KIT 發問於 科學及數學化學 · 1 十年前

NaOH and NH3

Why do some precipitates formed re-dissolve in excess sodium hydroxide

solution or in excess ammonia solution?

1 個解答

  • 1 十年前

    The precipitates of amphoteric hydroxides can be re-dissolved in excess sodium hydroxide. This is because these insoluble hydroxides can react with excess hydroxide ions to form anions, and the sodium salts of these anions are soluble in water. Common amphoteric hydroxides include aluminium hydroxide, zinc hydroxide and lead(II) hydroxide.

    Al(OH)3(s) + NaOH(aq) → Na[Al(OH)4](aq)

    or: Al(OH)3(s) + OH^-(aq) → [Al(OH)4]^-(aq)

    Zn(OH)2(s) + 2NaOH(aq) → Na2[Zn(OH)4](aq)

    or: Zn(OH)2(s) + 2OH^-(aq) → [Zn(OH)4]^2-(aq)

    Pb(OH)2(s) + 2NaOH(aq) → Na2[Pb(OH)4](aq)

    or: Pb(OH)2(s) + 2OH^-(aq) → [Pb(OH)4]^2-(aq)

    The precipitates of some insoluble hydroxides or oxides can be re-dissolved in excess ammonia solution if these precipitates can form soluble complexes in excess ammonia solution. Such precipitates commonly include zinc hydroxide, copper(II) hydroxide and silver oxide.

    Zn(OH)2(s) + 4NH3(aq) → [Zn(NH3)4]^2+(aq) + 2OH^-(aq)

    or: Zn(OH)2(s) + 4NH3(aq) → [Zn(NH3)4](OH)2(aq)

    Cu(OH)2(s) + 4NH3(aq) → [Cu(NH3)4]^2+(aq) + 2OH^-(aq)

    or: Cu(OH)2(s) + 4NH3(aq) → [Cu(NH3)4](OH)2(aq)

    Ag2O(s) + H2O(l) + 4NH3(aq) → 2[Ag(NH3)2]^+(aq) + 2OH^-(aq)

    or: Ag2O(s) + H2O(l) + 4NH3(aq) → 2[Ag(NH3)2]OH(aq)

    資料來源: 老爺子