Interesting chemistry exam questions - 4: Why does adding a base to some salt solutions cause precipitation to occur and then disappear, over and over again? Answer already exists
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This post was last edited by yjqin1 on 2015-8-24 21:11. There is an aqueous solution containing 10% ammonium chloride, as well as 0.1% of copper(II) chloride, ferrous(II) chloride, chromium(III) chloride, or nickel(II) chloride. Please note: it is “or” not “and”. When you add a very concentrated sodium hydroxide solution to it, a precipitate forms once a certain amount has been added ； As the concentrated alkali solution was added, it was observed that the precipitate gradually disappeared, eventually vanishing completely ； It’s not over yet; as more concentrated alkali solution is added, precipitates start to form again, and more and more of them appear ； Upon adding more concentrated alkali solution, the precipitate disappeared again. In the case of a mixed solution of 10% ammonium chloride and 0.1% magnesium chloride or ferric chloride(III), precipitation occurs as soon as concentrated alkali is added, and it does not disappear again. Please explain why there is this phenomenon of precipitation, the disappearance of the precipitation, further precipitation, and then the disappearance of that further precipitation.
These are copper complexes, nickel complexes, or chromium complexes formed at different pH values, whereas iron and magnesium cannot form complexes
What an interesting inorganic chemistry exam question – it’s surprising that not many people are interested in it. Standard answer: Take a mixed solution of copper chloride and ammonium chloride as an example. Copper ions can complex with ammonia as well as with hydroxide ions. There is a quantitative relationship. The solubility of copper hydroxide is very low; generally, when the pH is raised to 5-6 by adding alkali, significant precipitation of copper hydroxide occurs. At this pH value, ammonia exists almost entirely in the form of ammonium ions; thus, as a weak base, ammonium ions have a pKa of 9.2. When more sodium hydroxide is added, for example at pH values of 8.5–10.0, a considerable amount of ammonia exists in its free form. This free ammonia combines with copper ions to form soluble copper-ammonia complexes, which further reduces the concentration of dissolved copper ions and thus leads to the dissolution of the copper hydroxide precipitate. As more sodium hydroxide is added, the pH of the aqueous solution rises to 10 or even above 11; almost all ammonium ions are converted into free ammonia, and an adequate amount of free ammonia allows copper hydroxide to dissolve completely. However, as more sodium hydroxide is added, the concentration of hydroxide ions increases further, and free ammonia can no longer compete with these hydroxide ions; as a result, copper hydroxide precipitate begins to form again. However, when more sodium hydroxide is added, raising the concentration of hydroxide ions to over 5 mol/L, copper ions complex with the hydroxide ions to form soluble [Cu(OH)3]2- ions, so the solution becomes clear again. Speaking of which, these repeated precipitation-dissolution phenomena stem from the complexing properties of copper ions with ammonia, as well as their complexing properties with hydroxide ions; in other words, copper ions possess amphoteric properties. Trivalent chromium, divalent iron, and divalent nickel ions are all amphoteric. The two most well-known metal elements are aluminum and zinc, precisely because they have a stronger tendency to form complexes with hydroxide ions. Freud considered most people to be mentally ill, while chemists regard a considerable number of metal ions as amphoteric ions.
The teacher’s perseverance is truly admirable. The participants in the forum come from various backgrounds, and due to work-related constraints, the discussions tend to be more practical in nature. Moreover, the recent downturn in the industry has affected the forum as well, resulting in a less vibrant atmosphere for exchanges. I hope the teacher will continue to share more knowledge