Identify the 2 metal cations through reactions

  • Thread starter Thread starter TT0
  • Start date Start date
  • Tags Tags
    Metal Reactions
Click For Summary

Discussion Overview

The discussion revolves around identifying two metal cations based on a series of chemical reactions involving sodium hydroxide and hydrochloric acid. Participants analyze the behavior of various metal ions in solution, focusing on their precipitates and color changes during the reactions.

Discussion Character

  • Homework-related
  • Exploratory
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • One participant suggests that the reddish-violet solution indicates the presence of Mn2+, which leads them to eliminate certain options for the second cation.
  • Another participant points out that the description of the experiment is unclear and notes the ambiguity between "reddish-violet" and "purple-violet" solutions.
  • A different participant infers that the colorless filtrate suggests the second cation is likely not a transition metal, proposing Mg2+ as a candidate based on the behavior of the precipitate.
  • One participant questions the sequence of precipitate formation and dissolution upon the addition of hydrochloric acid, prompting further exploration of the reactions involved.
  • Another participant introduces the concept of amphotericity, suggesting that zinc hydroxide may play a role in the observed reactions.
  • A participant attempts to outline the reaction equations involving zinc and expresses uncertainty about their correctness, suggesting that the first cation is Mn2+ and the second is Zn2+.

Areas of Agreement / Disagreement

Participants express differing interpretations of the experimental results, particularly regarding the identity of the metal cations. There is no consensus on the correct pair of ions, and multiple competing views remain throughout the discussion.

Contextual Notes

Participants highlight the importance of specific details in the experiment, such as the color of the solutions and the behavior of precipitates, which may affect their conclusions. The discussion reflects varying levels of understanding regarding amphotericity and solubility products.

TT0
Messages
210
Reaction score
3

Homework Statement


A coloured solution, known to contain two metal ions, was treated with excess cold sodium hydroxide solution. When filtered a whitish solid, slowly changing to brown, was retained on the filter paper and a colourless solution collected as the filtrate. Dropwise addition of hydrochloric acid to the filtrate produced a white precipitate which dissolved in excess acid. Treatment of the residue from the filter paper with a solution of a strong oxidiser produced a reddish-violet solution.

Indicate any pairs of ions which on testing as above leads to the observed changes.
  1. A Zn2+ and Mn2+ ions
  2. B Mg2+ and Zn2+ ions
  3. C Mn2+ and Mg2+ ions
  4. D Fe2+ and Zn2+ ions
  5. E Mn2+ and Fe2+ ions

Homework Equations

The Attempt at a Solution


"Treatment of the residue from the filter paper with a solution of a strong oxidiser produced a reddish-violet solution." I am pretty sure this indicates Mn2+ is present as I think it turns purple. This cancels out B and D. This means that the second cation when treated with excess NaOH becomes a whitish solid, slowly changing to brown. I think this is Fe as when exposed to air it rusts and turns brown. I am pretty sure this logic is wrong though. Could someone give me some advice?

Thanks!
 
Physics news on Phys.org
Please reread the experiment description, as you are ignoring a very important part of the information given (plus, you are confused about which cation changes color during the experiment).

Sadly, the description is not clear and I see two fitting answers. Matter of difference between reddish-violet and purple-violet solution.

200px-Ferrate_and_permanganate_solution.jpg
 
  • Like
Likes   Reactions: TT0
I think the information I am ignoring is that "a colourless solution collected as the filtrate". This probably means that the cation left in the filtrate is not a transition metal as I think they generally don't give colourless solutions. There are 4 cations that they give us: Zn2+, Mn2+, Mg2+, Fe2+. So I guess Mg2+ is the second cation.

Since excess NaOH is added in the first step, that means a zinc precipitate won't form so this leaves us with Fe2+ and Mg2+ as the first cation. Since the whitish solid turns brown, I am guessing it is Fe2+ as when iron oxidises, it rusts which is brown. There is no Fe2+ and Mg2+ option though so I guess I am wrong. This leaves us with Mn2+ and Mg2+ which is option C. Is this correct? If it is, could you give me your thought process as my logic seems terrible.

Cheers!
 
Think about the filtrate: why on addition of an acid does the white precipitate appear first, then disappears?
 
  • Like
Likes   Reactions: TT0
The filtrate contains the metal cation, hydroxide anions, and sodium cations. When a bit of HCl is added, I can't see any combination that will precipitate other than the metal hydroxide. However, the metal hydroxide is clearly soluble as it was part of the filtrate. That means when HCl is added the metal hydroxide becomes insoluble probably because of solubility products. I have no clue though. I have tried googling but can't find anything relevant. Could you please explain this to me?
 
Again, you are forgetting amphotericity.
 
  • Like
Likes   Reactions: TT0
I have googled about amphotericity. From the knowledge that the most common hydroxides that are amphoteric are zinc and aluminium hydroxide. So I guess the reaction before the addition of HCl, after addition of a bit of HCl and after addition of HCl is:

Before HCl: Zn2+ + 4OH- ↔ Zn(OH)42-
After dropwise HCl: Zn(OH)42- + 2H+ ↔ Zn(OH)2 + 2HCl2O
After excess HCl: Zn(OH)2 + 2H+ ↔ Zn2+ + 2HCl2O

Is this the correct reaction equations?

Since first cation is Mn2+ and second is Zn2+ is A the answer?

Thanks a lot!
 

Similar threads

Corrosion & pH: Explaining Observations of Iron Nail
  • · Replies 1 ·
Replies
1
Views
4K
What Is the Identity of Metal M in M(ClO)2 Based on Iodometric Titration Data?
  • · Replies 3 ·
Replies
3
Views
4K
Where did I go wrong? Back Titrations: Calculating % Antimony in Stibnite Ore
  • · Replies 5 ·
Replies
5
Views
7K
Graviemertic analysis of Shulphate content on Fertiliser
  • · Replies 3 ·
Replies
3
Views
5K