Introduction to physical chemistry / by James Walker.

  • Walker, James, Sir, 1863-1935.
Date:
1919
Catalogue details

Licence: In copyright

Credit: Introduction to physical chemistry / by James Walker. Source: Wellcome Collection.

    292/462 (page 270)
    A balanced action of the same sort, but with the point of balance or equilibrium towards the other end of the reaction, is to be found when a solution of hydrogen sulphide is added to a solution of zinc chloride. A white precipitate of the metallic sulphide is formed, but even though a very large quantity of hydrogen sulphide is present, the precipitation is never complete, and a very moderate quantity of free hydrochloric acid will prevent the precipitation altogether. It is thus possible, by suitably choosing the conditions, to effect a separation of cadmium from zinc by means of sulphuretted hydrogen, although in the two cases we are dealing with balanced actions of precisely the same type. If some dilute hydrochloric acid is added to the solution of the mixed metallic chlorides, hydrogen sulphide will precipitate the cadmium almost completely, and pre- cipitate practically none of the zinc salt, even though it is present in great excess. If ammonium hydroxide solution is added to a solution of a magnesium salt, say magnesium chloride, part of the metal is precipitated as magnesium hydroxide, in accordance with the equa- tion MgCl2 + 2NH4OH = Mg(OH)2 + 2NH4C1, but the precipitation is never complete, for the reverse action Mg(OH)2 + 2NH4C1 = MgCl2 + 2NH4OH occurs simultaneously, and a state of balance results. In analytical practice we have usually excess of ammonium chloride present from the beginning, so that the addition of ammonium hydroxide produces no precipitate in the solution of magnesium salt. The reverse action can be easily studied by shaking up freshly-precipitated magnesium hydroxide with a solution of ammonium chloride, when the magnesium hydroxide will be found to dissolve (cp. Chapter XXX.). We may consider such cases of chemical equilibrium from the same standpoint as we adopted in Chapter X. for physical equilibrium. In the last instance given above, viz. the addition of ammonium hydroxide solution to magnesium chloride solution, if we mix definite quantities of solutions of definite strengths, the precipitation of magnesium hydroxide will come to an end when the reaction has proceeded to a certain ascertainable extent. Equilibrium is then reached, and the system undergoes no further apparent change. We may still conceive the opposed reactions to go on as before, but at such a rate that exactly as much magnesium hydroxide is formed by the direct action as is reconverted into magnesium chloride by the reverse action. Now at the beginning of the action there is no magnesium hydroxide or ammonium chloride present at all; these are only formed as the direct action proceeds. There is therefore at the beginning no reverse action. It is obvious that if a state of balance