4.23    describe reversible reactions such as the dehydration of hydrated copper (II) sulfate and the effect of heat on ammonium chloride

copper(II) sulphate example

If solid hydrated copper (II) sulfate (blue) is heated, it loses water from the crystals to form anhydrous copper (II) sulfate (white) and steam is given off:

hydrated copper (II) sulfate        ®         anhydrous copper (II) sulfate          +    water

(blue)                                                   (white)

CuSO4.5H2O(s)              ®                     CuSO4(s)                      +    5H2O(l)

Adding water to hydrated copper (II) sulfate reverses the above reaction (see Section 2.27)

anhydrous copper (II) sulfate      +    water                      ®         hydrated copper (II) sulfate

(white)                                                                                      (blue)

CuSO4(s)                      +   5H2O(l)                    ®         CuSO4.5H2O(s)

ammonium chloride example

If the white solid ammonium chloride is heated, it decomposes to give the colourless gases, ammonia and hydrogen chloride.

NH4Cl(s) → NH3(g) + HCl(g)

If the gases are allowed to cool, they recombine to form solid ammonium chloride again.

NH3(g) + HCl(g) → NH4Cl(s)

If you heat ammonium chloride in a test-tube, you see a white solid form near the mouth of the tube, where it is cooler, and the decomposition reaction has run in reverse. You need to know these observations and be able to explain them in terms of the reversible reaction.