Amphoteric hydroxides are hydroxides which show double properties, that is in reactions with acids can behave as the bases, and in reactions with the bases – as acids. Metals with a valency of II, III or IV are a part of such hydroxides. How to prove amphoteric character of any hydroxide?
It is required to you
- - soluble salt of zinc, for example, chloride ZnCl2 zinc;
- - strong alkali, for example, NaOH sodium hydroxide;
- - strong acid, for example, salt – HSl;
- - two flasks with shlify;
- - two drop funnels with shlify.
Instruction
1. To prove amphoteric character of hydroxide review an example. For this purpose use hydroxide of Zn (OH) 2 zinc.
2. In both flasks with shlify pour an equal small amount of solution of chloride zinc. Insert into the first flask a drop funnel with sodium hydroxide solution, carefully unscrew the crane and slowly begin to flow alkali to zinc salt. Almost at once the friable white deposit is formed. It is an indicator that the following reaction proceeds: ZnCl2 + 2NaOH = Zn(OH)2 + 2NaCl
3. Further continue to flow sodium hydroxide solution to a zinc hydroxide deposit. After a while you will notice that the deposit begins to disappear gradually, and soon in a flask there will be only a transparent solution. What has happened? Hydroxide of zinc turned into soluble complex Na2ZnO2 connection. There was a reaction: Zn(OH)2 + 2NaOH = Na2ZnO2 + 2H2O
4. That is hydroxide of zinc reacts with strong alkali. But whether it reacts with acid? In other flask likewise begin to add sodium hydroxide solution. As soon as the friable white deposit is formed, replace a drop funnel and begin to pour in already hydrochloric acid, only do everything slowly. You will notice that the deposit will very quickly disappear. Why so it happened? Hydroxide of zinc reacted with hydrochloric acid, having formed soluble salt, there was the following reaction: Zn(OH)2 + 2HCl = ZnCl2 + 2H2O
5. As hydroxide of zinc reacts both with alkali, and with acid, it is amphoteric. Thus, you proved what was required.