Complete Text (Part 14)

sulphates, are water and S02. Moderately dilute nitric acid, especially when warm, dissolves zinc, iron, nickel, lead, copper, mercury, silver, arsenic and bismuth. Arsenic is oxidized to arsenic acid and the other metals form nitrates. So much of the nitric acid used as does not enter into the formation of the nitrate yields water and the gas NO, which . oxidizes in the air to red vapors of N204 or N02, or both, according to the temperature. It is commonly said that "red nitrous vapors" are formed when metals are dissolved in nitric acid. Gold and very dilute nitric acid dissolves iron and zinc, forming ferrous nitrate or zinc nitrate, together with ammonium nitrate and water. This fact is particularly interesting and instructive, for it will be seen that the ammonium nitrate can only be formed by changing the algebraic combining number of a part of the nitrogen from +5, which is the combining value it possesses as the acidic element of nitric acid, to —3, which is the combining value it has in any ammonium compound. This may most clearly be shown by the following equation: 4Zn+10HONO2=4Zn (N"08)2+H4NON02+3H20. The four zinc atoms before the metal is dissolved in the nitric acid have an algebraic com- bining number of 0. But in the four molecules of zinc nitrate which the metal forms, the zinc atoms have a total algebraic combining number of +8, for zinc in combination has a valence of 2. The four zinc atoms, therefore, gained eight units of combining value. The first nitrogen atom in the molecule H4NON02, or the nitrogen of the ammonium, 140 A CORRESPONDENCE COURSE IN PHARMACY has five bonds, four of which hold the four hydrogen atoms, while the fifth connects the H4N to the oxygen atom stand- ing between the two nitrogen atoms of the H4NON02, and as the four bonds holding the hydrogen are negative bonds, while the fifth bond must be a positive bond, and as —4 added to +1 makes -3, the nitrogen atom of the H4N has a value of -3. Inasmuch as that nitrogen atom was furnished by the original HON02, in which all the nitrogen has a value of +5, it follows that the nitrogen reduced lost eight units, for the difference between +5 and —3 is, of course, eight units. These eight units lost by the nitrogen atom are the eight units gained by the four zinc atoms. Concentrated nitric acid is not attacked by iron, but dis- solves lead, copper, mercury, silver, arsenic and bismuth. It is not affected by gold and platinum. It oxidizes tin to insoluble so-called meta-stannic acid, and antimony to insol- uble antimonious oxide. 247. The foregoing statements must not be construed to mean that metals which are not dissolved by the acids named may not be superficially affected to a considerable degree. Diluted sulphuric acid does take up copper and form copper sulphate, so that copper vessels are corroded by diluted sulphuric acid. But the diluted acid dissolves the metal so slowly and to such a limited extent that we would not think of using diluted sulphuric acid for the purpose of dissolving copper. Tin is not affected by sulphuric acid, but tinned iron or "tin plate," however heavily coated with pure tin, is com- paratively soon destroyed by not only very dilute sulphuric acid, but even by boric acid solutions and by very weak acetic acid, probably because the tin coating is not so impervious that the iron is absolutely protected. 248. Acids are readily attacked by metals if the salts formed by the reaction are soluble in the liquid. But metals CHEMICAL REACTIONS 141 cannot be dissolved in the acids if the salts formed are insoluble in the liquid. Thus, strong nitric acid does not attack iron, because the iron nitrate is not soluble in strong nitric acid, but diluted nitric acid does attack iron, because the iron nitrate is soluble in diluted nitric acid and in water. Again, lead is not soluble in moderately diluted sulphuric acid, because lead sulphate is insoluble both in diluted sul- phuric acid and in water. But the lead is acted upon by concentrated sulphuric acid, because lead sulphate is soluble in that acid when of sufficient strength. 249. Chemical reactions occurring in processes of manu- facture of chemical compounds are very generally of such character that the products formed are easily separable from each other. Were not this the case, they would be practically useless. A double decomposition resulting in the formation of one product soluble in the liquid in which the reaction takes place and another insoluble in that liquid is useful or practicable, because the insoluble substance is easily sepa- rated from the soluble. A reaction resulting in the formation of one product which is volatile and another which is not volatile is also workable, because the volatile product can be easily dissipated and separated from the non-volatile. Eeactions in which water is the only by-product are also useful, because water is volatile and can be eliminated, or, if the principal product is obtained dissolved in the water, it can be recovered from the solution by evaporation of the water and crystallization of the solid. Metallic salts are successfully made by the solution of the metal in the appropriate acid, because the by-product is either hydrogen or some other gas. Metallic salts are easily made from metallic oxides by dissolving these in acids, because the by-product is water. Salts can readily be made 142 A CORRESPONDENCE COURSE IN PHARMACY by saturating acids with carbonates of the metals, because the by-products are water and the gas C02. 250. Neutralization is effected in solutions by mixing acids and alkalies, or acids and alkali carbonates, or acids and bases, etc., in the requisite proportions, adding either the acid to the metallic compound or the metallic compound to the acid. Acid salts are also neutralized by alkalies and alkali carbonates. Whenever practicable, the point of exact neutralization of an acid by a base or of a base by an acid is determined by a color reagent. The most common and useful reagent of this kind and for this purpose is litmus, which is generally employed in the form of litmus paper, which is unsized paper dipped in a solution of litmus and then dried. Litmus is a blue pigment which is very readily turned red by acids and blue by alkalies. Bine litmus paper is made from the unaltered solution of the pigment, while red litmus paper is made from a litmus solution to which just enough pure hydrochloric acid has been added barely to turn its color red. Litmus paper can also be made in such a way that it is neither red nor blue, by carefully adding just enough of the hydrochloric acid to the litmus solution used. A liquid which turns blue litmus paper red is said to have an acid reaction on test paper; one that turns red litmus paper blue is said to have an alkaline reaction upon test paper. A liquid which does not change the color of either red or blue litmus paper is said to have a neutral reaction on test paper, or to be neutral to test paper. The test is made by touching a small strip of the test paper with the liquid. 251. Salts of normal structure formed by strong acids with weak bases have an acid reaction on test paper, but those formed by weak acids with strong bases have an alkaline reaction on test paper. Only salts formed by strong CHEMICAL REACTIONS 143 acids with strong bases or weak acids with weak bases have a neutral reaction on test paper. Salts still containing some of the replaceable hydrogen of the acid are said to be acid salts, or to have an acid structure. Bicarbonate of potassium is such a salt, but bicarbonate of potassium, although of acid structure, has an alkaline reaction on test paper, because potassium is one of the most powerful basic elements, while the carbonate radical is a very feeble acid-radical. A salt containing a larger proportion of the basic element than that contained in a salt of normal structure is called a basic salt. Subsulphate of iron is such a salt, though a solution of subsulphate of iron has an acid reaction on test paper, because iron is not sufficiently strongly basic to form salts of neutral reaction with such a powerful acid as sulphuric acid. A solution of alum has an acid reaction, because aluminum is very feebly basic, although alum aiso contains potassium. 252. When acids are saturated with the metal or with metallic oxides, hydroxides or carbonates, the proportions of these materials employed are determined beforehand according to the atomic and molecular weights, even if an excess of the metal or metallic compound is to be used. When salts of normal composition are to be prepared and the reaction on test paper does not indicate the composition, the exact theoretical proportions are used. When iron or zinc is dissolved in sulphuric acid, the metal is added in excess, because the acid cannot possibly dissolve any more of the metal than the quantity required to form the sulphate. But when mercury is dissolved in nitric acid, it is necessary that the proportions of mercury and nitric acid be carefully attended to, because if the nitric acid is in large excess, mercuric nitrate is formed, while with a less propor- tion of nitric acid, mercurous nitrate of normal structure is 144 A CORRESPONDENCE COURSE IN PHARMACY formed, and with a still smaller amount of nitric acid, a basic mercurous nitrate is obtained. 253. The proportions to be employed of the factors of a chemical reaction are, of course, indicated by the molecular weights and atomic weights. In constructing a working formula, it is therefore necessary first to write down the chemical equation that represents the reaction taking place in the process, and when this equation is properly balanced, the atomic or molecular weights will show the quantities required of the factors, and also the quantities obtained of the products. But the proportions found in this way are only the theoretical proportions, and they may not be work- able, because it is generally the case that the reaction is not complete under those conditions. If it is necessary that one of the factors in a chemical reaction be completely decomposed or consumed, then the other factor or factors must be used in greater proportion than that required by theory. If a double decomposition between A and B is to be effected, A must be used in excess over the theoretical proportion, if it is necessary that B shall be completely decomposed; if A must be completely decomposed, then B must be used in excess. In other words, the course and relative completeness of chemical reactions may be materially affected by the relative masses of the factors. When a solution of sodium sulphate and a solution of barium acetate are mixed in the proportions required for even or complete metathesis, barium sulphate and sodium acetate will be formed, and the reaction proceeds to completion, because the barium sulphate is insoluble. But if barium sulphate is placed in water containing a large amount of sodium carbonate in solution, the sodium carbonate will gradually decompose . the barium sulphate so that sodium CHEMICAL REACTIONS 145 sulphate and barium carbonate are formed, and the insoluble solid matter in the liquid will become a mixture of barium carbonate and barium sulphate. If the sodium salts, which are soluble, are removed from time to time and fresh portions of sodium carbonate added, the entire amount of barium sulphate can be finally converted into bariuin carbonate. Mass reactions of this kind are numerous. 254. The most common and numerous chemical reactions are double decompositions, and, as already shown, double decompositions are most readily effected between reagents in a state of solution, under Berthollet's law with regard to the formation of insoluble or less soluble products. In other words, they are precipitations. Other common reactions between acids and bases are also double decompositions. The student should therefore learn to write chemical reactions representing double decompositions. He should bear in mind that any double decomposition between two substances is simply a mutual interchange of radicals. It is like an exchange of partners in a quadrille ; two couples meet and exchange partners. Each factor in a chemical reaction such as is called double decomposition is a couple, consisting of the positive radical and the negative radical. The positive radical of one factor gives up its negative radical to the positive radical of the other factor and takes the negative radical from that factor in exchange. For example, sil- ver nitrate meets sodium chloride. The silver and the sodium are the positive radicals, the nitrate radical (N03) and the chlorine are the negative radicals. The silver gives up its N03 to the sodium, taking the chlorine in exchange. This double interchange may also be likened to an exchange of horses between two riders. A red man on a white horse and a white man on a red horse meet, and the two men 14G A CORRESPONDENCE COURSE IN PHARMACY exchange horses. Both men are still there and so are the horses, hut they have changed positions. That is precisely what takes place between the several radicals concerned in a double composition. When mercuric chloride and potassium iodide, both in solution in water, are mixed with each other, the mercury leaves the chlorine and takes up the iodine instead, while the potassium, giving up the iodine to the mercury, takes up the chlorine in exchange. We started with a combina- tion of mercury and chlorine and a combination of potassium and iodine; we finish with a combination of mercury and iodine and a combination of potassium and chlorine. The equation representing the reaction is as follows: HgCl2+2KI=HgI2+2KCl. The reason why two molecules of KI are necessary is that the mercury atom has a valence of 2, whereas the atoms of chlorine, potassium and iodine each have a valence of only 1, and as we must have the same number of positive bonds as of negative bonds in any molecule, it follows that a molecule of mercuric chloride must contain two chlorine atoms to the one mercury atom, and a molecule of mercuric iodide must contain two iodine atoms to the one mercury atom. The two chlorine atoms contained in HgCl2 require two potassium atoms to form potassium chloride, and two molecules of KI are required to furnish those two potassium atoms for the potassium chloride, as well as to furnish the two iodine atoms for the mercuric iodide. The easiest rule to follow is this: Find the valence of the positive radicals of the two factors^ and then tahe the number of atoms or molecules of each that . will give you a common multiple of the numbers expressing those valences. For example, if the valence of one of the positive radicals concerned is 1 and the valence of the other positive radical is 2, then multiply the factor containing the radical having CHEMICAL EEACTIONS 147 a valence of 1 by 2, and multiply by 1 that factor the positive radical of which has a valence of 2. If the valence of the positive radical of one factor is 2 and that of the other is 3, then multiply the 2 by 3 and the 3 by 2. In the reaction represented by the equation ]N"a2C03+CaCl2= CaC03+2NaCl, the student will see that the positive radicals are the Na and the Ca. The valence of ISTa is 1 and the valence of Ca is 2, but as the Na is multiplied by 2 already and since the formula of the sodium carbonate is Na2C03, one molecule of Na2C03 is sufficient, for the two sodium atoms together form two bonds, and the single calcium atom having a valence of 2 has also two bonds. The fact which the student should keep clearly in view is that the total number of bonds of the positive radical of one of the factors must be the same as the total number of bonds of the positive radical of the other factor. In order that the exchange may be even, it takes two five-dollar bills to match five two-dollar bills. Test Questions The number of questions in this and some of the following lessons may seem too great. Theoretically, it would be better if the lessons were more uniform in length, but as the real purpose of the Course is to insure a thorough understanding on the part of the student, the number of questions bears a close relation to the importance of the subject. It is not necessary that long answers should be written nor that all answers should be expressed in complete sentences. Occasionally "yes" or "no" may serve as an answer. Still, every answer must be clear and precise. If numerical computations are necessary, it is always desirable that the entire work of the problem should be submitted. If merely the answer is given and it is wrong, the instructor has no clue as to what is really the reason for the student's mistakes and so no assist- ance can be rendered. 148 A CORRESPONDENCE COURSE IN PHARMACY 1. What is meant by the factors of chemical reactions ? 2. How many factors are necessary in a chemical reaction ? 3. How many products are formed by any chemical reaction ? 4. What is meant by dissociation ? 5. How many factors are concerned in metathesis ? 6. How many factors are necessary in a synthesis? 7. What general expression is used to signify an alteration in the atomic linking of any molecule or molecules ? 8. What is the difference between an elemental factor and a compound factor ? 9. What is the algebraic combining value of an elemental factor in a chemical reaction ? 10. When two