Understanding the Le Chatelier Principle: Impact of Adding NaSCN

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In summary, the conversation discusses the dissolution of Fe(SCN)_3 in water and the resulting ion dissociation. The resulting solution is red due to the presence of Fe(SCN)^{2+} and yellow due to SCN^- ions. The addition of NaSCN would result in more red solution due to the increase in Fe(SCN)^{2+} ions. However, the equilibrium constant remains the same and the color of the solution may be affected by the presence of acids or bases. The explanation for this is related to oxidation potential and saturation points of the iron adduct.
  • #1
PPonte
http://img15.imgspot.com/u/06/61/13/out1141410754.gif

Someone dissolved [tex]Fe(SCN)_3[/tex] in water, in the recipient A.

This ion dissociates. The chemical equation is:

[tex]Fe(SCN)^{2+}[/tex] -->* [tex]Fe^{3+}[/tex] + [tex]SCN^-[/tex]
[tex]Fe(SCN)^{2+}[/tex] is red.
[tex]SCN^-[/tex] is yellow.

* The reaction is incomplete.


Select the recipient that has the resulting solution of the adition of NaSCN to the initial solution.

To add NaSCN is the same to add SCN mononegative ions to the solution. The reaction will produce more reactants, in this case, [tex]Fe(SCN)^{2+}[/tex]. Therefore, the resulting solution would be more red. Recipient B. I can understand but I can't agree. The resulting solution when reaches the equilibrium should have the same colour since the proportion of the concentrations of products and the concentration of reactants does not change because the equilibrium constant is the same although the rate of the direct reaction, when I perturbate the system, is lower than the rate of the inverse reaction. Am I missing something?
 
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  • #2
[tex]Fe(SCN)^{2+}[/tex] One has to put the 2+ in {} -> {2+} in order to group the exponent.

And [tex]Fe^{3+}[/tex]

Solution A would have a mix of [itex]Fe^{2+}[/itex] and [itex]Fe^{3+}[/itex].

Anyway, the solution with [itex]Fe^{2+}[/itex] is red, which occurs if the pH is more acidic.

I presume C is mostly [itex]Fe^{3+}[/itex] and adding Na would make it more basic, IIRC.

Of course, I am stretching my memory back 30+ years. :rolleyes:
 
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  • #3
Sorry, you know I appreciate very much your help, but that is not the point. Please, forget acids and bases. Remember Le Chatelier Principle:
If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium moves to counteract the change.

Follow my thinking and comment, please.

To add NaSCN is the same to add SCN mononegative ions to the solution. The reaction will produce more reactants, in this case, LaTeX graphic is being generated. Reload this page in a moment.. Therefore, the resulting solution would be more red. Recipient B. I can understand but I can't agree. The resulting solution when reaches the equilibrium should have the same colour since the proportion of the concentrations of products and the concentration of reactants does not change because the equilibrium constant is the same although the rate of the direct reaction, when I perturbate the system, is lower than the rate of the inverse reaction. Am I missing something?
 
  • #4
What I was trying to say is that Solution A contains a mix of FeSCN2+ (red) and Fe3+ (colorless) ions, and SCN-(yellow).

Solution A is orange or reddish-yellow because one 'sees' the color of both FeSCN2+ (red) and SCN-(yellow), because there sufficient Fe3+ (colorless) ions to allow for sufficient SCN- to show yellow.

Solution B is mostly FeSCN2+ (red) with equilibrium SCN-. The red dominates the yellow.

Solution C is mostly Fe3+ (colorless) ions, and SCN-(yellow). The Na drives the reaction from FeSCN2+ (red) to Fe3+ (colorless), and there is SCN-(yellow).

See http://www.polaris.nova.edu/~pomeroy/LE_CHAT.html particularly 3. A Complex Ion Equilibrium (Fe thiocyanide).
 
  • #5
The Na drives the reaction from FeSCN2+ (red) to Fe3+ (colorless), and there is SCN-(yellow).

How? To add NaSCN is to increase the concentration of SCN-. And if I increase the concentration of the products, the system will react in order to decrease the concentration of products consuming them and forming more reactants: FeSCN2+(red). Therefore, the solution is reddish.
 
  • #6
PPonte said:
How? To add NaSCN is to increase the concentration of SCN-. And if I increase the concentration of the products, the system will react in order to decrease the concentration of products consuming them and forming more reactants: FeSCN2+(red). Therefore, the solution is reddish.
Possibly because Na promotes OH-, which would tend to favor Fe3+ - IIRC, it has to do with oxidation potential.

See - of the Equilibrium Constant
for the Formation of FeSCN2+[/url]


One could add thiocyanic acid to the solution, but that would tend to promote FeSCN2+
 
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  • #7
Astronuc said:
Possibly because Na promotes OH-[/url], which would tend to favor Fe3+ - IIRC, it has to do with oxidation potential.

I didn't study yet oxidation power. But even if Na promotes OH- it does not react, since the chemical equation is:

[tex]Fe(SCN)^{2+}[/tex] --> [tex]Fe^{3+}[/tex] + [tex]SCN^-[/tex]

But if you could explain how... Thanks :smile:

See - Determination of the Equilibrium Constant
for the Formation of FeSCN2+
It requires some advanced knowledges. Wikipedia gave a hand.

One could add thiocyanic acid to the solution, but that would tend to promote FeSCN2+
Yes. Because to add HSCN is to add SCN-. Correct?
 
  • #8
You can think of the iron adduct having a "saturation" point, as you may have learned with some relatively insoluble solids. If you add any of the ion components, there's going to be more precipitation. It's pretty much the same case here, at least, pertaining to what the problem is trying to illustrate.
 
  • #9
I am trying to find the explanation of why pH or the presence of Na vs H would affect the ionized state of Fe. I think it is somewhat like chromate/dichromate equilibrium, which is pH dependent.
 

FAQ: Understanding the Le Chatelier Principle: Impact of Adding NaSCN

What is the Le Chatelier Principle?

The Le Chatelier Principle is a concept in chemistry that states that when a system at equilibrium is subjected to a stress, it will adjust in a way that minimizes the effect of that stress and returns the system to equilibrium.

How does the Le Chatelier Principle apply to adding NaSCN?

When NaSCN is added to a system at equilibrium, it acts as a source of the SCN- ion. According to Le Chatelier's Principle, this will shift the equilibrium towards the formation of more Fe(SCN)2+ in order to minimize the effect of the added SCN- ions.

What is the overall impact of adding NaSCN on the equilibrium system?

The addition of NaSCN will cause the equilibrium to shift towards the formation of more Fe(SCN)2+, resulting in an increase in the concentration of this species and a decrease in the concentrations of Fe3+ and SCN- ions.

How does the concentration of added NaSCN affect the equilibrium system?

The concentration of added NaSCN will determine the extent to which the equilibrium shifts towards the formation of more Fe(SCN)2+. A higher concentration of NaSCN will result in a greater shift towards the product side of the equilibrium.

Can the Le Chatelier Principle be applied to other chemical reactions?

Yes, the Le Chatelier Principle can be applied to any reversible reaction in which the reactants and products are in equilibrium. It can also be applied to physical processes such as changes in pressure, temperature, or volume.

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