Question

C. Effect of adding a strong acid base to an amphoteric hydroxide

1. Obtain approximately 20.0 mL of 1.0 M zinc nitrate solution and place it into a 150 mL beaker.

2. Add 6 M sodium hydroxide (with mixing), in a drop-wise fashion, until a reasonable amount of solid appears.

3. Divide the mixture from step 2 into approximately two equal portions. (This mixture contains the amphoteric hydroxide.)

4. To one of the two portions, continue to add 6 Msodium hydroxide (with mixing) until you see a distinct change in the mixture. Note how much sodium hydroxide solution was added. (Recall: 20 drops  1 mL)

5. To the other portion, add 6 M nitric acid (with mixing) until you see a distinct change in the mixture. Note how much nitric acid solution was added. Dispose of the solutions in the appropriate waste receptacle

results for part c are as follow

effect of adding a strong acid or ase to an amphoteric hydroxide

obsrvation after mixng the two reagent-------white precipate fromed

first portion the solution got thick and was cloud llike.

the second portion

obervation after adding 1mL HNO3------------------ the precipate broke apart to smaller pieces onnce a good amount was added

QUESTION the context of part C of this experiment, explain what an amphoteric hydroxide can do that:

• acetic acid can’t do

• aqueous ammonia can’t do

• sodium chloride can’t do

Answer 1

An amphoteric hydroxide can react with either acids or bases, while acetic acid, aqueous ammonia and sodium chloride cannot react with both.

Zinc hydroxide is an amphoteric hydroxide. If we have Zn²⁺ ion in solution and we start to increase the pH by adding a strong base, we initially produce an insoluble compound. Continued addition of hydroxide allows another equilibrium to occur in which a complex ion is formed between the zinc ion and the hydroxide which results in the solid Zn (OH)₂ dissolving

2NaOH + Zn(NO₃)₂ -> Zn(OH)₂ + 2NaNO₃

Zn(OH)₂ + 2NaOH -> Na₂ZnO₂ + 2H₂O

Addition of a strong acid would also dissolve solid Zn (OH)₂

2 HNO₃ + Zn (OH)₂ → 2 H₂O + Zn(NO₃)₂

While mixing a strong base with a weak base, contentedly ignore each other in solution. So NH₃does not react with NaOH. It reacts with HNO₃ forming ammonium nitrate.

  NH₃ (aq) + HNO₃ -> NH₄NO₃

Acetic acid reacts only with NaOH

NaOH + CH₃COOH → CH₃COONa + H₂O

NaCl does not with acids or bases in ordinary condition.