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Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Byrne on December 17, 2005, 04:47:10 PM

Title: Acids and Bases: how do I predict whether reactants or products will dominate
Post by: Byrne on December 17, 2005, 04:47:10 PM: 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^-
Title: Re:Acids and Bases...
Post by: Borek on December 17, 2005, 07:11:20 PM: Stronger acid protonates conjugated base of the weaker acid.
Title: Re:Acids and Bases...
Post by: Byrne on December 18, 2005, 12:25:32 AM: '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.
Title: Re:Acids and Bases...
Post by: Borek on December 18, 2005, 05:20:10 AM: In other words - stronger acid will be dissociated, weaker will be not.
Title: Re:Acids and Bases...
Post by: Donaldson Tan on December 18, 2005, 08:22:10 AM: Quote from: Byrne on December 17, 2005, 04:47:10 PM
(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..
Title: Re:Acids and Bases...
Post by: Byrne on December 18, 2005, 04:19:10 PM: 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.
Title: Re:Acids and Bases...
Post by: Borek on December 18, 2005, 04:51:25 PM: 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 (http://www.chembuddy.com/?left=BATE&right=dissociation_constants) link, or visit google.