1

Hence the specific volume-i.e. solution is 971.

of the potassium nitrate

sp. grav.

But if no volume-change had occurred, the specific volume of this solution would have been 965. Hence when equivalent masses of potash and nitric acid interacted in dilute aqueous solution an expansion occurred which can be represented by 006. Similarly when equivalent masses of sulphuric acid and potash interacted in dilute aqueous solution the expansion which occurred can be represented by 003; and when equivalent masses of the two acids and potash interacted the expansion which occurred can be represented by '005.

The observed value of the ratio : 1 x shews that the affinity of nitric acid towards potash is double that of sulphuric acid. It is here assumed that the only change which occurs when the two acids and the base are mixed is formation of the two normal salts; as a matter of fact the change is complicated by an interaction between the sulphuric acid and the normal potassium sulphate formed. (comp. Thermal methods of measuring affinities, in this Chapter.)

CHAPTER IV.

METHODS OF DETERMINING THE CONSTITUTIONS OF

COMPOUNDS.

ASSUMING the molecular formulae of a series of compounds to have been determined, the reactions of formation and decomposition of the compounds are studied, and the relations between the compounds established by these reactions are expressed in formulae which rest on the atomic and molecular theory and more especially on the hypothesis of valency.

Determinations of certain physical or physico-chemical constants for series of compounds also sometimes enable conclusions to be drawn concerning the constitutions of these compounds; for instance, determinations of the specific volumes of gasifiable carbon compounds, or of the rates of etherification of alcohols and acids, have thrown light on the constitutions of many compounds.

Exp. 1. Chemical method of investigating the constitutions of compounds. (s. Pattison Muir's Principles of Chemistry, Book I.; especially Chap. II. sects. 3 and 4.) The molecular formulae of the compounds studied in the following Exps. are assumed to be known. Details of the preparation of the compounds will be found in manuals of organic chemistry; the student should consult one of these manuals, and draw up a plan for the preparation of each compound: only the outlines of the scheme of experimental study are given here.

(1) Prepare absolute alcohol, CHO. (2) Dissolve sodium in a portion of the alcohol; prove that hydrogen is evolved; evaporate the liquid in vacuo; prove that the crystals obtained have the composition CH ONa. 3C,HO, by heating a weighed quantity to 180° and noting the loss of weight (the loss is alcohol), and estimating sodium in the residue. (3) Compare the reactions between (a) PCI, and H2O, and (b) PCI, and CHO.

5

The results of Exps. (1), (2) and (3) suggest the formula CH.OH for alcohol.

(4) From another portion of the alcohol prepare aldehyde, CHO. (5) From another portion of the alcohol prepare acetic acid CHO,. Prove acetic acid to be monobasic. (6) From a portion of the acetic acid prepare acetyl chloride, CH2OCI.

3

The results of Exps. (4), (5), and (6) suggest the formula CHO.OH for acetic acid.

3

49

(7) Mix dry sodium acetate with dry caustic soda, heat, and prove that methane, CH,, is evolved, and sodium carbonate remains. (8) From another portion of the acetic acid made in (5) prepare, i. monochloracetic acid C,H,CIO.OH; ii. trichloracetic acid C.C1.0.OH; and prove each acid to be monobasic. The results of Exps. (7) and (8) suggest the formula CH.COOH for acetic acid; and taken along with the former results, they suggest the formulae CH,.CH,OH and CH ̧.CHO for alcohol and aldehyde, respectively.

3

(9) From aldehyde prepare ethylidene chloride, C‚Í‚Cl ̧. (10) From ethylene prepare ethylene chloride, CH,CL (11) Prove that the ethylidene chloride prepared from aldehyde is different from the ethylene chloride prepared from ethylene, by comparing the boiling points and specific gravities of the two compounds.

Assuming the structural formula of the molecule of ethylene to be H,C-CH,, and the addition of chlorine to result in the formation of the molecule CIH,C-CH,Cl, the results of Exps. (9), (10), and (11) confirm the formula CH,.CHO for aldehyde, and hence the formula CH.CH,OH for alcohol.

(12) From aldehyde prepare alcohol by the action of nascent hydrogen.

Exp. (12) confirms the formula CH ̧.CH,OH for alcohol. Draw up an account of the experiments performed; and state clearly the reasoning employed, the assumptions made, and the conclusions arrived at regarding the constitution of the molecules of alcohol, aldehyde, and acetic acid.

Exp. 2. Rates of etherification of alcohols by acetic acid. When an alcohol is heated with acetic acid in molecular proportions a certain amount of the alcohol is changed to an ethereal acetate. The amount of change which occurs in one hour has been called the initial velocity of etherification, and the amount which occurs when the whole system has attained equilibrium has been called the limit of etherification. The values of these constants for series of alcohol, are connected

with the constitutions of the alcohols. (s. Pattison Muir's Principles of Chemistry, Book I. Chap. IV. sect. 4.)

Rates of etherification of ethylic, propylic, and normal primary butylic, alcohol. Read, Menschutkin, Recherches sur l'influence exercée par l'isomérie des alcools et des acides sur la formation des Ethers composés. Annales de Chim. et de Phys. (5). 20, 289.

About 5 grams of each alcohol; 10 or 15 grams of pure acetic acid; and a dilute baryta solution standardised against acetic acid, are required.

а

Mixtures of the alcohols with acetic acid in molecular proportions are heated in very small sealed tubes of about 1 c.c. capacity. About 4 or 5 grams of each alcohol is placed in a small stoppered bottle of about 20 c.c. capacity, which has been previously weighed; the bottle and its contents are weighed, and the mass of alcohol is thus accurately determined. The necessary quantity of acetic acid is calculated, and is then added from a small burette with a fine opening, graduated to c.c.; the weight of a drop of pure acetic acid delivered by this pipette is determined, and the exact quantity of acid can then be delivered by counting the drops after the greater part of the acid has been run into the alcohol.

C

The tubes to contain the mixture are of the shape shewn in Fig. 57. A piece of caoutchouc tubing, connected with an india-rubber ball and furnished with a screw-clamp, is attached to a; the ball is squeezed, and the other end of the tube is dipped into the liquid, which is then sucked into the tube by slowly releasing the ball. When the little bulb is about filled, the screw-clamp on the caoutchouc tubing is closed, and the tube is sealed off at b; by gently tapping the little bulb the liquid is caused to rise above c, and the tube is then sealed off at c. The small bottle containing the mixture of alcohol and acid is again weighed, and the weight of the mixture in the little bulb is thus determined.

Fig. 57.

Four tubes are to be filled with each mixture of alcohol and acid; making twelve tubes in all. The tubes are then to be suspended in a bath of glycerin, placed on a sand-tray, and

heated by means of a Bunsen-lamp furnished with a gas regulator and surrounded by a screen to stop draughts of air. The temperature of the glycerin is raised to 154°-156° and the tubes are then slowly immersed.

Six tubes, being two of each series, are removed at the end of an hour, and the remaining six are removed after 48 hours. When each tube is cold it is placed in a stoppered bottle containing 30-40 c.c. pure alcohol and 4 or 5 drops of a very dilute alcoholic solution of rosolic acid; the tube is broken by agitating the bottle, and the acetic acid which has not been decomposed is determined by the standardised baryta solution.

As the amount of acetic acid originally present is known, it is easy to calculate the percentage of acid, and hence the percentage of alcohol, which has been changed to an ethereal salt. The mean of each pair of experiments after etherification has proceeded for one hour is taken as the initial velocity of etherification, and the mean of each pair after 48 hours as the limit of etherification.

The experiments of Menschutkin proved that the systems consisting of acetic acid and ethylic, propylic, or butylic, alcohol attain their final equilibrium after about 48 hours at 155°. Menschutkin obtained the following results :—

The specific volume of a gasifiable compound is usually molecular weight

defined as

spec. grav. of liquid at B. P.

The specific volume of a solid compound is usually defined as reacting weight

spec. grav. of solid

Determinations of these constants often throw light on questions regarding the constitutions of compounds. (s. Pattison Muir's Principles of Chemistry, Book I. Chap. IV. sect. 3.)

Specific volumes of liquids. Read, Ramsay on the Volumes of liquids at their boiling-points obtained from unit