Anybody good with equilibrium reactions?

Question

Consider an unknown indicator, HInd. When you add sodium hydroxide (NaOH) to the indicator solution, it turns red. When you add hydrochloric acid (HCl) to the indicator, it turns yellow. What color are HLnd and Ind? Explain using a balanced chemical reaction.

This is one of my questions on my pre-lab report, and I don't understand how to approach this problem. I know that the general form for acid-base indicators is HInd (aq) <--> H+ + Ind- , but where do I go from here?

Thanks in advance!

Answer 1

The indicators are in themselves weak acids. They dissociate:

HInd ↔ H+ + Ind-

You will see that this is an equilibrium reaction.



Now you add some HCl to the indicator solution: You increase the {H+] because the HCl disociates to produce a lot of H+ ions, Because you now have excess H+ ions in solution, the equilibrium:



HInd ↔ H+ + Ind-



is shifted to the left. More HInd is formed. In this case HInd, the undissociated indicator molecule, has a yellow colour, so the solution turns yellow.



Now you repeat the experiment, but you add NaOH to the indicator solution. This adds a large excess of OH- ions to the solution. This causes the equilibrium

HInd ↔ H+ + Ind-

to shift to the right- the OH- ions combine with the H+ ions causing more and more indicator to dissociate until the equilibrium is reestablished. This means that you now have a high concentration of Ind- ions in solution. The Ind- ion has a red colour, so the solution turns red.

Answer 2

HInd + NaOH -> Na+ + Ind- + H2O



So Ind has red colour.



Ind- + HCl -> HInd + Cl-



So HInd has yellow colour.



(It is considered that the indicator in aqueous phase exist as HInd <-> H+ + Ind-)

Answer 3

Berkeley Summer Session ? lol ..