[Byrne]

In the following reaction, how do I predict whether the reactants and products will predominate at 25 degrees Celcius?

(i) HCO₃^- + HS^- <-> H₂CO₃ + S^2-

(ii) C₂H₃O₂^- + HNO₂ <-> HC₂H₃O₂ + NO₂^-

[Borek]

Stronger acid protonates conjugated base of the weaker acid.

[Byrne]

'protonates?'

I don't think I've ever heard that term before, but I'm assuming it means the stronger acid donates a proton to the conjugate base of the weaker acid.  I still don't understand how this helps me determine whether the reactants or products will dominate.

[Borek]

In other words - stronger acid will be dissociated, weaker will be not.

[Donaldson Tan]

(i) HCO₃^- + HS^- <-> H₂CO₃ + S^2-

Which is the stronger acid? bicarbonate or hydrogen sulphite?

since carbonate anion is a stronger base than the sulphite anion, then HS^- must be a stronger acid than HCO₃^-, therefore the bicarbonate anion will be protonated.

Try to follow this line of reasoning for the 2nd reaction..

[Byrne]

Okay, I think I understand how to determine which side the reaction favors if you know which acid is stronger, but how do you determine that?  Normally we are given the K_A value, so it's easier to determine which is the stronger acid.

[Borek]

And if you are not given their values you are hopelessly lost? There are tons of books, links, sites with these values. Check on the left or follow Ka and Kb link, or visit google.