Theoretical Chemistry
(1) Definitions.—An element is a substance that cannot be decomposed.
A compound is a substance that can be decomposed into other different substances; and if the decomposition goes far enough, these substances will be elements.
A mixture is made up of two or more components (elements and compounds or both), physically put together. It differs from a compound whose compounds are chemically united.
(2) Laws.—Law of Definite Proportions: All specimens of a compound contain the same elements in the same proportions.
Law of Multiple Proportions: When two compounds consist of the same elements, the proportion of one is a simple multiple of the proportion of the other.
Law of Combining Proportions: Each element enters into all its compounds by a fixed proportional weight.
The fundamental laws of chemistry are proved by experiment.
(3) The Atomic Theory.—The atomic theory teaches that matter is composed of minute particles which themselves cannot be divided, but which unite to form molecules which can be divided.
A molecule, then, is the smallest amount of a substance that can exist in a free state.
The diameters of molecules have been ascertained by Jeans to be—
| Hydrogen | 20.3 |
| Nitrogen | 29.1 |
| Oxygen | 27.3 |
These figures express number of billionths of a meter.
An atom is an indivisible particle of an element, and goes to make up the molecule.
(4) Chemical Notation.—The symbols used to represent the different elements (e.g. H for hydrogen, O for oxygen, etc.), when used in chemical compounds, refer to the number of atoms which go to make up the molecule of that particular compound. For example, the expression H2SO4 means that in one molecule of that acid there are 2 atoms of hydrogen, 1 of sulphur, and 4 of oxygen.
(5) Molecular Weights.—To determine the molecular weight of a compound it is necessary to know Avogadro’s Law: Equal volumes of all gases under the same conditions contain the same number of molecules; and Molecular Weight = Vapor Density × 2.
(6) Reactions.—A reaction or chemical equation is a method of representing a chemical change.
In chemistry we have three kinds of reactions, namely:
(1) Analytical reaction, which is the breaking up of compound bodies into simple, e.g., H2CO3 can be broken up into its components, H2O and CO2, e.g., H2CO3 = H2O + CO2.
(2) Synthetical reaction is the building up of a compound body by the union of two or more simple bodies, e.g., H2 + O = H2O and H + Cl = HCl.
(3) Metathetical reaction consists in the interchange of two radicals in two substances, e.g.,
2HCl + Zn = ZnCl2 + H2. Here the H of the acid is replaced by the Zn.
KCl + AgNO3 = AgCl + KNO3. Here the Ag and the K change places.
(7) The Chemical Arithmetic by which from the molecular weights of two substances, and the weight of one substance we are enabled to get the weight of the required substance is called Stoichiometry.
Example: Required the amount of zinc necessary to generate 10 grams of hydrogen.
Atomic weights of H, Cl, and Zn are respectively 1, 35.5, and 65.3.
The reaction is as follows:—
Zn + 2HCl = ZnCl2 + H2, and shows that 2 atoms of H are used for every 1 of Zn.
| (Mol. Wt. Zn.) | (Mol. Wt. H2) | (Wt. Zn.) | (Wt. H2.) | |||
| 65.3 | : | 2 | = | x | : | 10 |
65.3 × 102 = x = 326.5 grams of Zn.
(8) Berthollet’s Law.—Berthollet established the following law, which is of great importance. When two substances can form a substance insoluble or volatile, under the condition of the reaction, that substance will be formed till one of the factors is exhausted.
(9) Radicals.—A radical is an atom or group of atoms which changes places in a reaction. A compound radical is made up of different sorts of radicals, as NH4.
A basic radical is a metal, or a compound radical which behaves like a metal, e.g., Zn and NH4.
(10) Hydrates.—A hydrate is a substance formed from water by replacing half of its hydrogen by a radical, e.g., H2O + 2Na = 2NaOH + H2, where the sodium has taken the place of one atom of hydrogen.
(11) Base.—If a hydrate is formed by a basic radical, the hydrate is called a base, e.g., ZnO2H2.
(12) Alkali.—An alkali is a soluble base, e.g., NaOH, KOH, NH4OH, LiOH.
(13) Acid.—An acid is a substance containing hydrogen which may be replaced by a basic radical, e.g., 2HCl + Zn = ZnCl2 + H2.
(14) Salts.—A salt is a substance formed from an acid replacing its hydrogen by a basic radical, e.g. 2HCl + Zn = ZnCl2 + H2.
An acid salt is a compound derived from an acid which has not all of its hydrogen replaced, e.g., 2NaCl + H2SO4 = NaHSO4 + HCl + NaCl.
(15) Chemical Nomenclature.—Termination “—UM” is now applied to all Metals, though the older-known metals retain the former names, e.g.—Aluminium, Tellurium, etc.
Termination “—IDE” denotes a Binary Compound, that is, a substance composed of only two elements, e.g., Sodium Chloride (NaCl).
Termination “—OUS” is applied to the first of two elements when it exists in a greater proportion than in another combination with the same element, e.g., one atom of phosphorus and three atoms of chlorine form Phosphorous Chloride.
Termination “—IC,” when the first exists in a lesser proportion, e.g., one atom of phosphorus with five atoms of chlorine form Phosphoric Chloride.
Prefixes “MONO—,” “BI—,” “TRI—,” etc., indicate the proportion of the latter of two elements, and are sometimes used instead of the above termination. Thus phosphorous chloride may also be called Phosphorous Tri-Chloride; so one atom of carbon with one atom of oxygen is Carbon Monoxide.
Prefix “HYPO—” (under) and “PER—” (over), specify compounds formed by the same two elements containing less (or more) of an element than is in the usual compound.
Nomenclature of Salts.—From the common acids we get the following salts:—
| HCl | forms chlorides. |
| HNO3 | forms nitrates. |
| H2SO4 | forms sulphates. |
| H2S | forms sulphides. |
| H2CO3 | forms carbonates. |
| H2O | forms no salts. |
| H2SiO4 | forms silicates. |
| H3PO4 | forms phosphates. |
A rough rule for the nomenclature of acids may be made from the above. Acids with the prefix hydro and the suffix ic form salts in ide; with suffix ate, salts in ate; with suffix ous, salts in ite.
(16) Basicity.—The basicity of a substance is measured by the amount of hydrogen which it contains that can be replaced by a basic radical, e.g., H2SO4 is di-basic, i.e., the two atoms of hydrogen can be replaced by a basic radical. H2SO4 + CaCl2 = CaSO4 + 2HCl.
(17) Quantivalence.—The quantivalence of an element is measured by the number of atoms of hydrogen it combines with or replaces. E.g., Na is univalent, for when added to HCl it replaces one atom of hydrogen; Ca is bivalent, for, as seen in the above reaction, it replaces two atoms of hydrogen.
(18) Test for a Chloride.—To test for HCl or any chloride, add to the solution to be tested a little AgNO3, and if a chloride is present, a milky-white precipitate will be formed. The reaction is as follows: HCl + AgNO3 = AgCl (white precipitate) + HNO3. A metal almost invariably changes places with hydrogen.
Caution.—In diluting H2SO4 add the acid to the water; for the evolution of heat from the process will cause the water to boil, and reversing this process will cause the liquid to boil over and possibly result disastrously.
(19) Impurity in H2SO4.—Commercial sulphuric acid contains PbSO4 as an impurity. This gives it the colored appearance, plumbic sulphate being soluble in strong sulphuric acid.
(20) H2S.—Sulphuretted hydrogen is somewhat soluble in water, slightly poisonous, and is a reducing agent.
(21) Carbonic Acid.—H2CO3 does not exist as an acid. We infer its existence from the presence of its salts. Na2CO3 + 2HCl = 2NaCl + H2CO3, but the H2CO3 is so unstable that it breaks up at once into H2O and CO2.
(22) Test for a Carbonate.—To test for a carbonate, treat the substance with an acid; CO2 is formed; pour the gas into a solution of lime-water, and a white insoluble precipitate is formed, CaCO3.