Henry Watts.

A Dictionary of chemistry and the allied branches of other sciences, Volume 3 online

. (page 21 of 86)
Online LibraryHenry WattsA Dictionary of chemistry and the allied branches of other sciences, Volume 3 → online text (page 21 of 86)
Font size
QR-code for this ebook


of a solution containing both chlorides and bromMes with nitrate of silver, the whole

of the bromine is contained in the first portions of the precipitate ; the similar obaer-

ration of Field (Chem. Soc Qn. J. x. 284), and the decomposition of chloride of

silver when brought in contact with an aqueous solution of bromide or iodide of

potassium, described by the same chemist ; and particularly the violent decomposition

of chloride of silver, attended with evolution of heat, which takes place, as obswed

by Deville (Ann. Ch. Pharm. cL 198), when aqueous hvdriodic add is poured upon iU

The precipitation of bromide or iodide of silver, to the exclusion of the chlonde, in

cases of fractional precipitation with nitrate of silver, and the convefsion of chlonde

of silver into bromide or iodide by contact with an aqueous solution of iodide of

potassium, may be considered as essentially the same phenomenon. When the latter

transformation is expressed by an equation, the substances whose formulas appear in

the iHft-hand member of the equation are those wluch waSfT decomposition ; while

those which are formed appear on the light-hand side ; and on comparinff the ealoti^

metric values of the seversi terms, making due allowance for their state of solution or

otherwise^ we find that the sum of the catorimetric values on the left-hand side of the

equation is less than the sum of the calorimetric values of the substances on the zight-

bftnd ; thns indicating that, althou^ the conversion o( chloride of silver into bromide

or iodide by simple substitution would be attended with absorption of heat, the com*

bined efifect of all the actions which take place in the experiments under consideration

m to eauM an evolution of heat

• TbeM Dombcrs rvprewnt the formaUon oC aqueous soluUom of iwCaah and loda: all the real raiir
o anbjdroua conpouads.

l2



Digitized by



Google



11$



HEAT.



Thus we hare

AgPl + KBr » AgBr + Kd
Calorimetrie Tallies 84800 85678 26618 97091

>-^ , ^ ^ Y

Sums : 120478 122709

therefore: heat evoWed in the reaction » 122709 - 120478 - 2231 grainm^-degr»

A«C1 -h KI » Agl -f ECl
Calorimetrie Talues 34800 72479 18661 97091



Sums: 107279 116742

therefore : heat eToWed in the reaction » 116742 — 107279 •- 8463 gramme-dm
Similarly, for the reaction of aqueoos hydriodic acid on chloride of silTer, we na

Agl + Ha
18661 40192



Aga -f m -

Calorimetrie valaes 34800 16004



Sums :



49804



68843



therefore: heat eyolred in the reaction — 68848 - 49804 - 9039 t

The heat eyoWed in a considerable nnmber of processes of oxidation has beei
directly determined by Far re (J. Pharm. Chim. [3] zxiy. 241, 311, 412 ; Jahrei
1863, pp. 22 et seqA for the most part by effisctinff the oxidations in presence of n
in the mercurial ouorimeter, by means m hypoduorons acid, whose heat of forma
calculated for one equivalent of chlorine, he found by preliminary experiments i
— 7370 units.

For the details of this investigation we must refer to the original pap ers ; the
important results obtained are as follows, calculated always for one equiyalent d
element operated upon.



SuteUnce.


Product.


Heaterohred.


Ratio.


YeHow phosphorus )
Red phosphorus (


Aqueous phosphoric add .


5 209476
) 181230


4-86


IN


Yellow phosphorus


Phosphoric anhydride


190490






»f »f • •


Phosphorous acid


140394


2-91


1-


»» If • •




48302


1- '


Arsenic


Arsenic aod .


110787


164


„ • • .


Solid opaque arsenious add


76363




»f • •


Dissolved opaque arsenious )
add . . . (


71690


1-


Phosphorus .


Pentachloride .


100373




»» ...


Trichloride


94804




Arsenic


Trichloride


71883




Nitrogen


Nitrous oxide .


-8724




Nitric oxide .


Nitric add . .


20666




Nitrous add .


If 11 • • •


27269




Sulphurous acid .


Sulphuric add .


27839




Sulphur (portion of
flowera of sulphur in-








soluble in sulphide of '


If „ • . .


64110




carbon) J








Plastic sulphur


II tt • • •


67211


3-32 1-


Sulphur (prectnitatad]
from hyposu^uiite of -


II II • • •


63407


\


sodium) . j






y.


Sulphur . . 1


Sulphurous add (FaTre )
and Silbermann) . J


^373


1-96 V


Sulphur
Selemum •


Hyposulphurous add
Seleniousadd . .


20233
23206


1*66


«, . . • •


Selenicadd .


86360


Oxalic add . .


Carbonic anhydride .
Carbcmic oxide (Favre and >
Silbermann) . . 5


30140




Carbon


14838


1-


» • • • •


Oxalic anhydride


33410


2-26


II • • •


Carbonic anhydride (Favre
and Silbermann) .


48480


3-27


Chlorine


Chloric anhydride .


-66234





Digitized by



Google



DETERMINATIONS OF HEAT OF COMBINATION. 117

It ii olmoiu that, eren ffoppoang the prindple rxpoa wfaieh these indirect deter-
ininartonii are made to be quite oone^ ana all the interfering drcnmstaneee to hare
been taken into aooonnt, the resolta arrired at cannot be of the.aame degree of aoenracr
as those obtained by more direct methods: fbr the total uncertainty attaching to ail
the independent data from which they are deduced will be accumulated upon them.
Horeorer, although a comparison of the heat of chemical action, as determined directly,
with that deduced indirectly, generally shows, in the simple cases in which it is possible
to institute such a. comparison, a degree of agreement which may be taken as a con-
flimation of ths coanectness of the principles whereby the calcnlat^M! result is azriyed
at,, and as proving that, at least in these cases, no modifying circumstance of any great
impcntanoe has beisn oTerlooked in the calculation, still it cannot be considered as prered
by existing experiments that the cold of decomposition is o/iooys equal to the heat of
combination. On the other hand, it seems i>robable^ on general grounds, that the
ealorimetric eibct of a giren chemical process is not an ab«>lutel^ constant quantibr,
but that it is to some extent dependent on accompanying physical conditions. In
particular, it seems likely that it must Taiy more or less with xke temperature at iHiich
the chemical action takes place^ and therdbre—to take a particnlar example— that the
heat ewhed by the combination of mercunr with oxygen, at a comparatiTely low tem-
perature, is not eqnal to the heat abtorbed by the decomposition of oxide of mercury at
a higher temperature The equality of these two quantities of heat would inTohe also
the equality of the spedfio heat of oxide of mercury with the sum of the specific heats
of its elements ; otherwise, the quantity of heat needed to raise the temperature of a
giren quantity of mercury and oxygen from the ordinary temperature to a temperature
above that at which oxide of mercury is decomposed, would be different, if the two
substances remained unoombined dunng the process, from what it would be if they
first entered into combination and afterwards separated again ; and since the final con-
dition of the mercury and oxygen would be the same in each case, the difiference
bKween the Quantities d best would hare disappeared without producing any apparent
efiect In tne particular case that has been taken as an example, it is possibk that
the spedflc heats of the elementary bodies are together equal to the specific heat of the
compoimd, or that the difference may be compensated by the difference in the latent
heats of Taporisation of oxygen at the temperature of combination and at that of de-
composition. But, although this may be the case, experimental proof that it is bo is
still wanting ; and even if it were airarded here, or for any other particular substance,
there would still not be sufikient warrant for assuming as a g|eneralhr established fact
that the cold of decomposition is equal to the heat of combination, independetttly of ths
conditions under whicn these processes respectively occur. In connection with these
connderations, it is perhaps worth while to drew attention to the fact that the heat of
combustion of hydrogen, deduced by Joule horn the eleetzolysis of water at the ordinary
temperature, is decidedly less than that found by other experimenten who determined
directly the heat of combustion at a high temperature (see p. 114). These experiments
are perhaps not sufficiently comparable in other respects to justify us in attaching
much importance to this result ; but, taking it as it stands, it is in harmony with the
fret that the specific heat of water-vapour is less than the sum of the specific heats of
its constituents.

The laws by which Thomson oonsidem that the deyelopment of heat in chemical
action is governed, appear to inyohe yiews similar to those aboye explained. The
frndamental propositions assumed by him are as follows: — The intensity of tho
chemical emgy of the same body at a constant temperature is imchangeable ; the
whole quantity of heat deyeloped in a ^yen chemical action is a measure of the chemioBl
eneigy therein exerted, and is proportional to the difference between the total diemical
enmy of the reagents and the total chemical energy of the products.

Meat produced by Chemical Deeomfosition, — ^I%ero are a few phenomena which are^
at least apparently, exceptions to the general rule that heat is evolyed in combination
and absorbed in decomposition. It was found, for instance, by Fayre and Silbermann,,
that charcoal eyolves more heat by burning in nitrous oxide than by burning in pure
oxygen. The excess of heat in this case could only be due to heat evolTM in tho
decomposition of the nitrous oxide^ and direct experiments proyed that this decompo-
sition was attended with evolution of heat They found frrther that heat is produced
in the decomposition of peroxide of hydrogen by platinum, and from the experiments
of Favre, already recorded, it appears that heat is absorbed in Uie oxidation dT chlorine
to hypochlorous or to chloric acid. Perhaps these and similar phenomena may be
brought into harmony with the general rule, by taking into consideration the eyolution
of heat due to the combination of two similar atoms to form a molecule, and the
corre sp onding absorption of heat due to the separation of the atoms constituting a
molecule of an elementary body.

The influence of high temperature in promoting chemical action^as, for instancy



Digitized by



Google



118 HEAT. .

between Cajgpn and hydrogen, which combine when heated, but are without i
each other in the cold— ia not an exception of the kind here referred tow It ia zsthie
thennometric than a calorimetric phenomenon ; hig^ temperatore being a fKvoaaHi
condition for <^emical action in these caaee, bat in no way its canae. At wtrj I
temperatores chemical action appeaze in all caaea to be mneh retarded : mm one in
tiataon, it may be mentioned thtSb when liqneAed ammonia, cooled to abomt •-66^,
ponred upon concentrated solphoric add, the two li^nida form separate lajera, and
some time no action takea place between them (Loi r and Dri on). Fhoeidiflraa a
iodine, which combine energieticaUy and with Tiyid combustion at common t euipwrmU n
may be shaken together in a tube co(ded by a freeong mixtoie, without ezarCii^ 1
slightest action on each other.

5. Selatiom of Heat to EUetrioity,

atootrietl^ vrodncmA toy WiMitf—Puro'eiectricU\
has been already said on this sn^'ect in the article £i
to add the reaolts arrived at by GFaoffain (Ann. GL
the pyro-electricity ci tonrmalmea. Se finds that this
laws :— (1.) When any number of tourmalines are m
name, they fonn a battezy which prodocefl^ under any
of electria^ exactly equal to the sum of the quantiti<
would produce separatdy under the same circumstances.

(2.) When several prisms of equal section are superposed, the quantity of electric
developed by the combination is merely e^ual to that which woula be f umiahed, uk
the same conditions^ by any one of the pnsms taken separately.

(3.) The quantity of electricity produced by a pism of tourmaline is proportional
its sectional area, and is independent of its length.

(4.) The quanti^ of electricity which a tourmaline derelopes when its temperati
is lowered a given number of degrees, is independent of the length of time in vhi
the cooling is effected.

(5.) When a tourmaline is heated a given number of deareea, the ouanlil^ of elc
trici^ developed is precisely the same as that which womd be proauced if it wc
cooled to an eoual extent

Oaugain finds that the development of electricity, consequent on the alteration
the tempmture of a tourmaline, is very much more marked if one pole of the czyst
is placea in electric communication with the earth, than when the crystal ia oomplike
insulated.

T^ermO'Eieotrieity, — ^When two ban of dissimilar metals are soldered together i
each end, a current of electricity genendly circulates through the circuit formed 1
them, as long as the two soldering points are at different temperatures. The electi^
motive force developed at each joint is a function of the temperature, which, :
the cases that have been specially examined, may be represented by the formu
J?«i a+bt-^rci*^ where t is the temperature, and a, b, and c are constants depending <
the nature of the metals used. Consequently, the resultant electromotive force, wh«
the temperatures of the joints are t and t respectively, may be represented by

which shows that the current vanishes not only when the joints are at the same ten
perature, or when t b t^, but also when the sum of their temperatures attains a vali

that is constant for the same pair of metals,— that is when t+t^^t^-. (A ve ii ar in

c
Fogg. Ann. cxix. 406, 637.)

Beat prodttoed toy Sleetrleity«— The laws regulating the development of ho
by the sudden discharge of frictional electricity, or electricity of high tension, hm^
abeady been treated of in the article Elbctricitt (ii. 895), and in the same artic
(p. 470) it has been shown that the heat produced by a continuous current of electridt
is the exact equivalent of the chemical action whereby the current is generated. 1
the application of this rule, the total chemical action throughout the circuit must 1
taken into account, electrolytic decomposition being counted positive or negative aoooit
ing as the direction of the current which would be generated by it, were it to tal
place independently of the battery, is the same as that of the battery or the oontrar
As ftirther explained in the article referred to, the heat developed in the unit of time b
an electric current is related to the intensity of the current and to the resistance <
the circuit in the manner expressed by the formula :

H'^PR.



Digitized by



Google



HEAT AND BIECHANICAL ENERGY. 119

Ttom this it loUowi that, by diminiihing the zeeistaneeof the dzcait, the eleetro-motire
force lemainiDg the tame, tne total heat, ff, can be augmented indefinitely, the inten-
•ity. and therefofe also the chemical action in the battery, increasing in proportion
to the diminution of resistance. Bat in any single ^rtion of the cizcuit, whose resist-
ance is r, the deyelopment of heat will attain a maximum for a psrtioular Talue of r.

The heat produced in this part will in feet be A — /V = — — _r, the value of

which htst eacpression reaches a maximum when Siarj B being the constant rsfistanoe
of the remainder of the circuit, including the battery. Bence, in order that as large a

Suantity of heat as possible may be generated in a giyen time in the eonducting wire,
be resistance r of the conducting wire must be made equal to the permanent
resistance B of the remainder of the circuit. And, since the heating power of
the current represents its total eneigy, it follows that the same proportion between
the external and internal resistance of the circuit is the most adTaatageous that can
be adopted lor causing the current to prodnoe changes of any other kind outside the
battery.

Thermal phenomena of a particular kind occur when an electric current, instead of
trarersing a homogeneous conductor, passes from one substance to another. Then, in
addition to the deyelopment of heat corresponding to the resistance of the conductor,
according to the laws stated in the last pangrai^ a further deTelopment or absorption
of heat takes place, depending upon the direction of the current If the current crosses
the point of junction of two dissimilar portions of the circuit, in the same direction as
the thermo-electno current which would be generated by the application of heat to this
point, then the heat deyeloped there is leas than corresnonds to the resistance of that
portion of the circuit, or, in other words, heat is absorbed ; if the current passes the
point of junction in the opposite direction, the heat generated is more than what cor-
responds to the resistance.

With a junction formed of the same two metals, the deTelopment or absorption of
heat thus occasioned is proportional to the intensity of the current by which the junc-
tion is trsTersed. Witn combinations of different metals, the extent to which this
phenomenon occurs Taries with the poeitions of the metals in the thermo-electric
series (ii. 412), being greatest for those combinations which give the most intense
thermo-electric currents.

6. Belatiom of Heat to Meohanieal Energy,

In preceding parts of this article, we hate seen that increase of Tolume and of elastic
force are amon^ the most uniTersal of the effects of heat. In this way heat is constantly

5 reducing motion, and when this motion is concentrated in a giren solid body, and a
efinite direction is giyen to it, as in the steam-engine, heat becomes by far the most
important artificial source of mechanical power that we possess. ConTersely, inuumer-
abu familiar &cts supply us with illustrations of the i^oduction of heat at the expense
of mechanical work. Thus, in all machines there is a certain loss of mechanical
enogy, the work performed by the machine never representing the full equivalent ot
the work needed to drire it That portion of the mechanical energy supplied to the
machine which is wasted, so fiur as the perfbrmasce of useful work is concerned, is
expended in o r e rc om ing ^e passiTS resistances by which the motion of the machine is
opposed ; such, for instance, as friction between contiguous surfaces not moving with
toe same ydodtr, or the rigidity of straps or cords lued to transmit movement from
one part to another. But, whenever motion is produced in opposition to friction, or a
rigid body is forcibly bent, heat is generated ; the mechanical energy expended in pro-
ducing these effects is lost to the purposes of the machine, but the heat evolved is its
representative. Percussion is another means by which mechanical energy can be de-
stroyed and heat generated in its stead. These and similar fiMts have long attracted
the attention of ^iloeophers, and great importance has been attached to them in re^
fsrence to most of the theories that have been suggested ftcfm. time to time to explain
the nature of heat and the effects which it produces. The following is a quotation
from Black's Lectures on ChenUetry (vol. i. pp. 31, 32), and relates to Bacon's
investigations into the nature of heat contained in his treatise Deformd Calidi: ^

" The only conclusion, however, that he ** (Lord Yerulam) " is able to draw from the
iHiole of his facts, is a veiy general one, vix. that heat is motion.

'* This conclusion is founded chiefljr on the consideration of several means by which
heat is produced, or made to appear, in bodies ; as the percussion of iron, the faction
of solid bodies, the c<dlision of fiint and steel.

*' The first of these examples is a practice to which blacksmiths have sometimes
recourse for kindling a fire ; they take a rod of soft iron, half an inch or Isss in thick-
ness, and, layinff tlM end of it upon their anvil, they turn and strike that end very
quicidy on its different sides, with smart blows of a hmmm«. It Vflfl7 soon beoom<«



Digitized by



Google



120



HEAT.



ted hot, and can be employed to kindle ahayingi or wood». or other very oombfoatibk
matter.

"The heat prodneible hy the strong friction of solid bodies, oocnrs often m sqbi
parts of heayy machinery, when proper care is not taken to diminish that firietion ai
much as possible, by the interposition of lubricating substances ; as in the axles g^
wheels thi^ are heayy themselyes, or heayily loaded. Thick fiorests are said to hsn
taken fire sometimes, by the friction of branches against one another in stoimj weather
And sayages, in different parts of the world, have recourse to the friction of pieces o
wood for kindling their fires. ....

** The third example abore adduced in the collision of flint and steel, is imiTenall^
known.

" In all these examples, heat is produced or made to H^pear suddenly, in bodia
which have not reoeiyed it in the usiul way of communication from others^ and the onlj
cause of its production is a mechanical foroe or impulse, or mechanical yioleoce."

f^om these and many other similar phenomena, it is abundantly dear that, hj thi
expenditure of heat, mechanical work can be eflfected, and that heat can be prodnoed h}
the expenditure of work. Dismissing, therefore, for the present, all considentioiis u
to the ultimate natore of heat, we may say that heat and work are mutually oonyertiUe,
and we haye in this section to consider the conditions which regulate the tranafomiatioE
of one of these forms of energy into the other.

H^ork iMTodneed bj Beat. — ^The measure of the work done by heat, when il
causes expansion, is obyiously the product of the resistance oyercome into the mpaa
throuffh which the expansion takes place; that is, if r denote the resistance, and v and t
the initial and final yolumes of the body which undergoes expansion, the work done h}
the heat will be represented by

^ - r(i^ - V).

In estimating the resistance by which the expansion of a giyen body is opposed, it it
needfol to take into account^ not only the external pressure upon its sorfoce, but also
as has been already pointed out (pp. 40 and 73 X tne molecutsf forces which tend U
maintajn unaltered the relative positions of its ultimate partides. The work, jL, per
formed by heat when it causes expansion, must therefore be considered as maide up oi
the internal work, a^ performed in opposition to internal molecular forces, and ihi
external work, a^ o^iended in oyerooming external pressure. In solid and liquic

bodies the ratio, ^, of the internal to the external work is yery considerable, but in i

perfect gas, the resistance to expansion arising from the mutoal attractions of thi
molecoles is insensible, and therefore also the expenditure of work in oyerooming thx

resistance is inaenmble: hence, in this case - ^ — 0, or o^ » if . But since it is tbi

external work only which is ayailable for the production of external effects, the oon^
sideration of the transformation of heat into mechanical energy is much simpler in tlM
ciase of gases than in that of liquid or solid bodies ; it is frirther much the mosl
important practically, for althou^ the heat employed in the performance of interna]
work upon solids or liquids is not necessarily lost ultimately, it is, for the time at
least, totally unavailableu We may perhaps compare the traxisfonnation of heat intc
work with the commercial transformation of labour into wealth : then, a pc
will represent a medium of transformation comparable with an industrial proo
requires no preyious expenditure on the psrt of the workman to enable him to
his labour for wages ; while a sdid or liquid will represent a medium of trand
comparable with a process whidi can only be earned out by the preyious in
of a considerable capital The capital thus "sunk" may be realiBed again
future time, but it represents a certain quantity of labour, which, for the tii
cannot be tzansfonned into wealth.

In order that work may be continuonaly performed by heat, it is not enooj
portion of vr or other gas should be expanded once for all in a cylinder dc
piston, or in any other similar apparatoa. It is true that motion would be 1
duced, and that this might be the frill equiyalent of the heat expended, but th
would soon eome to an end. Ftectically, the extent to which a gas can be <
•o as to produce motion, is yery limited, both by the size of the appazatos th
laed and by the temperatures that are attainable. In order that the proces
continuous, the action of the machine, by means of wluch the transformation ii
must be periodic: after a certain cyde A changes, all its parts must return to



Online LibraryHenry WattsA Dictionary of chemistry and the allied branches of other sciences, Volume 3 → online text (page 21 of 86)