Equilibrium constant rules help.

[madcat8000]

I was wondering where i could find a source for the rules to find Kc from a chemical formula? My professor tonight was changeing the formula like this: H2+Br2+2HBr then showed that 2H2+2Br2=4HBr...and getting different Kc values for each. I think that this is straight out wrong but she's the nice sort so i thought id just email her some proof instead of making an *** out of myself. If i remember correctly only lowest common denominator formula can be used and absolutely no fractions at all. Could i be wrong(doubt it).

 

[Borek]

You are right that we usually prefer lowest possible integer coefficients in the reaction equation. Trick is - it doesn't matter. While you will get different value for Kc, as long as you are consistent in the way you write reaction quotient, your final results - if you calculate equilibrium concentrations - will be the same. Same can be said about stoichiometric calculations - it is their ratio that is important, not absolute values of coefficients.

However, there is at least one important reason to follow the standard - you don't have to convert published Kc values, you always know what they mean, and when you publish them, others also know what you mean. It is like speaking the same language.