Complete Text (Part 6)

with oxygen and how many of them with hydrogen ? LESSON FIVE To the Student: Considerable attention has been given in this lesson and others that follow to elaborating for you the course of reasoning by which certain laws were established. It is thought that everything has been made sufficiently clear, but it is too much to expect that you will be able to follow the reasoning at first reading. We expect that you will have to go many times over the text, until not only the words become familiar to you in their meaning, but the trend of thought becomes as natural to you as it is to the writer. Do not forget your dictionary or the treatment of corresponding subjects in the school text-books. All will help you to a clearer understanding of the text. VII Definite Combining Proportions and the Atomic Hypothesis 133. Proust pointed out that all chemical compounds contain their component elements in fixed and invariable proportions. The following examples illustrate this : a, 1 gram of hydrogen unites with 16 grams of oxygen to form the compound called hydrogen dioxide, commonly known as peroxide of hydrogen. These proportions can- not be altered. And a mass of 17 grams of hydrogen dioxide always consists of 1 gram of hydrogen and 16 grams of oxygen. A 2-gram mass of hydrogen unites with a 16 -gram mass of oxygen to form the compound called water. And all water, wherever found or however produced, when de- composed yields hydrogen and oxygen in the proportion 53 54 A CORRESPONDENCE COUKSE IN" PHARMACY of 1 part of the first named to 8 parts of the other element. 1). 1 gram of hydrogen unites with 35.5 grams olthe element called chlorine to form 36.5 grams of hydrogen chloride, commonly called hydrochloric acid gas. In the following table are given in grams the combining proportions of several different elements and the product obtained from the combination. Eead from left to right, the first and third columns give the names of the elements ; the second and fourth, their combining weights; the fifth, the name of the product ; and the sixth, the weight of the product. The horizontal lines separate the compounds into related groups : Element No. OF Grams Element No. OF Grams Product Grams in Product Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen Nitrogen 14 2x14 14 2x14 2x14 2x14 Hydrogen Oxygen Oxygen Oxygen Oxygen Oxygen 3 16 16 3x16 2x16 5x16 Hydrogen Nitride (Ammonia) Hyponitrous oxide Mononitrogen mon- oxide (nitrosyl) Nitrogen trioxide Nitrogen peroxide Nitrogen pentoxide 17 44 30 76 60 108 Nitrogen 14 Chlorine 3x35.5 Nitrogen trichloride 120.5 Manganese 55 Chlorine 2x35.5 Manganous chloride 126 Manganese Manganese Manganese 55 2x55 55 Oxygen Oxygen Oxygen 16 3x16 2x16 Manganous oxide Manganic oxide Manganese dioxide 71 158 87 Carbon Carbon 12 12 Oxygen Oxygen 16 2x16 Carbon monoxide (Carbonyl) Carbon dioxide 28 44 Carbon 12 Hydrogen 4x1 Methane (marsh gas) 16 Carbon 12 Sulphur 2x32 Carbon disulphide 76 Sulphur Sulphur 32 32 Oxygen Oxygen 2x16 3x16 Sulphur dioxide Sulphur trioxide 64 80 Sulphur 32 Manganese 55 Manganese sulphide 87 PKOPOKTIOtfS AND THE ATOMIC HYPOTHESIS 55 Element No OF Grams Element No. OF Grams Product Grams in Product Sulphur 32 Mercury 200 Mercuric sulphide 232 Mercury Mercury 200 2x200 Oxygen Oxygen 16 16 Mercuric oxide Mercurous oxide 216 416 Mercury Mercury 200 200 Chlorine Chlorine 35.5 2x35.5 Mercurous chloride Mercuric chloride 235.5 271 Sulphur 32 Chlorine 4x35.5 Sulphur tetrachlo- ride 174 Carbon 12 Chlorine 4x35.5 Carbon tetrachloride 154 From the foregoing facts, which have been determined by repeated experimentation, it appears that the relative com- bining masses of the several elements are simple multiples of definite values. The relative combining mass of hydrogen is 1 or a multiple of it. oxygen is 16 chlorine is 35.5 nitrogen is 14 manganese is 55 carbon is 12 sulphur is 32 mercury is 200 These comparisons might be extended to include every chemical compound known, with the same results — definite combining proportions by weight of all the different elements in all the compounds which they severally form with one another. 134. The combining proportions of hydrogen and chlorine whenever they unite to form hydrogen chloride are invariably as 1 part of hydrogen to 35.5 parts of chlorine, or 35.5 times as much chlorine as hydrogen by weight. Hydrogen and chlorine cannot be made to combine in any other pro- portions. And if a mass of 36.5 kilograms of hydrogen chloride be decomposed it will give 1 kilogram of hydrogen and 35.5 kilograms of chlorine. 56 A CORRESPONDENCE COURSE IN PHARMACY 135. Two hypotheses were formulated by John Dalton to express the definite combining proportions of the elements as exemplified in the preceding paragraphs : A. The Law of Definite Proportions. — Any given chemical compound always contains the same component elements and in the same mass proportions. B. The Law of Multiple Proportions. — Whenever any two elements unite with each other in more than one mass pro- portion, simple multiples of a fixed mass unit of either unite with a fixed mass unit or with multiples of a fixed mass unit of the other element. Any two compounds containing the same two elements but in different proportions have different properties and are differ- ent compounds. When the compound contains more than two elements the proportions are also equally simple and definite. 136. The Atomic Hypothesis. Dalton explained the definite combining proportions of the elements by adopting the ancient hypothesis that all matter is composed of indivisi- ble individual particles, and by assuming that all such parti- cles of any one element have the same mass but that the particles of one element have a different mass from that of the particles of any other element: Each element consists of indivisible atoms of fixed mass. 137. If this atomic hypothesis be accepted as true, then the fixed chemical combining proportions by weight are thereby explained and seen to be the inevitable result of the fixed atomic masses. If, on the other hand, the atomic theory be rejected, then the fixed combining weights of the elements remain unintelligible, for no other sufficient explanation thereof has yet been made. The atomic hypothesis is a lucid and reliable working theory, and the system of chemistry built upon it leads to fixed results which may be expected and realized with absolute certainty and uniformity. PROPORTIONS AKD THE ATOMIC HYPOTHESIS 5? All known facts of chemistry agree with the atomic theory. 138. Atomic Weight. The numbers expressing the rela- tive masses of the atoms of different elements are called their atomic weights. The unit of expression of atomic weights is the mass of the hydrogen atom. The specific atomic weight of hydrogen is, therefore, 1 and that of oxygen is 16, because an atom of oxygen weighs 16 times as much as an atom of hydrogen. The atomic weight of chlorine is 35.5; that of nitrogen is 14; that of manganese is 55; that of carbon is 12; that of sulphur is 32; and that of mercury is 200. The atomic weights are the smallest relative masses of ele- ments that can enter into chemical combination with other elements. A table of the elements and their atomic weights was given in Lesson Four, VI, paragraph 122. 139. Molecular Weight. As all molecules consist of atoms and as all atoms have fixed masses, it follows that the mole- cule of any given element or chemical compound must also have a fixed mass, which is the sum of the masses of the atoms contained in it. A few elemental molecules consist of single atoms, and the molecular weight of any element having monatomic molecules is of course identical with its atomic weight. The molecule of hydrogen consists of two atoms of hydro- gen. Hence, as the atomic weight of hydrogen is 1, its molecular weight is 2. A molecule of ordinary oxygen contains two atoms. Hence, as the atomic weight of oxygen is 16, its molecular weight is 32. But there is another form of oxygen called ozone, each molecule of which consists of three oxygen atoms ; the molecular weight of ozone is, therefore, 48. The molecule of water contains two atoms of hydrogen and one atom of oxygen; the molecular weight of water is accordingly 18. 58 A CORRESPONDENCE COURSE IN PHARMACY 140. Vapor Densities. The specific weights of all gases and vapors are expressed in nnits of the density of hydrogen. The specific weight of hydrogen in the gaseous state, or its vapor density, is 1. The vapor density of any other gas or vapor is the quotient obtained when the weight of any given volume of it is divided by the weight of the same volume of hydrogen. One liter of hydrogen at 0° 0. weighs 0.09 gm. ; one liter of oxygen at 0° C. weighs 1.43 gm. ; a liter of chlorine at 0° C. weighs 3.17 gm. ; and a liter of nitrogen at 0° 0. weighs 1.26 gm. Hence, as the vapor density of hydrogen is 1, that of oxygen must be 16, that of chlorine must be 35.5; that of nitrogen 14. These numbers coincide with the atomic weights ; we shall presently learn why. 141. Avogadro's Law. Equal volumes of all gases or vapors at the same temperature and under the same pressure contain the same number of individual particles of matter.* Now, as the vapor density of hydrogen is 1 and its atomic weight also 1, and as its molecule contains two atoms so that its molecular weight is 2, we see that its molecular weight is twice its vapor density. The vapor density of oxygen is 16, for one liter of it weighs 16 times as much as one liter of hydrogen at the same temperature and pressure. The molecular weight of oxygen is 32, for its molecule contains two atoms and its atomic weight is 16. Hence the molecular weight of oxygen is twice its vapor density, just as the molecular weight of hydrogen is twice the vapor density of that element. The vapor density of ozone, however, is not 16 but 24. Why ? Because each individual particle or molecule of ozone consists of ♦This hypothesis is usually expressed as follows: "Equal volumes of all gases contain the same number of molecules." But this statement is incon- sistent with the definition of the term molecule (IV, par. 68), which refers to the smallest particle of any kind of matter, as the molecule. PROPORTION'S AND THE ATOMIC HYPOTHESIS 59 three atoms of oxygen, and as equal volumes of all gases contain the same number of individual particles of matter, each individual particle of ozone must weigh 24 times as much as each individual particle of hydrogen, and as each particle or molecule of hydrogen weighs 2, since it consists of 2 atoms, the individual particle or molecule of ozone must weigh 48 and must consist of three atoms of oxygen. It is true that, in a way, a molecule of ozone is a triatomic molecule of oxygen; it contains only oxygen. But the molecule of oxygen is diatomic, and the molecular weight of oxygen is 32. Below 500° C. the vapor of iodine weighs 126.5 times as much as the same volume of hydrogen at the same temper- ature and pressure. Hence that iodine vapor must consist of diatomic particles, or particles consisting of two atoms each. But at 1700° the vapor of iodine has a density of only 63.25, or weighs only 63.25 times as much as an equal volume of hydrogen at the same temperature and pressure; this iodine vapor at 1700° 0. must accordingly consist of particles weighing only half as much as the par- ticles of iodine vapor below 500° 0. Therefore, the particles of iodine at 1700° must contain only one atom each instead of two. The question may then be asked: is the diatomic particle of iodine its molecule, or is the mon- atomic particle its molecule ? The answer must be that the molecule of iodine is its atom, and that the molecular weight of iodine is identical with its atomic weight, for the monatomic particles of iodine are the smallest particles of that element exhibiting the specific properties by which the individuality of iodine is determined. Ferric chloride is a chloride of iron composed of iron and chlorine in the proportion of 56 parts of iron to 106.5 parts of chlorine. Its vapor at temperatures below 700° weighs 162.5 times as much as the same volume of hydrogen; this 60 A CORRESPONDENCE COURSE IN PHARMACY corresponds to the formula Fe2Cl6, and the weight of each particle must be 325. But the vapor of ferric chloride at 1000° C. weighs only 81.25 times as much as hydrogen, which proves that the individual particles of the compound at that temperature must consist of FeCl3. The molecule of ferric chloride is, therefore, now represented as FeCl3 and the molecular weight is given as 162.5. It was formerly represented as Fe2Cl6 and its molecular weight was then, of course, put down as 325. It would be confusing to recognize two different molecules and two different molecular weights. The smaller particle is then adopted as the molecule. The double molecule Fe2016 may be represented as (FeCl3)2. 142. Gay-Lussac's Proposition. — Gaseous elements combine in simple volume proportions, and the volumes of the products hear simple relations to the volumes of the component elements. This conclusion is self-evident from the law of Avogadro and Dalton's laws of combining proportions by weight. One liter of hydrogen and 1 liter of chlorine combine to form 2 liters of hydrogen chloride, because the molecules of hydrogen, chlorine and hydrogen chloride are all diatomic, or contain two atoms each. One liter of oxygen and 2 liters of hydrogen combine to form 2 liters of water vapor, because while the molecules of hydrogen and oxygen contain two atoms each, the molecule of water contains three atoms (=HOH), or is triatomic. Three liters of hydrogen with 1 liter of nitrogen must produce only 2 liters of ammonia, H3N, because while the hydrogen and nitrogen molecules are diatomic the molecule of ammonia contains four atoms, or is tetratomic. 143. Specific Heat. The relative quantity of thermal energy (heat) required to raise the temperature of a given mass of any substance one degree is called the specific heat of that substance. The specific heat of water is the unit in which the specific PROPORTIONS AND THE ATOMIC HYPOTHESIS 61 heat of any other substance is expressed. Therefore the specific heat of water is 1, and it signifies the quantity of heat energy required to raise the temperature of one weight unit of water one degree. The specific heat of mercury is 0.0319, because only yMff o as much heat energy is required to raise the temperature of mercury one degree as is necessary to raise the temperature of an equal quantity of water one degree. 144. The Law of Dulong and Petit. — All atoms have the same capacity for heat. This means that it requires exactly the same amount of heat energy to raise the temperature of any atom of any kind one degree. The specific heat of any element is inversely as its atomic Weight. The product obtained by multiplying the atomic weight of any element by its specific heat is a constant number ; it is approximately 6.4, and that number is called the atomic heat. Hence when 6.4 is divided by the specific heat of any element the quotient must be approximately the atomic weight of that element. The atomic weight of any element can, therefore, be approximately deduced from or verified by its specific heat. [6.4 -*- 0.0319=200.] 145. Neumann and Regnault proved that the specific heats of compounds are inversely proportional to their molecular weights (just as we have seen that specific heats of elements are inversely as their atomic weights). The sum of the atomic heats of the atoms of any molecule is the molecular heat of that molecule. Hence, when the molecular heat of any substance is divided by 6.4 the quotient is the number of atoms contained in the molecule, whether elemental or compound. Molecular weights can, therefore, be deduced from or verified by the specific heats of substances. 62 A CORRESPONDENCE COURSE IN PHARMACY 146. There are, furthermore, other methods by which molecular weights can be verified. These methods would be wholly out of place in elementary lessons like these, but their existence is referred to simply to indicate that the atomic theory is amply confirmed by many facts in chemical physics which have been discovered and demonstrated independently of one another and of the atomic hypothesis itself. * Test Questions In general, it is not expected that you will refer to your text in the preparation of your answers to Test Questions. We have indi- cated some things that should be committed to memory. Other things should be thoroughly understood. At the same time there may be occasions when the use of your text is almost necessary in the solution of problems. You may not remember, for instance, the numbers which are necessary. Under such circumstances you are at liberty to refer to the text of your lesson, but should never do so for principles and laws. If you know and understand a law you can apply it. The purpose of these test questions is to see whether you know and do understand. If you deceive us, even unintentionally, you suffer the consequences, for, unless you are per- fectly fair with us, we cannot give you the assistance we should like to render. It is what you know, not what you can take from a book, that we wish to determine. If your papers are not fairly prepared, you lose the best part of that for which you paid when you enrolled in the School. 1. If a given chemical compound consists of carbon