INTRODUCTION TO CHEMISTRY

  1. Devopments in Chemistry after Avogadro.
    1. The Method of Equivalent Weights.
      1. History:
        Berzelius made many contributions to chemistry in the period from 1811 to 1826 among which was the use of letter symbols for chemical elements. He also determined the atomic weight of 43 elements which he published in 1818. He devoted 10 years to the acquisition of reliable data on equivalent or combining weight of the elements. With the greatest accuracy he determined the percent composition of some 2000 compounds.
      2. Definition of Equivalent weights.
        The gram of equivalent weight of an element may be defined as weight in grams of that element which will combine with 8 grams of oxygen or its equivalent. Although oxygen is the basis of equivalent weights it is not necessary to use oxygen in determining the equivalent weight of an element. If the equivalent weight of another element is known, it can be used to determine the unknown equivalent weight, if the element of the known equivalent weight combines with it. For example: Find the gram equivalent weight of silver given that in silver oxide 13.5 grams of silver combines with one gram of oxygen. Solution: Set the following proportion between the silver and oxygen: 
        13.5     x
----  = ---  and solve for x: x = (13.5)(8) = 108.0 grams.
  1      8
        Thus the gram equivalent weight of silver is 108.0 grams.
      3. Valence.
        The concept of valence was introduced in 1852 by Edward Franklin (1825-1899). He attempted to state the combining capacity of elements. Valence is a measure of the combining capacity of an element and is defined as the number of atoms of hydrogen which will combine with one atom of that element or the number of atoms of hydrogen replaced by one atom of that element in a compound. The valence is positive if it replaces hydrogen and negative if it combines with hydrogen. For example: the valence of oxygen is -2 because one atom of oxygen combines with two atoms of hydrogen. The valence of sodium is +1 because one atom of sodium replaces one atom of hydrogen from hydrogen chloride (hydrochloric acid) to form sodium chloride (salt).
      4. The Law of Equivalent Weights or Combining Weights:
        The atomic weight of an element is equal to the product of its valence and its Equivalent Weight:
        A = NE,
        where A is the Atomic weight of an element, N is the valence of that element, and E is the Equivalent Weight of the element.
    2. Dulong and Petit's Law (1819).
      Both Pierre Dulong (1785-1838) and Alexis Petit (1791-1820) were professors at the Ecole Polytechnique who both observed this law. It states that the product of the specific heat of an element and its atomic weight is approximately 6 for nearly all known solid elements. The specific heat of an element is the amount of heat measured in calories which will raise the temperature of one gram of the element one degree centigrade. For example: the specific heat of silver is 0.056 calorie per gram. So by Dulong and Petit's Law the atomic weight of silver should be 6/0.056 = 107 approximately. In view of the approximate nature of this law, we see that the value of the atomic weight of silver is 108.0 and its valence is +1, since by the Law of Equivalent Weights the atomic weight of an element divided by its equivalent weight is its valence. It is positive since silver replaces hydrogen in nitric acid forming silver nitrate.
    3. Cannizarro's Method of Atomic Weight Determination.
      On September 3, 1860, because of the uncertainty about the determination of the atomic weights a conference was called at Karlsrule, Germany. This First International Chemical Congress was chaired by Jean Dumas (1800-1884) who had worked out an ingenious method for the precise determination of the densities of vapors. His method made possible, for the first time, the determination of gas densities of many substances normally liquid or solid at room temperature. The conference was attended by every important chemist in the world. The conference settled nothing. As the chemists prepared to leave in as great confusion of mind as when they had arrived, Angelo Pavesi of the University of Pavia stood at the door and handed out copies of a little pamphlet in Italian written by his friend Stanislao Cannizzaro (1826-1916), professor of chemistry at the University of Genoa. The little pamphlet was entitled Sketch of a Course of Chemical Philosophy (The English translation). It had originally appeared in the Nuovo Cimento in 1858 but had been reprinted at Pisa in 1859. A copy was given to the German chemist Lothar Meyer. When Meyer returned home he read the pamphlet and, in his own words, "The scales seemed to fall from my eyes. Doubts disappeared and a feeling of quiet certainty took their place." He was so impressed that he made Cannizzaro's views the basis of his influential text book, Modernen Theorien der Chemie which was first published in 1864. Cannizzaro's method of atomic weight determination had cleared up the confusion.
      1. Cannizzaro's Interpretation of Avogadro's Hypothesis. He believed that acceptance of Avogadro's Hypothesis that equal volumes of gases, whether elements or compounds, contain equal number of particles was necessary for progress in the determination of atomic weights. He, however, drew a clear distinction between determining the relative molecular weights from gas density data using Avogadro's Hypothesis and determining relative atomic weights.
      2. The essence of Cannizzaro's method of determining relative atomic weights of an element was to determine the relative molecular weights from the gas densities of as many gaseous or vaporizable compounds of that element as could be prepared. From the analysis of these compounds he found the smallest weight of the element contained in the molecular weight of the various gaseous compounds of the element. This smallest weight was accepted as the relative atomic weight of an atom of the element. The table below will illustrate the first part of Cannizzaro's method. Let us consider first just those substances containing hydrogen in Part A of the table. In column 1 there is listed the gas densities (grams per liter), all at the same temperature and pressure, of the various substances listed in first column. In column 2 are the percentage of hydrogen by weight in each of these gases, as determined by chemical analysis. Column 3 contains, for each substance, the product of gas density and percentage of hydrogen, that is, column 1 multiplied by column 2 (converting percentage to a decimal). This gives us in column 3 the weight of hydrogen present in one liter of the substance. Now an inspection of column 3 reveals that the numbers are integrally related so that if each number in column 3 is divided by the smallest number (that of hydrogen chloride), an integer is found. These are listed in column 4. Parts B and C of the table contain a similar analysis for oxygen and chlorine, repectively. 
                   CANNIZZARO'S METHOD OF MOLECULAR FORMULA DETERMINATION
------------------------------------------------------------------------
|                   |   1      |     2      |    3       |    4        |
|                   | Density, | Percentage | Product    | Values in 3 |
|                   |grams per | of element | of values  | divided by  |
| Substance         |  liter   | by weight  | in 1 and 2 | least value |
------------------------------------------------------------------------
|            A. Hydrogen and its gaseous compounds                     |
------------------------------------------------------------------------
| Hydrogen          |  0.0659  |   100.00   |   0.0659   |     2       |
| Hydrogen chloride |  1.19    |     2.76   |   0.0329   |     1       |
| Water             |  0.589   |    11.2    |   0.0659   |     2       |
| Ammonia           |  0.557   |    17.7    |   0.0986   |     3       |
| Methane           |  0.524   |    25.1    |   0.132    |     4       |
------------------------------------------------------------------------
|            B. Oxygen and its gaseous compounds                       |
------------------------------------------------------------------------
| Oxygen            |  1.05    |   100.00   |   1.05     |     2       |
| Water             |  0.589   |    88.8    |   0.523    |     1       |
| Sulfur dioxide    |  2.09    |    50.0    |   1.05     |     2       |
| Carbon monoxide   |  0.916   |    57.1    |   0.523    |     1       |
| Carbon dioxide    |  1.44    |    72.7    |   1.05     |     2       |
------------------------------------------------------------------------
|            C. Chlorine and its gaseous compounds                     |
------------------------------------------------------------------------
| Chlorine          |  2.32    |   100.0    |   2.32     |     2       |
| Hydrogen chloride |  1.19    |    97.2    |   1.16     |     1       |
| Chloroform        |  3.90    |    89.1    |   3.48     |     3       |
| Methylene chloride|  2.78    |    83.5    |   2.32     |     2       |
| Carbon            |          |            |            |             |
|   Tetrachloride   |  5.03    |    92.2    |   4.64     |     4       |
------------------------------------------------------------------------
        All gas densities in this table are given for 100°C and one atmosphere pressure.

        How did Cannizzaro explain these striking numerical data? First, he argued, we must accept the truth of Avogadro's Hypothesis that equal volumes of gases contain equal number of particles. Hence, in comparing densities of gases we are comparing the weights of equal number of molecules. Then when the density of a gas containing a certain element is multiplied by its weight percentage of that element in that gas, the result is the weight of that element per unit volume. The numbers in column 3 of Part A thus represent the weights of hydrogen present in a fixed number of molecules. Now if we compare these weights in a series of substances that contain that element, we should expect them to be different because the number of atoms of that element should vary from substance to substance. And these weights should be integer multiples of some smallest weight, since the atoms of the element in different substances are in integer ratio to each other and to one atom in the substance containing one atom per molecule. Then dividing each weight in column 3 by the smallest weight in column 3 should give us the number of atoms present in one molecule of each substance. Those are the integers in column 4.

        This data and Cannizzario's interpretation of it give a confirmation of the second part of Avogadro's Hypothesis that the molecules of gaseous elements may contain more than one atom of the element. In the table in Part A the weight of hydrogen in one liter of elemental hydrogen gas is exactly twice the least weight, that of hydrogen chloride. Thus hydrogen gas does not consist of individual atoms, but of molecules each containing two atoms. And from the results in Parts B and C, we can conclude that oxygen and clorine also consists of diatomic molecules. Cannizzaro's method had at long last provided a means to establish the number of atoms in the molecules of gaseous substances. The smallest physical particle, the molecule, and the smallest chemical unit, the atom, of an element are not necessarily identical.

      3. Cannizzaro's determination of atomic weights.
        Having determined the number of atoms of an element per molecule of the element, he thus was able to determine the relative molecular weights of other compounds. The compound of water, for example, appears twice in the table, once in part A and once in part B. Its weight of hydrogen (Part A) is twice the least weight of that element, and its weight of oxygen (Part B) is equal to that element's least weight. According to Cannizzaro's interpretation, hydrogen and oxygen atoms must be in present in a 2:1 ratio in the water molecule. Using the known analytical result that the weight ratio of hydrogen to water is is nearly 1:8, he concluded that the oxyen atoms must therefore be 2 × 8 = 16 times heavier than hydrogen atoms. Thus the relative atomic weight of oxygen is 16.
      4. The significance of Cannizzaro's Method.
        Basic to Cannizzaro's entire scheme of atomic weight determination was the assumption that each list of compounds of a given element included at least one compound whose molecules contained only one atom of that element. If the list does not include such a compound, the weights in column 3 will still be multiples of some smallest weight. And the method may still be used for that element. The acceptance of Cannizzaro's proposal brought order out of the previously confusing assignment of atomic ratios and atomic weights.
    4. The Periodic Classification of the Elements.
      As the number of elements increased in the early 19th century and as their chemical and physical properties became increasingly better established, there naturally arose attempts to find some sort of relationship between the elements.
      1. Families of Elements.
        Early in the 19th century it was observed that even though no two elements have the same properties, some of them do display similarities so clear that they might be said to belong to the same "family." With valence as guide, chemist were able to delineate such groups and to clarify their "family characteristics." The following are some of the families and some of their properties.
        1. The Halogens. The elements fluorine, chlorine, bromine, and iodine, listed here in order of increasing atomic weight, and having atomic weights of 19 (F), 35.5 (Cl), 80 (Br), and 127 (I), constitute the family of elements called the halogens. They are all non-metals and have a valence of -1. Although these four elements have striking dissimilarities (for example, the first two are greenish and greenish-yellow gases, the third is a deep red liquid, and the last dark violet volatile solid) they also have similar properties. They all combine violently with many metals to form white, crystalline salts (halogen means "salt former") having similar formulas, such as NaF, NaCl, NaBr, and NaI, because they have the same valence. With hydrogen, all four elements form simple compounds that dissolve in water and form acids. All four, when vaporized under under ordinary conditions, form diatomic molecules. While these chemical properties are the same for all four elements, they have other properties that vary progressively with increasing atomic weight. The melting point and boiling point increase with atomic weight. The solubility in water decreases with atomic weight as does the chemical activity, such as speed of reaction and amount of energy released. Thus the members of the familly have varying similarities as well as the same properties.
        2. The alkali metals family consists of the elements lithium, sodium, postassium, rubidium, and cesium, listed here in order of increasing atomic weights, ranging from 0.94 (Li) to 133 (Cs). They all are very active metals, capable of liberating hydrogen from water, forming an alkali or base (LiOH, NaOH, KOH, RbOH, and CsOH). All of them have the single valence of +1, and hence form compounds with other elements with similar formulas: LiCl, NaCl, KCl, RbCl and CsCl. If exposed to air these metals combine readily with the oxygen of the atomsphere. They also exhibit regular gradations in other properties, such as melting point, density.
        3. The alkaline earth metals family has as members the elements beryllium, magnesium, calcium, strontium, barium, radium, listed here in order of increasing atomic weights, ranging from 9.2 (Be) to 226 (Ra). They are all active metals, although less than the alkali metals, and like the alkali metals they exhibit increasing activity with increasing atomic weight. In their compounds they show a single valence of +2. All form chloride that are water soluble and carbonates that are insoluble in water.
        4. The elements oxgyen, sulfur, selenium, tellurium, and polonium are members of the same family called the oxygen group. Here the family properties are not as striking as the above families. Oxygen and sulfur are strongly nonmetallic, but the last three show increasing metallic properties, although not "true" metals. Oxygen has the group characteristic valence of -2, which is very nearly oxygen's only valence. The other elements of the group have additional valences as well as -2. All the members of this group form compounds with hydrogen.
      2. The Periodic Law.
        By the middle of 19th century at least five distinct families of elements were known. Several attempts were made to find regularities among these families and the other elements that did not seem to fit into any family.
        1. In 1829 the German chemist, Johann Wolfgang Dobereiner (1780-1849), observed that within each of several chemical families, the atomic weight of any element is equal, or nearly equal, to the average of the atomic weights of its two immediate neighbors. For example, in the alkali metals, 23.0 for Na is halfway between 6.94 for Li and 39.11 for K; in the halogen family, 79.9 for Br is roughly the average of 35.5 for Cl and 127 for I. It looked as if the atomic weights within any particular family might be in arithmetic progression. But no physical meaning could be made of it. But Dobereiner's "triad" stimulated (in part) the study of the chemical families.
        2. In 1863 the English chemist, J. A. R, Newlands (1836-1898), proposed a "law of octaves," according to which the properties of the elements are repeated at equal intervals of eight when the elements are arranged in order of increasing atomic weight. In his own words, Newland said, "the eighth element, starting from a given one, is a kind of repetition of the first, like the eighth note in an octave of music." But this proposal of Newland was of limited use. The scheme breaks down entirely after Newland's 17th element.
        3. The Perodic Law of Elements. The Russian chemist, Dimitri Ivanovich Mendeleyev (1834-1907), in 1868 expanded Newland's law of octaves into the periodic law of elements. If the word "periodic" is understood to mean "repeating at intervals," the intervals being the periods of the repetition, the best statement of The Periodic Law is that of Mendeleyev himself: "The properties of the elements are in periodic dependence upon their atomic weights." That is, if the elements are arranged in order of increasing atomic weight, the properties of the elements will be observed to be repeated at intervals. The intervals are not all the same as suggested by Newland but increase as the atomic weights increase. Such properties as valence, atomic volume, melting point, etc. all exhibit this characteristic stated in the periodic law.
      3. The Periodic Table.
        In the publications of Mendeleyev and, starting a little later, of the German chemist, Julius Lothar Meyer (1830-1895), they set forth a periodic classification of the elements in the form of a table.
        1. Definition of Periodic Table:
          A periodic table of the elements may be defined as a classification wherein all the elements are arranged according to increasing atomic weight in order to show the periodic recurrence of the physical and chemical properties of the elements. The early tables of Mendeleyev were arranged according to increasing atomic weight; modern tables are arranged according to increasing atomic number of the element, which is the number of protons in the nucleus of an atom of that element. Also the rows and columns in the table have changed; in Mendeleyev's early tables the rows are the families and columns are periods; in modern tables the rows are periods and the columns are groups; the families are paired together into groups (families are subgroups). In modern tables there are two kinds of families: the main families which usually have a single valence and transition families which have 1, 2 or 3 valences and are metals.
        2. Prediction of Elements.
          As Mendeleyev arranged his periodic table he found families that had missing members in some periods. Realizing these were yet undiscovered elements, he left holes in his table for these missing undiscovered elements. In addition, on the basis of its family characteristics and the periodic repetition of the properties of the elements, he predicted the properties of the missing elements. He predicted in 1871 the element which he called eka-silicon (discoveried by C. Winkler of Freiberg, Germany, in 1886 and named Germanium) and the element he called eka-aluminium (discoveried by Lecoq de Boisbaudran in 1875 and named Galium). Another of his missing elements was discovered by Nilson in 1879 and named Scandium.
  2. The Language of Chemistry.
    1. Symbols: chemical symbols are associated with elements; they are abbreviations.
      1. A symbol identifies an element.
      2. A symbol stands for one atom of that element.
      3. A symbol stands for one gram-atomic weight (gram-atom) of an element.
        1. Definition: A gram-atomic weight of an element is that quantity of the element in grams equal to its atomic weight. For example, the gram-atomic weight of oxygen is 16.000 grams and the gram-atomic weight of hydrogen is 1.0080 grams.
        2. Importance: The gram-atomic weight of any element contains the same number of atoms.
      4. A symbol stands for one Avogadro's Number of atoms which is equal 6.02 × 10+23 atoms per gram-atomic weight.
    2. Formulas: chemical formulas are associated with compounds; they are not a recipe for how to make the compound.
      1. A formula tells us what elements are in the compound.
      2. A formula tells us the ratio of the atoms of each element in the compound. These are called simple formulas; all compounds have a simple formula. For example, the formula of salt or sodium chloride: NaCl. The subscripts on the symbols in a formula indicate the number of atoms of the element subscripted.
      3. A formula tells us how many atoms of each element are in a molecule of the compound, if the compound is made up of molecules. Not all compounds are molecular, that is, made up of molecules. The formula of a molecular compound is called a molecular formula. A molecular formula is a simple formula, but not all simple formulas are molecular formulas. For not all substances are made up of molecules. For example, common salt or sodium chloride is not made up of molecules, but is a pile of ions, which are electrically charged atoms. In addition to simple and molecular formulas, there are also semi-structured and structured formulas.
      4. A formula tells us the molecular weight of the compound if the compound consists of molecules and the formula weight of the compound if the compound does not consist of molecules.
        1. Definition of molecular weight: the relative weight of a molecule of compound compared to the relative weight of the oxygen molecule taken as 32.0000; it has no units because it is a ratio.
        2. Calculation of molecular weight from molecular formula: For example, the formula of water tells us that there are two atoms of hydrogen and one atom of oxygen in each molecule. 
          Hydrogen: 2 ×  1.008 =  2.016
Oxygen:   1 × 16.000 = 16.000
                           --------
                            18.016 = molecular weight of water.
        3. Definition of formula weight: the sum of the atomic weights of each element multiplied by the subscript appearing on each symbol in the formula.
      5. A formula stands for or represents one gram-molecular weight (gram-mole or mole) of the substance.
        1. Definition of gram-molecular weight: A gram-molecular weight of a substance is that quantity of the substance in grams equal to its molecular weight.
        2. Importance of gram-molecular weight: a mole of any substance contains the same number of molecules.
      6. A formula stands for or represents one Avogadro's Number of molecules.
      7. A formula tells us the percentage composition of a compound.
    3. Equations: chemical equations are associated with the chemical reactions.
      1. An equation tells us how to make a compound or compounds; it is a recipe.
      2. An equation tells us what substances are necessary for the reaction; these are called the reactants. The reactants are on the left side of the equation.
      3. An equation tells us what substances are produced by the reaction; they are called the products of the reaction. The products are on the right side of the equation.
      4. An equation must be balanced, that is, the sum of the atoms in the reactions must equal the sum of atoms in the products.
      5. The coefficients of each formula in the equation tell us the number of molecules of each substance if they are molecular.
      6. The coefficients of each formula also tell us the volumes of the reactions and products if they are gases.
      7. An equation tells us the weights of the reactants and products to complete the reaction. There are three kinds of calculations:
        1. weight-weight calculations:
        2. weight-volume calculations:
        3. volume-volume calculations: