Question

This question is about zinc and compounds of zinc.

A student produces pure crystals of zinc chloride by reacting zinc oxide with
hydrochloric acid.
The equation for the reaction is:
ZnO(s) + 2HCl(aq)=ZnCl₂(aq) + H₂O(l)
The student adds zinc oxide to hydrochloric acid until the zinc oxide is in excess.
Give one observation that the student could make to show that the zinc oxide is
in excess.
[1 mark]
Why is excess zinc oxide used rather than excess hydrochloric acid?
[1 mark]
Name one other compound that the student could add to hydrochloric acid to produce
zinc chloride.
[1 mark]
Describe how the student should obtain crystals of zinc chloride from a solution of
zinc chloride.
[2 marks]

Answer 1

Observation to show excess zinc oxide:

If the student observes that there is some undissolved solid zinc oxide remaining in the reaction mixture even after stirring for a long time, it indicates that zinc oxide is in excess.

Reason for using excess zinc oxide:

Excess zinc oxide is used rather than excess hydrochloric acid because zinc oxide is insoluble in water, while hydrochloric acid is soluble. Therefore, adding excess hydrochloric acid would result in excess acid remaining in the solution and contaminating the pure crystals of zinc chloride.

Another compound that could be added to hydrochloric acid to produce zinc chloride:

Zinc sulfate (ZnSO4) could be added to hydrochloric acid to produce zinc chloride.

Obtaining crystals of zinc chloride from a solution:

The student can obtain crystals of zinc chloride from a solution by evaporating the solvent, typically water, to a smaller volume, allowing the solute to become more concentrated. The solution should then be left to cool slowly to allow the crystals of zinc chloride to form. The crystals can be separated from the solution by filtration, washed with cold water, and dried to obtain pure crystals of zinc chloride.