Page:Outlines of Physical Chemistry - 1899.djvu/176

 ��OUTLINES OF PHYSICAL CHEMISTRY

��The following are some heats of solution :

��(KC1, n aq

(NaCl, n aq

(NH 4 C1, n aq

(EI, n aq

(ECN, n aq

(LiCl, n aq

(LiNO„ n aq

��(ENO a, n aq) =

(NaNO„ n aq) =

(NH 4 NO„ n aq) =

(KC10„ n aq) =

(E,S0 4I n aq) =

(KjjCrO,, n off) =

��- 8-3 Cal.

- *7 „

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��A certain number of salts crystallise with molecules of water. It is necessary to admit that they have a par- ticular avidity for the solvent, and on solidifying they re- tain a certain proportion of it. The heat of solution of these salts taken in the anhydrous condition is generally positive and in all cases higher than that of the hydrated compound.

Examples :

(Nal, n aq) = 1-3 Cal.

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��(FeCl^ n aq) =

(FeCL, 2aq, n aq) =

(GoC\ 2 ,naq) =

(CoCl, 6aq, n aq) = -

(NajSO,, n aq) = 0-44 „

(Na 2 SO 4 10aq,nog) = -18-2 „

��180 Cal.

��The difference between the heats of solution of the anhydrous and the hydrated salts is termed the heat of hydration (application of the principle of the initial and final states).

Neutralisation Phenomena

The heat of neutralisation of a base by an acid is gene- rally determined in dilute aqueous solution. The calori- metric operation can then be easily carried out, and, since the conditions of the experiment are almost the same for all salts, the thermal effects found are comparable amongst themselves.

��1 The equations are written according to Thomsen's method, n aq *= much water, 100 to 200 molecules.

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