How Does Adding Water Affect the Color Change in CaCl2 . 6H2O + HCl Solution?

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Discussion Overview

The discussion revolves around the effects of adding water to a solution of CaCl2 . 6H2O and HCl, particularly focusing on the resulting color change and the implications for chemical equilibrium. Participants also explore the effects of various reagents on the equilibrium of a mixture of FeCl3 and KSCN, examining how these changes influence color and equilibrium shifts.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested
  • Mathematical reasoning

Main Points Raised

  • Some participants question whether the initial solution was CaCl2 or CoCl2, indicating potential confusion about the chemical species involved.
  • There is a suggestion that the color change from purple to pink upon adding water may be due to dilution, although some participants propose that it could also represent a shift in equilibrium.
  • One participant mentions that adding water could be considered a source of "stress" to the system, prompting questions about the direction of any equilibrium shift.
  • In discussing the mixture of FeCl3 and KSCN, participants note that adding KCl likely results in dilution, while the addition of KSCN is expected to push the reaction forward due to its presence as a reactant.
  • Participants discuss the formation of various colored complexes in the Fe3+/SCN- equilibrium and the potential reactions involving OH- when NaOH is added, suggesting that mixed complexes may form.
  • There is mention of stability constants for iron thiocyanate and iron chloride complexes, indicating that thiocyanate complexes are more stable, which may influence the observed color changes.
  • One participant expresses uncertainty about how to investigate the effects of reagents on equilibrium reactions, particularly regarding the role of KSCN.
  • Another participant shares their reasoning about the bonding of Fe3+ with OH-, suggesting that the absence of precipitate indicates the presence of mixed complexes.

Areas of Agreement / Disagreement

Participants express differing views on whether the color change is solely due to dilution or if it indicates a shift in equilibrium. There is no consensus on the effects of adding various reagents to the equilibrium of the FeCl3 and KSCN mixture, with multiple competing perspectives presented.

Contextual Notes

Participants reference the need to consider stability constants and side reactions, indicating that the discussion may involve complex interactions that are not fully resolved. The influence of dilution versus equilibrium shifts remains a point of contention.

Who May Find This Useful

This discussion may be of interest to students and educators in chemistry, particularly those exploring chemical equilibria, complex formation, and the effects of dilution on solution chemistry.

dekoi
(Excuse spelling of thread name).
1.) If i originally had CaCl2 . 6H20 + HCl solution (which is deep purple in color), then added distilled water which caused a rose pink color, then:

Is there a source of "stress"? That is, is adding water also adding stress to a system? Or is the change in color from purple to pink simply a matter of dilution (water causes the solution to dilute, thus making the purple color faint and hence pink)?

Also, if there is in fact a stress, what is the direction of the equilibrium shift?

2.) If i have a mixture of 0.2 M FeCl3 and 0.2 M KSCN (mixed together) in four beakers, and did the following to each:
a.) Add KCl.
b.) Add Fe(No3)3
c.) Add KSCN.
d.) NaOH
Then what would be the stress in each and the direction of the equilibrium shift? For a.), the color was changed from light amber to an even lighter amber. For b.) the color was changed to red. For c.) darker red. And for d.) Lemon-yellow.

I guess my second question is a more general one. I don't seem to understand how to investigate the effects of the reagents on the equilibrium reaction. The only one i seem to understand is KSCN, because KSCN is in fact part of the original reactants; thus, adding KSCN will contribute to the forward reaction.

Thank you.
 
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Are you sure it was CaCl2, not CoCl2?

Second question is not an easy one..

a - probably just dilution, although Cl- is a complexing agent too
b & c - changes in Fe3+/SCN- equilibrium, there are six different complexes in this solution, all colored.
d - Fe3+ reacts with OH-, as long as Fe(OH)3 doesn't precipitate I will suspect some mixed complexes Fe(SCN)n(OH)m are created.

To really understand what is going on in the solutions you should compare all stability constants to see what side reactions are taking place. This is not a thing you want to do by hand...

Chemical calculators for labs and education

BATE - pH calculations, titration curves
CASC - concentration conversions, solution preparation
 
Borek said:
Are you sure it was CaCl2, not CoCl2?

Second question is not an easy one..

a - probably just dilution, although Cl- is a complexing agent too
b & c - changes in Fe3+/SCN- equilibrium, there are six different complexes in this solution, all colored.
d - Fe3+ reacts with OH-, as long as Fe(OH)3 doesn't precipitate I will suspect some mixed complexes Fe(SCN)n(OH)m are created.

To really understand what is going on in the solutions you should compare all stability constants to see what side reactions are taking place. This is not a thing you want to do by hand...


Borek
--
Chemical calculators for labs and education
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BATE - pH calculations, titration curves
CASC - concentration conversions, solution preparation
Yes, i meant CoCl2. Sorry for the confusion.

Your second answer regarding KCl and its dilution of the solution seems to be in accord with my lab results. However, i don't understand how you determined that it will only dilute the solution and not truly effect the equilibrium. Would you mind sharing?

Also, how did you determine that Fe3+ will bond with OH- in d.) ?
 
1.) If i originally had CaCl2 . 6H20 + HCl solution (which is deep purple in color), then added distilled water which caused a rose pink color, then:

Is there a source of "stress"? That is, is adding water also adding stress to a system? Or is the change in color from purple to pink simply a matter of dilution (water causes the solution to dilute, thus making the purple color faint and hence pink)?

Also, if there is in fact a stress, what is the direction of the equilibrium shift?

ah, La Chatelier lab.

[tex]Co(OH_2)_6^{2+}_{(aq)} + 4Cl^{-} \rightleftharpoon Co(Cl)_4^{2-}_{(aq)} + 6H_2O[/tex]

adding water to the system, towards what direction do you think it will shift towards, the left or the right? Which rate will increase further, the forward or reverse? It's not dilution in this case, because the water will actually react, it's consumed.

2.) If i have a mixture of 0.2 M FeCl3 and 0.2 M KSCN (mixed together) in four beakers, and did the following to each:
a.) Add KCl.
b.) Add Fe(No3)3
c.) Add KSCN.
d.) NaOH
Then what would be the stress in each and the direction of the equilibrium shift? For a.), the color was changed from light amber to an even lighter amber. For b.) the color was changed to red. For c.) darker red. And for d.) Lemon-yellow.

I guess my second question is a more general one. I don't seem to understand how to investigate the effects of the reagents on the equilibrium reaction. The only one i seem to understand is KSCN, because KSCN is in fact part of the original reactants; thus, adding KSCN will contribute to the forward reaction.

you can think of disturbance in terms of there influence on the forward and reverse reactions.

why don't you write out the equation first and do a similar analysis to the one I did above. It's always the first step, observe the equation.
 
Last edited:
dekoi said:
Your second answer regarding KCl and its dilution of the solution seems to be in accord with my lab results. However, i don't understand how you determined that it will only dilute the solution and not truly effect the equilibrium. Would you mind sharing?

Stability constants for iron thiocyanate complexes are much larger then for iron chloride complexes - log of first constant (for FeL(2-) complex, where L stands for ligand) is 3.02 for thiocyanate and 1.48 for chloride, log of second constants are 4.64 and 2.13 respectively.

Also, how did you determine that Fe3+ will bond with OH- in d.) ?

Call it chemical intuition :)

More seriosuly - Fe(OH)3 precipitate easily from the solutions. If you don't see precipitate, but you see some color change there is only one explanation - thiocyanate are complexing strong enough to not allow precipitation (yet). But the color have changed, so the thiocyanate complex is not dominating in the solution. Some mixed complex with both OH- and SCN- ligands will be the easiest way to explain the situation. But that's only guess!

Chemical calculators for labs and education

BATE - pH calculations, titration curves
CASC - concentration conversions, solution preparation
 

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