Addition of species on both sides of an equilibrium reaction

[stonecold]

You guys are just saying that when you add an acid, you forget that the solvent is water, and just treat it like H+ has been added right?

But if you add straight water then this counts as reactants, hence greater [h2o], and therefore equilibrium will shift to the right and favour H+

I think...

[fady_22]

You guys are just saying that when you add an acid, you forget that the solvent is water, and just treat it like H+ has been added right?

But if you add straight water then this counts as reactants, hence greater [h2o], and therefore equilibrium will shift to the right and favour H+

I think...

It makes more sense to think of adding water as a dilution, and not as the addition of a reactant, because technically it isn't a reactant-- it doesn't directly change the value of Ka, but rather decreases the concentration of all other species, which in turn changes the Ka value.

In this question, you have to remember that "a small amount" is added, and so dilution is not significant here. If the question had said "1 L of 0.1 M HCl is added to a solution of 0.1 M ethanoic acid" then it would be difficult to predict the direction of the net reaction, either to oppose dilution or to oppose the addition of H+.

[stonecold]

thanks. probably better to just discount the water in the reaction all together...

[kenhung123]

Wow this is so confusing...but wouldn't small amount also imply [H+] is insignificant if [H2O] is also :S
I always though % ionisation increases as H2O increase, reaction shifts to the right.