What Side of the Bromothymol Blue Equilibrium is Favored?

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Discussion Overview

The discussion revolves around the equilibrium of bromothymol blue, an acid-base indicator, specifically questioning which side of the equilibrium is favored when the solution is in the bottle. The scope includes theoretical understanding and application of acid-base chemistry.

Discussion Character

  • Homework-related
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant suggests that the equilibrium favors the reactants because the solution is blue, indicating a low concentration of H+ ions.
  • Another participant states that bromothymol blue is a weak acid with a pKa around 7, implying a specific behavior in equilibrium.
  • A third participant challenges the relevance of the initial equation provided, hinting at a different interpretation of the equilibrium.
  • Further discussion questions the nature of bromothymol blue as an acid despite its blue color, emphasizing that being an acid does not necessarily correlate with low pH solutions.
  • Another participant clarifies that bromothymol blue is a weak acid that dissociates and has a conjugate base, with the color change depending on the pH of the solution.

Areas of Agreement / Disagreement

Participants express differing views on the interpretation of the equilibrium and the properties of bromothymol blue, indicating that multiple competing views remain and the discussion is unresolved.

Contextual Notes

There are limitations regarding the assumptions about the concentration of bromothymol blue in solution and the definitions of acidity and pH in this context. The relevance of the equations provided is also questioned without resolution.

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Homework Statement


Bromothymol blue is an acid-base indicator. When in solution, it forms the following equilibrium:
HInd(aq) ⇌ H+(aq) + Ind-(aq)
What side of the equilibrium is favored while the solution is in the bromothymol blue bottle?

Homework Equations



Keq = [(C)c(D)d]/[(A)a(B)b]

The Attempt at a Solution



I think the equilibrium favors the reactants because since it is blue, then bromothymol blue is a base so there is small amounts of H+ ions so the K expressions is a small number divided by a bigger number which results in a small number.

Can anyone confirm that my thinking is correct? Thanks!
 
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Yes. Bromothymol blue is a weak acid, with pKa around 7.
 
The equation you give is not the relevant one. The right one would also suggest answer.
 
Thank you, but why is Bromothymol Blue an acid even though it is blue?
 
Badgeray said:
Thank you, but why is Bromothymol Blue an acid even though it is blue?

You are mistaking several things at the same time. Being an acid doesn't mean its solutions have low pH. First, it is a weak acid, second, its solutions - as used in the analytical chemistry - have very low concentration.

What is important is that - as a weak acid - it dissociates, it has a conjugate base (which has a different color than the acid itself), and the ratio of concentrations of both forms depends on the solution pH.
 

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