Equilibrium and indicator equation

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Discussion Overview

The discussion revolves around the equilibrium reaction of an acid-base indicator, specifically the equation HIn ⇌ H+ + In-. Participants explore the effects of increasing alkali on the position of this equilibrium, questioning the implications of such changes and the interpretations presented in course materials.

Discussion Character

  • Debate/contested
  • Conceptual clarification
  • Homework-related

Main Points Raised

  • One participant suggests that increasing alkali should favor the reverse reaction, leading to an increase in HIn as it reacts with H+ to minimize the change.
  • Another participant questions how the reverse reaction would minimize the effect of the change and how [In-] could increase by reacting with H+.
  • A third participant points out a lack of clarity regarding what is meant by "alkali," asking whether it refers to an alkali salt or an alkali base.
  • A later reply reiterates the initial question and expresses confusion about the terminology used in discussing equilibrium, suggesting that the concept of equilibrium is often misunderstood.
  • This reply also encourages the original poster to consider the equilibrium constant and the effects of adding OH- to the solution, implying that this addition will affect the equilibrium dynamics.

Areas of Agreement / Disagreement

Participants express differing views on the effects of increasing alkali on the equilibrium position, with no consensus reached on the correct interpretation of the changes involved.

Contextual Notes

There is ambiguity in the term "alkali," which could refer to different substances, and participants have not fully clarified the assumptions behind their arguments regarding the equilibrium shift.

haha0p1
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The equation for indicator is:
HIn ⇌H+ + In-
Which reaction (forward or reverse) will be preferred when I increase the Alkali.
According to me, The reverse reaction should be preferred as the reaction will try to minimize the effect of the change. Hence In- (i.e the conjugate base) will be increased as it will react with H+ to form HIn.
But the coursebook says that the position of the equilibrium will move to the right. Kindly explain why is that so?
 

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How would the reverse reaction minimise the effect of the change?
How is [In-] increased by reacting with H+?
 
Not enough information to answer. Alkali what? Alkali salt? Alkali base?
 
haha0p1 said:
The equation for indicator is:
HIn ⇌H+ + In-
Which reaction (forward or reverse) will be preferred when I increase the Alkali.
According to me, The reverse reaction should be preferred as the reaction will try to minimize the effect of the change. Hence In- (i.e the conjugate base) will be increased as it will react with H+ to form HIn.
But the coursebook says that the position of the equilibrium will move to the right. Kindly explain why is that so?
The question should be in the homework section. I think it is a good question, deserving some attention by chemistry teachers. I think when I first met this subject at school I felt confused, the reason being what I saw as ambiguous expressions like "the equilibrium will move to the right" I was thinking of equilibrium as weights on a lever… but never mind.

Help yourself by writing out the equation for the equilibrium constant relating [HIn] to the dissociation products molarities [H+] and [In-]. "Adding alkali" means adding something like NaOH, I.e. essentially adding OH- to the solution, which will react with H+ there, and directly or indirectly with HIn (it matters not which) to do - what? So think again through what will happen – you have got it wrong.

I think it is better to work out your own way of thinking and talking about these things (which you are likely to meet again again) rather than someone else's way of expressing it.
 

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