Aqueous zinc chloride (ZnCl2) reacts with aqueous sodium hydroxide (NaOH) to produce zinc hydroxide ( Zn(OH)2 ) and sodium chloride (NaCl). Zinc hydroxide is a white colour precipitate and it is also an amphoteric hydroxide. Therefore, zinc hydroxide is dissolved when excess NaOH is added to the precipitate.
From the balanced equation, we can say, 1 mol of zinc chloride reacts with 2 mol of sodium hydroxide and produce 1 mol of zinc hydroxide and 2 mol of sodium chloride.
This equation can be balanced easily from following steps.
ZnCl2 + NaOH = Zn(OH)2 + NaCl
Remember that zinc is an amphoteric element and zinc hydroxide is an amphoteric hydroxide.
When aqueous NaOH is added to ZnCl2 solution drop by drop, at one time, Zn(OH)2 precipitate is formed. If you add more aqueous NaOH to the solution which contains the precipitate, precipitate is dissolved and give a colourless solution due to formation of sodium zincate (Na2ZnO2). Sodium zincate is a colourless aqueous solution.
Sometime above equation is written as below.
If you slowly add HCl acid to the [Zn(OH)4]2+, you can see a white precipitate is formed in the solution. That is zinc hydroxide precipitate. If you add more HCl acid, that white precipitate is also dissolved and give colourless ZnCl2 solution.
Due to presence of excess NaOH, reaction will end up with soluble sodium zincate.
Questions asked by students
Zinc hydroxide is the given precipitate when Zinc chloride and Sodium hydroxide are mixed. But, if excess Sodium hydroxide is added, you will not see the formation of white precipitate because Zinc hydroxide is soluble in excess Sodium hydroxide. Similar to excess Sodium hydroxide case, if you use concentrated sodium hydroxide, there is the possibility of dissolving the Zinc hydroxide precipitate.
Potassium hydroxide and Barium hydroxide are alternatives for Sodium hydroxide if you need to prepare Zinc hydroxide precipitate.
Option 1: If you know the concentratins of each solution, you can do a calculation to check Ksp value related to the Zn(OH)2.
In the final solution, if [Zn2+] * [Cl-]2 exceeds Ksp of Zn(OH)2, a precipiates is formed. When [Zn2+] and [Cl-] are taken, you should assume that there is no precipiate and all Zn2+ and Cl- ions exist in aqueous solution.
You can do the experiment by starting from ZnO. Find dilute HCl bottle. Remember that it is good to use solid ZnO to prevent any impurities. Measure the weight ZnO and put it in a beaker. Then slowly add dilute HCl solution until all ZnO is dissolved. When all ZnO is dissolveed, ZnCl2 solution is given as the following reaction.
Now, you can continue the rest of the experiment with by adding NaOH to ZnCl2 solution.