Salts and their reactions : a text-book of practical chemistry / by Leonard Dobbin and Hugh Marshall.

  • Doddin, Leonard, 1858-
Date:
1912
Catalogue details

Licence: In copyright

Credit: Salts and their reactions : a text-book of practical chemistry / by Leonard Dobbin and Hugh Marshall. Source: Wellcome Collection.

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    electric charges indicated; if there is a like number of positive and of negative charges represented on one side, the same must be the case on the other, although the actual number on the two different sides need not be the same ; if, on the other hand, there is an excess of one kind of charge on one side of the equation, there must be the same excess on the other. Thus Ba + S04 = BaS04 (precipitation of barium sulphate from solutions of a barium salt and of a sulphate) ; Mg->2 0H' = Mg(0H)2 ' (precipitation of magnesium hydroxide from solutions of a magnesium salt and ot a hydroxide); Cu + Fe = Fe” + Cu (precipitation of metallic copper from a cupric salt by means of metallic iron); 2Fe” + Cl2 = 2Fe- + 2Cr (conversion of ferrous salt into ferric salt by means of chlorine). Interaction of Acids and Bases. The typical method of forming a salt by the interaction of an acid and a hydroxide is, as has already been observed, a particular kind of double decomposition. If the acid (a strong one) and the base (an alkali) are in dilute solution, then they will be to a large extent ionised to begin with, and the resulting salt, being also in dilute solution, will also be mostly ionised. Taking the case of hydrochloric acid and potassium hydroxide as an example, and representing it by the equation (H- + cr) + (K- + OH') = (K* + cr) + H^O it is evident that the anion of the acid and the cation of the alkali are practically unaffected by the action, as only a small proportion of them will unite to form molecules of the salt; and we therefore have the result, that the process of neutralisation consists merely in the mutual discharge of hydrogen ions and hydroxyl ions with formation of molecules of water. This explains why, in dilute solution, the heat of neutralisation is independent of the particular acid and base involved, provided they are both relatively strong, i.e , highly ionised. The heat' of neutralisation in such cases is merely the heat effect of the change represented by the equation In the case of comparatively weak acids and weak bases, how- ever, where ionisation in moderately dilute solution is still very