Practical Proofs of Chemical Laws: A Course of Experiments Upon the ...

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Preface

These experimental proofs (or more properly verifications) of quantitative laws were undertaken ■O by pupils after the qualitative composition of the

■— principal substances employed had been carefully

L dealt with in the accompanying lecture course,

^ Practical Physics went on side by side with the

practical chemistry course.

The pupils whose results are quoted in the text were mostly between twelve and eighteen years of age, A book of results was kept so that each pupil could compare his results with others obtained under similar conditions.

The pupils had i^ hour at a time in the laboratory, and attended twice a week,

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Google vi Preface

I have not been satisfied with quantitative experiments unless they yield good results in the hands, not only of the teacher but of the pupils. The results quoted in the text are those obtained by the pupils.

I am not aware that a satisfactory standard as to the accuracy required for such experiments has yet been laid down. Within i per cent, is certainly sufficient, but the standard may vary to some extent according to the nature of the law or problem investigated.

Perhaps the standard is best determined by historical considerations, for the history- of a science is recapitulated by the learner. If the pupil can verify a law to such a degree of approximation as first served to convince the scientific world of its truth, he may generally be satisfied with his work. I have quoted in the text the results of early historic experiments side by side with those obtained by pupils. Although in these early experiments the error is often large, yet there is less difference than the learner may have supposed

Preface vii

between the accuracy of the first approximations which have obtained the provisional assent of the scientific world at the beginning and towards the end of the nineteenth century, respectively.

As far as possible no numerical data, whether chemical or physical, were assumed. The density of hydrogen and the proportion by weight in which hydrogen and oxygen combine are neither assumed nor determined in these experimental verifications of the Laws of Combining Proportions. It is not necessary that equivalent weights should be referred to that of hydrogen, and the experiments cannot be done with the same accuracy as is attainable in the case of other elements.

The use of atomic and molecular formulas is inadmissible in an examination of the facts upon which the atomic and molecular theory is based. Chemical equations and formula: have therefore been excluded.

The course is, I believe, suitable for first-year's students at colleges as well as for the upper forms of schools.

viii Preface

I have to acknowledge valuable suggestions received from other chemists while this course was in preparation. My thanks are due more particularly to Dr. J. B. Cohen, of the Yorkshire College, Leeds, and to Mr. G. Stallard, of Rugby.

VAUGHAN CORNISH.

Chapter II The Law Of Conservation Of Mass

Exercise I.— The Use of (he Balance .... Exercise II.— The Complete Synthesis of Silver Sulphide . Calculation and Statement of Results . . . . ■ Note upoti eaity Experiments relating to this I^w, and upon Stas' Determinalions

CHAPTER III Deductions, bearing upon this Law, from Exercise 11. . 17

Exercise III.— The Determination of the Proportion by Weight in which Silver and Chlorine combine when

Chlorine Gas acts upon Silver 19

Exercise IV.— The Determination of the Proportion by Weight in which Silver and Chlorine combine when a solution of Hydrochloric Acid acts upon a solulioQ of Silver Nitrate 22 l;,L.OO^^IC

X Contents

Calculation and Statement of Results 24

Note upon early Experiments relating to this Law, and upon

Stas' Determinations 25

Exercise IV. (a).— The Determination of the Quantities of Hydrc^en evolved on the Solution of Zinc in Sulphuric Acid and in Hydrochloric Acid respeclivelf . . . 2S

Chapter IV The Law Of Equivalent Proportions

Scope of the Experiments in this Chapter .... 31

Exercise v.— The Determination of the combining Proportions of Silver and Bromine by the action of Hydrobromic Add upon Silver Nitrate 3j

Exercise VI.— The Determination of the Percentage of Silver in Silver Nitrate by the reduction of Silver Nitrate in Hydri^en . 34

Exercise VII.— The Preparation of a Solution of Silver Nitrate containing a known We^ht of Sliver per Cubic Centimetre 35

Exercise VIII. — The Determination of the Proportion in which Potassium and Chlorine are combined in Potassium Chloride 3S

Exercise IX.— The Determination of the Proportion in which Potassium and Bromine are combined in Potassium Bromide. . . 41

Calculation and Statement of Results obtained in Verification

of the Law of Eijuivalent Proportions .... 41

Note upon early Experiments relating to this Law and upon

Stas' Work 43

Chapter V

Exercise X.— The Determination of the constancy of the Proportion between the Weights of Potassium and Chlorine in Potassium Chloride and in Potassium Chlorate .

Note upon Slas' Determinations relating to the 5iecond Part of the Law of Defitiile and of Constant Proportions . .

Chapter VI The Law Of Multiple Proportions

Verificacion of the Law by E^cperiments upon the two

Chlorides of Copper . , . . . -SI

Exercise XI.~The Preparation of the two Chlorides of

Copper in a slate of Purity 53

ExbrciseXII.— TheDetenoinitionof the Copper in the two

Chlorides 55

Exercise XIII.— The Delerminalion of the Chlorine in the

two Chlorides 56

Calculation and Statement of Results obtained in Verification

of the Law of Multiple Proportions . . . . 6i A Second Method, alternative to thai contained in the last

three Exercises, for Verifying the Law of Multiple

Proportions 64

Exercise XIV. —The DetemiinaiLon of the Weights of Bro-

mine contained in known Weights of Mercuious Bromide

and of Mercuric Bromide ...... 65

Note upon eariy Experiments relating to the Law, and upon

Stas' Criticism of these Experiments . . . . 68

Chapter VII

THE LAW OF SIMPLE VOLUMETRIC PROPORTIONS IN THE CHEMICAL REACTION OF GASES

Upon the Molecules of Gases 70

Exercise XV.— The Determination 0/ the Volume of Nitrt^en

obtained by the Decomposition of a known Volume of

Ammonia . ..7a

Calculation and Statement of Results 77

Note upon early Experiments relating to the Law of Simple

Volumetric Proportions 78

Upon the Relation of the (Physical) Molecule of Nitrogen Gas

to the (Chemical) Atom of the Element Nilr<^en . . 79 Upon the relative Weights of the Chemical Atoms . . . 81 Upon the Choice of a Unit to which the Weights of the

Atoms are referred 81

Upon Prout's Hypothesis 84

Contents

Chapter VIII

EXERCISES SUPPLEMENTAKV TO THE COURSE ILLUS- TRATING THE SCOPE OF THE TERM EQUIVALENCE IN CHEMISTRY

Upon Diagrams illustrating Equivalence

SupPLEMKNTARV Exercise I.— The Determination of the Volume of Hydrogen evolved during the Solution of a known Weight of Zinc in dilute Sulphuric Acid .

V Exercise II.— The Determination of (he

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PRACTICAL

PROOFS OF CHEMICAL LAWS

Chapter I

STATEMENT OF DALTON'S ATOMIC THEORY AND OF THE LAWS OF COMBINING PROPORTIONS

Dalton's atomic theory of chemical action may be stated as follows :—

When a chemical action takes place, what we observe on the lai^e scale is the total effect of a vast number of similar actions occurring between ultimate particles, or atoms, of the substances. The atom of each chemical element has its own specific mass. These chemical atoms are beyond our powers of vision, and we have no means of dealing with them individually, Dalton's theory remains therefore a theory only, and has not been raised to the rank of a statement of observed facts. The theory is, however, based upon observed facts ascertained by experiment.

B L, . h.L.LXV^lC

The laws of chemical combination by weight are the experimental basis of Dalton's theory.

The later discovery, by Gay-Lussac, of the simple ratios between the reacting volumes of gases (the ' Laws of Combination by Volume ') led to the development of the atomic theory in its present form, in which we suppose the existence of two orders of particles, the molecule and the chemical atotn. The modern development of theoretical chemistry is due in great measure to the theory of atoms and molecules, the theory itself being based, as has been said, upon the laws of combining proportions, gravimetric and volumetric. These are both included in the term Laws of combining proportions.

The methods of chemical analysis are founded directly upon these experimental laws and upon the law of conservation of mass, the results obtained by analysis being independent of the atomic theory.

These laws are therefore the foundation of the' greater part both of practical and of theoretical chemistry.

We proceed to enunciate the laws before giving' the description of experiments by which each of them may be verified.

I. The Law of Conservation of Mass. — The law, in its bearing upon chemistry, may be stated in a general form as follows : —

' The total mass of the substances taking part in any chemical process remains constant' {Oslwalct).

We will for the purpose of this course state the law in a less general form, in which it can be readily verified, as follows : —

When elements combine together cfumically, the. mass of the compound formed is equal to the sum of the masses of the elements before combination.— mass is almost always measured by tveighing, we shall substitute the more familiar term weight in the following statements of laws.

2. The Law OF Definite and of Constant Proportions. Part I. {definite proportions).— When two elements combine to form a particular compound substance, they do so in a definite, fixed proportion by weight, which is independent of the manner in which their combinatioms brought about ; and Part II. (constant proportions'). This proportion remains constant in compounds which contain also other elements.

3. The Law of Equivalent Proportions. The weights of two elements which are equivalent (i.e. of equal value) in any chemical reaction are equivalent in all.

The meaning of this general statement of the law may be illustrated by special statements applicable to particular cases which require dififerent experimental methods for their verification. >

First special statement (applicable to the case of elements each of which is capable of combining with each of the others, e.g. silver, chlorine, and sulphur) : .

The weights of two elements (eg, sulphur and

I;. L.oo^lc chlorine) which combine with a certain fixed weight of a third element (e.g. silver) are in the proportion in which those two elements (sulphur and chlorine) combine with one another. — These weights of sulphur and chlorine are said to be equivalent to one another.

Second special statement (applicable to the case of elements, some of which do not combine together, e.g. silver, potassium, chlorine and bromine) :

The weights of tivo elements (e.g. silver and potassium) which combine with a given weight of a third element {e.g. chlorine) will also combine with another fixed weight of a fourth element (eg. bromine), — The experimental verification of this statement of the law is given in Chapter IV,

4. The Law of Multiple Proportions, — It sometimes happens that there are two (or more) different substances formed from the same elements. By different substances we mean materials which .differ in a marked degree in their physical characters (e.g. in density, boiling point, melting point, and so forth). To such cases the law of multiple proportions applies. To simplify the wording of the law, we will frame a statement suited to the case of substances containing only two elements.

Statement of the Law of Multiple Proportions. — If there be more than one substance formed by the combination of two elements, then, taking the weight of one element as fixed in each substance, the weight of the other element in the second compound bears a simple proportion to the weight of tltat element in the first. — When the student has fully grasped the meaning of the word equivalence in chemistry, which is generally not until he has had some experience of experimental work, he will find the following statement useful as a summary of all the laws of chemical combination by weight ;

Elements combine together in the proportion {or ratio) of their equivalent weights, or in the proportion of whole multiples of their equivalent weights.

5. The Volumetric Law of Combination OF Gases may be stated thus: — The volume of an element in the gaseous state bears a simple proportion to tlu volume of the compound gas of which it is a constituent.

CHAPTER 11

THE LAW OF CONSERVATION OF MASS

Statement. — When elements combine together chemically the mass of the compounds formed is equal to the sum of the masses of the elements before combination.

Mass is the one property of matter which remains absolutely constant in every state of chemical combination. The Balance is the instrument employed in the comparison of masses, and a knowledge of the systematic method of weighing must be acquired before undertaking quantitative chemical experiments.

Exercise I. — The Use of the Balance. Apparatus required. — A chemical balance, the pointer of which will move one division on the scale when i mgrm. is placed on the empty pan, and which will bear a load of 50 grams on each pan. Becker's 5ar. balance fulfils these conditions. A set of weights from 50 grams to i mgrm. It is well that each pupil should have a set of fractions of a centigram. These can be obtained, in platinum, for about is.

Determination, of ilie Zero point. — Raise the beam of the balance from its support by turning the knob which is on the front of the balance case. The beam now swings freely. The balance is adjusted by the instructor, not by the pupil, so that the pointer swings tteat'fy evenly on either side of the central line on the ivory scale. It is required to determine exactly the point on the scale which . marks the position of equilibrium, or the Zero point of the balance. This is best done, not by .waiting for the pointer to come to rest; but by observing the swings on either side of the central point. The amplitude of the swings steadily diminishes owing to friction. Thus taking 10 as the value of each division, calling the central point * 100, and reckoning through from left to right, we may observe the following extreme positions of the pointer ;

Left hand Righl hand the parts of a division being estimated by eye. Now 85 is further from the Zero point and 87 is nearer to the Zero point than no. We may

assumethat -~= 86 is as far on the left of the Zero point as 1 10 is on the right. The Zero point is found by taking the mean of the two numbers 86 and 1 10 ; thus -.

= 98 (Zero point)

DeUrmination of Sensibility. — In weighing a body it will generally be found after adjusting the weights as nearly as possible that the pointer swings nearly, but not quite, evenly about the Zero point. The weights being placed on the right-hand pan, and the body to be weighed on the left-hand pan, then if the Zero-point is 98 and the pointer is found to swing evenly about the position 90, it is evident that the weights are somewhat too heavy. We require to know what extra weight in one pan produces a deflexion of 8, i.e. yjthfi of a division. To determine the sensibility with empty pans, place one milligram on the right-hand pan and observe three or five swings. Suppose these to be

Rigbl hand Left hanil the balancing- point is evidently

The Zero point being 98, the sensibility with a very small load is 10. Thus, in the example given, if the weight of the body was small (say 5 grams) we conclude that the weights on right-hand pan were too heavy by Wths of a milligram. The true weight of the body is therefore obtained by deducting ■0008 gram from the amount of the weights on the right-hand pan.

To determine the sensibility tvitk a load of 20 grams in each pan. — The sensibility generally decreases with heavier loads. In the experiments given in this course the loads are generally either small (less than 5 grams) or about 20 to 25 grams in each pan — e.g. in weighing porcelain crucibles. To determine the sensibility in this second case, place a 20-gram weight in each pan and determine the balancing- point. This may not be exactly the Zero point, since the two weights may not be exactly equal. Suppose the balancing-point to be 96. Place 001 gram (i milligram) on the right-hand pan and again determine the balancing- point. Suppose this to be 88. The sensibility with a load of 20 grams in each pan is 8.

Record of the results of the exercise. — The observations of swings and the results for sensibility are to be clearly stated in the pupil's note-book. They should be taken down in the first place in a small pocket note-book, in which all weighings, as well as observations made during the actual progress of experiments, should be entered at the time, to be copied out afterwards in a laiger note-book. Weighings should on no account be entered on loose sheets of paper ; the results of experiments are often lost in this manner.

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Exercise II. — Verification of the Law of Conservation of Mass by the complete synthesis of silver sulphide.

This experiment shows that when silver and sulphur are heated together, they combine chemically, producing a new substance (silver sulphide), and that tlie weight of the silver sulphide formed is equal to the sum of the weights of the silver and sulphur from which it was produced.

Apparatus and substances required — Fine silver wire on a reel — that used in surgery is the best Re-sublimed sulphur, as obtained from the dealers, but freed from moisture by being kept on a clock glass in a desiccator over strong sulphuric acid. The clock-glass must be replaced in a desiccator when a portion of the sulphur has been taken out for the experiment. Glass tubing of soft glass, not combustion tubing ; the most convenient width of bore is i to rs cm. It should be sealed up at one end and be of a length not less than 1 6 to 20 cm. A cork to fit this tube, through which passes, somewhat loosely, a piece of glass tubing about 2 to 3 cm, diameter. Carbonic acid apparatus. Footblowpipe and bellows. An oven or gas-furnace in which a tube of about 10 cm. length can be placed and heated to about 500° C. In default of this, the tube may be covered with sand and heated on a sand-tray. Weighing-tube, watch-glass, glazed paper, camel-hair brush or, better, a stiff feather, carbon bisulphide, sliarp three-cornered file, scissors.

Method of conducting the experiment. — Make the glass tube thoroughly dry by moving it from side to side and rotating in a luminous gas dame, at the same time blowing air through from the bellows. Measure off a length of the silver wire which has been found to weigh about one gram. Cut this up with scissors into short pieces, about ■5 cm. length, and weigh accurately. This substance may be weighed on the bare pan of the balance. Transfer the silver wire to the tube. We now have to weigh out about enough sulphur to combine with the whole of the silver. For i gram silver take 1 5 to -20 grams sulphur. The quantity taken must be accurately weighed. The sulphur may be placed in a small stoppered weighing tube which must be dry. This is weighed with its contents, and the sulphur is then shaken out carefully, little by little, into the tube which contains the silver. Should any sulphur fall outside the tube it is received on the glazed paper and can be brushed into the tube. If the tube has not been properly dried the sulphur will adhere to the sides. The weighing-tube which contained the sulphur is corked and weighed again, the difference of the two weighings being the weight of sulphur taken.

If the tube were sealed up while containing air some of the sulphur would burn on heating, in order to avoid this complication the air is displaced by carbonic acid before sealing. The gas is passed in through the narrow glass tube carrying the cork, the cork of course not being fitted into the wider tube. The current of gas is turned on slowly in order to avoid the risk of scattering the sulphur over the sides of the tube. The tube is full of carbonic acid when a taper is extinguished at its mouth. Now stop the current of carbonic acid and fit in the cork ; draw the narrow tube through the cork so that it projects not more than i to 2 cm. into the wider tube. The wider tube must now be sealed up. In the operation of sealing, begin by warming the tube above the luminous flame of the blow-pipe ; then bring it into the luminous flame, keeping it constantly turned. When the tube is covered with soot the blast may be put on, but very gently at first. When the tube softens draw it out slightly, then lower the flame and heat strongly, rotating the tube during the operation, so ' that the sides fall together. When this has taken place cool slowly in the luminous flame until the tube is again coated with soot. !f the pupil has no previous practice in glass-working this part of the exercise should be rehearsed with an empty tube to master the manipulation. This practice of rehearsing may with advantage be adopted whenever a quantitative experiment involves a new piece of manipulation. Everything is now ready for bringing about chemical combination by heating. The method of doing this has already been described. The time for completing the reaction depends partly upon the thickness of the wire. It is best to arrange matters so that the weighings and sealing are completed at the end of

L, . l;,L.OOglC a lesson, so that the heating of the tube may be left to go on for a few hours in order to ensure completion. The remaining determinations can be done in the next lesson.

After heating, it is found that the silver and sulphur have been converted into the black, shining crystalline silver sulphide, if an excess of sulphur was taken this will be found collected on the part of the tube which was coolest. We now have to weigh the silver sulphide and the excess, if any, of sulphur. With a sharp file make a deep transverse cut at the middle of the tube. Touch this with a fine point of glass which has been heated till quite soft in the blow-pipe flame. In this way the tube may be readily cut into halves. The sulphide is readily detached from the tube and is brought on to a weighed watch-glass and weighed. The tube must be carefully brushed out, the watch-glass being placed on the sheet of glazed paper. The accuracy of the results obtained in this experiment depends upon the attention given to the details of manipulation, the chemical part of the experiment not presenting any difficulty. If there is any excess of sulphur, weigh the pieces of glass to which the sulphur adheres, dissolve off the sulphur with carbon bi-sulphide, in a draught chamber, and weigh again. The difference gives the weight of the sulphur.

Calculation and statemtnt of results. — Examples are given from the results obtained by two pupils. The weights are in grams.

The first pupil found

Sulphur . . 'IZ33 (no excess of sulphui)

Sum '9534 Sulphide . '9531

The weight of the sulphide is found to be less than that of the elements from which it was formed by 3 parts in 9534. Regarding the exercise as intended to verify a law believed to be mathematically exact, we may say that this represents the experimental error. This should always be expressed as a percentage. To find the percentage in the above case divide 3 by 9534 and multiply by 100. The result is, very nearly, '03. It must be written with the minus sign, since the weight of the compound formed is found to be less than it should be ; therefore we write

Experimental error — '03 per cent.

The second pupil found

Sulphur laVen , -2963 Excess sulphur , ■1476

Excess sulphur lell '1476 Sulphide . . 11476

Therefore sulphur useil

in combinalion . -14S7

Sum of wcighls of elements iiefore combination . 1 '1487

Weiglit of compound fonned 11476

Difference -001 1

Therefore experimental error — -10 p. c.

The average error of thirteen experiments recorded by ten pupils was 'S per cent One of the experiments exceeded 2 per cent, in error, and another exceeded i per cent. In no case was the error +"", i.e. the weight of the compound was never found to be greater than the sum of the weights of its constituents.

Note upon early experiments relating to the Law of Conservation of Mass, and upon Stas' determinations. — W. Black (' Experiments upon Magnesia Alba,' published 1782, pp. 66-68) converted 120 grains of chalk, by heating, into quicklime, of which 68 grains were left. The lime was thrown into a solution of carbonate of soda, by which means chalk was again formed, which weighed 118 grains. The final weight was, therefore, found to be less than the original weight by 2 parts in 120 r experimental error — i'6 per cent.

Lavoisier {' Elements of Chemistry,' translated by Dr. Kerr, published 1792) decomposed mercuric oxide by heat, weighed the mercury obtained, and measured the volume of oxygen. He also determined the density of oxygen. The experimental error appears to have been about — i per cent., but the statement of results leaves some uncertainty. Lavoisier stated {loc. cit.) the law of conservation of mass in connection with his experiments upon the fermentation of sugar in presence of water and yeast. He found that the weight of the products formed was equal to the weight of the substances decomposed. The numbers given are apparently not those actually determined, but rather what he considered they ought to have been if experimental error were eliminated. The numbers, therefore, are not available for calculating the approach to accuracy attained in the experiment

For modern work upon this law, see Stas' ' Nouvelles Recherches sur les Lois des Proportions Chimiques,' experiments upon the complete synthesis of silver iodide, and complete analysis of silver iodate, which confirmed the law within a very small margin of experimental error.

Chapter III The Law Of Definite Proportions

Statement of the Lav/. — When two elements combine to form a particular compound substance, tliey do so in a definite, fixed proportion by weight, which is independent oft/ie manner in which their combination is brought about.

This is the first part of the law of definite and of constant pi-oportions ; for the second part, see Chapter V.

The significance of this law lies in the persistence with which elements adhere to certain proportions of combination in spite of variation in the conditions under which combination occurs. The conditions of combination may vary in the following respects ; —

1. Relative masses of the reacting substances,

2. Pressure under which the reaction occurs.

3. Temperature at which the reaction occurs.

4. Chemical composition of the substances by the reaction of which the combination is brought about.

The fact that the combining proportion does not depend upon the relative masses present may c be verified by a comparison of the results obtained by different pupils in the last exercise. Thus, taking the two examples already quoted : —

First pupil . Sulphur l : Silver 6732 Sulphur I Silver 6732 Second pupil . „ i : „ 3-333 ,, i „ 6777

the difference between the combining proportions obtained by the two pupils is -66 per cent., the combining proportions being constant within '66 per cent, when the proportion between the substances taken was varied from 2 to i.

The second condition (variation in pressure) is tested to some extent by the same experiment, since the pressure in the tube depends, other things being equal, on the excess of sulphur present.

The third and fourth variations of condition (viz. of temperature, and of the chemical composition of the reacting substances) are tested by the following experiments upon the proportion by weight in which silver and chlorine combine together. The last (fourth) mode of varying the conditions is especially important as showing that the combining proportions are independent not only of physical but also of chemical conditions.

In the first experiment (Exercise III.), the elementary gas chlorine acts on metallic silver at a temperature of about 400° C. to 500° C. In the second experiment (Exercise IV.) the combination of the two elements, chlorine and silver, is brought about by the action of a solution of hydrochloric acid upon silver nitrate at a temperature rather below 100° C. Exercises III. and IV. may be worked side by side. Such an arrangement as the following will save time. Weigh out the materials and start the evaporation in Exercise IV. While this is going on, weigh out for Exercise III. and start the heating in chlorine. Then return to Exercise IV., which can be finished. The chlorination in Exercise III. is, if pos.sible, left going for some hours, the weighing and calculation being postponed till the next lesson.

Exercise \\l.— Determination of the proportion in ■which silver and chlorine combine when chlorine gas acts upon metallic silver.