Identifying Unknown Solutions in Chem Lab: NH3 + CuSO4 and NaI + CuSO4 Reactions

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Discussion Overview

The discussion revolves around identifying unknown solutions in a chemistry lab, specifically focusing on the reactions involving NH3 with CuSO4 and NaI with CuSO4. Participants explore the expected products, colors, and states of the precipitates formed in these reactions.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant presents the reactions involving NH3 and CuSO4, and NaI and CuSO4, questioning the correctness of the equations and the observed precipitates.
  • Another participant affirms the color of Copper(II) Sulfate and suggests showing oxidation states to explain color variations in metals.
  • A subsequent reply questions the need for a plus sign between Cu(NH3)4 and SO4 and asks about the state of I2 in the second reaction.
  • Some participants argue that iodine is brown and discuss its properties, including its state and role as an indicator.
  • Another participant challenges the expectation of precipitation in the first reaction, suggesting that the observed blue precipitate may be due to a side reaction with hydroxide ions.
  • There is a claim that iodine is purple, not brown, and that the expected outcome of the second reaction should be a clear solution with a white precipitate of CuI.
  • One participant questions the source of hydroxide ions in the discussion and suggests that neither reaction should produce a precipitate.

Areas of Agreement / Disagreement

Participants express differing views on the expected outcomes of the reactions, particularly regarding the formation of precipitates and the colors of the products. There is no consensus on the correctness of the proposed reactions or the states of the products.

Contextual Notes

Some participants note potential misbalancing of equations and the role of oxidation states in determining colors, but these points remain unresolved. The discussion includes assumptions about the reactions that are not explicitly stated.

ldixon
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Chem Lab "ID of Unknowns"

I had to find 5 unknown solutions in a recent chem lab and now I am doing the equations, but I am not sure about two of them:

Deep blue percipitate
NH3 (aq) + CuSO4 (aq) -------> Cu(NH3)4 SO4 (s)

brown heavy percipitate
4NaI (aq) + 2CuSO4 (aq) -------> 2CuI (s) + I2 (s) + 2Na2SO4 (aq)

Are these reactions correct? I got percipitates for both of them.
 
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Copper(II)Sulphate is a blue color, crystalises and often used to grow crystals. As is Copper(II)Nitrate.

looks good :)

You should show the oxidation states of the elements ( the metals ) because it is the key to the color. Metals in different oxidation states are often different colors. This is why they are used extensivley for titrations as dyes & indicators.
 
Last edited:
Deep blue percipitate
NH3 (aq) + CuSO4 (aq) -------> Cu(NH3)4SO4 (s)

Should there be a + between the Cu(NH3)4 and SO4?

brown heavy percipitate
4NaI (aq) + 2CuSO4 (aq) -------> 2CuI (s) + I2 (s) + 2Na2SO4 (aq)

Is the I2 solid or aqueous? What gives the brown color?
 
No, i don't think so. Its a crystalline salt of copper, and its solid. The ions are packed in a lattice.

Iodine is brown (and i think aqueous? I think you have mis-balenced that equation, not sure), the color comes from the metals electronic structure, absorption of photons of certain energies and not others means that it appears brown, it acts like a color filter.

Changing the oxidation state of the metal or the electronic configuration through complex formation changes the frequency of photons that it absorbs, and so too its color.
 
3trQN said:
No, i don't think so. Its a crystalline salt of copper, and its solid. The ions are packed in a lattice.

Iodine is brown (and i think aqueous? I think you have mis-balenced that equation, not sure), the color comes from the metals electronic structure, absorption of photons of certain energies and not others means that it appears brown, it acts like a color filter.

Changing the oxidation state of the metal or the electronic configuration through complex formation changes the frequency of photons that it absorbs, and so too its color.

Iodine is brown, actually a very deep brown liquid. You are correct
 
Stevedye56 said:
Iodine is brown, actually a very deep brown liquid. You are correct

isn't iodine some kinda indicator like phenothayleine (mind the spelling) ?
 
For the first reaction: you should not be expecting any precipitation - tetra-amino cuprate is indeed deep blue in colour but water soluble. The blue precipitate observed was a side reaction of Cu2+ with OH- as the solution become more and more basic on the addition of NH3:

Cu2+(aq) + OH-(aq) -----> Cu(OH)2(s)

Which is a also blue in colour.For the latter reaction:

Something's werid... You should not expect a brown colour because none of the species in the reaction is brown. Iodine is purple, not brown (well, bromine is brown...). There should have been nothing more than a clear solution with white precipitate of CuI(s).
 
Where did you get the OH- from? So the neither one of these reactions should be a percipitate?
 

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