BY D
T
TiILBEK
- I t temperatures below 3 2 6' the moiitening. of anhydrous sodium sulphatc TT ith 11 ater causes an evolution of heat 0 1 1 ins to the formation of the decahydrate I t was found by d e Coppet' that heat is evolved a t higher temperatures Sodium itilphate n a s dehydrated a t rooo and placed in a thimn allcd ?lass bulb The bulb mas placed in a flask containing. a thermometer and a little n a t e r The flask, n i t h its contents, was placed in a coiistant temperattire air-bath and allon-ed to come to ecluilibritim The bulb \\asthen broken and the riie of temperature noted The rise n a s 2 I O from an initial temperature of 39 g o , j 9' from 51 z", 6 4' from j r 6 " and I O 4' from 90 I O , the actual rise beiiiq q-eater the initial temperature, for the range of temperatures studied LIe Coppet explained this rise in temperature a t first by a5stiminq two isomeric forms of sodium sulphate, b u t he abandoned this later' rvithout ofiering any new explanation Juliti5 Thonisen considered t h a t a monohydrate i q formed, b u t that is krionn not to be the case Thomsen'i presentation of the matter is miileading He says t h a t the heat due to the formation of the monohydrate may raise the temperatiire above the boiling point of water \Yhile this ii true, it \\auld ha\-e been better to have stated also that the temperature does not rise a h o \ e 103 2' the boiling point o f the saturated solution T h e rise of temperature i i of course due to the fact thLit anhydrous soilitim culphate is less wluble a t high temperaturei than a t Ion tempernturei and coiisequeiitlydissolvesn ith evoltition of heat Thii explanation has already been qiven j 4 b u t the increa4iiq rise in temperature 17 ith increasiiiq initial
557 temperature is there accounted for by the assumption t h a t the heat of solution increases in amount with rising temperature. A glance a t the solubility curve shows t h a t this explicit assumption is not true. This difficulty disappears if one remembers that the rise in temperature in these experiments depends on the rate a t which heat is evolved as well as on the amount of heat evolved. At the higher temperatures of 90°, equilibrium is approached so much more rapidly t h a t the amount of heat evolved in a given brief interval of time is greater than a t 40’ through the total heat evolved u p to equilibrium is greater in the latter case. Cornell 1
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