Chemical Forums
Chemistry Forums for Students => High School Chemistry Forum => Topic started by: Byrne on December 17, 2005, 04:47:10 PM
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In the following reaction, how do I predict whether the reactants and products will predominate at 25 degrees Celcius?
(i) HCO3- + HS- <-> H2CO3 + S2-
(ii) C2H3O2- + HNO2 <-> HC2H3O2 + NO2-
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Stronger acid protonates conjugated base of the weaker acid.
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'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.
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In other words - stronger acid will be dissociated, weaker will be not.
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(i) HCO3- + HS- <-> H2CO3 + S2-
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..
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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 KA value, so it's easier to determine which is the stronger acid.
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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.