For example, Na+ is +1, Cl- is -1, Al +3 is +3, etc. The oxidized. Chlorine undergoes an oxidation number change from oxidation number of an element in its standard state is 0.
+7 to -1, or a gain of 8 electrons per atom - it has been reduced.
3. In a polar covalent molecule, the more electronegative In a balanced oxidation-reduction reaction, the electrons lost atom in a bonded pair is assigned all of the electrons must equal the electrons gained; therefore, four magnesium atoms shared between the two atoms. For example, in H-Cl, (each losing two electrons) are required to reduce one chlorine the chlorine atom is assigned both bonded electrons, atom from the +7 (as C1O,, - ) to -1 (as C1 - ) state. The equation making it identical to C1 - and giving it an oxidation num-is now balanced!
ber of -1. The hydrogen atom therefore has an oxidation number of +1 (in agreement with rule #1 as well).