The Student Room Group
Reply 1
because the bonds are harder to break and so the reaction is more exothermic....

I have no idea what the answer is, I'm just answering the question like Simon told me too!
Reply 2
Hoofbeat
because the bonds are harder to break and so the reaction is more exothermic....

I have no idea what the answer is, I'm just answering the question like Simon told me too!

Go back to your rocket :rolleyes:
Reply 3
...ssssshhhhh I've almost done it honest! But there are some maths questions on their way no doubt! :p:
Reply 4
1st thing is about ionic strength. It's the measure of total effect of all the ions in solution. It's the sum of concentration*square of valency of all ions in solution.
2nd thing is about Ka.
Ka = a[R-].a[H+]/a[RH] where a[X] is effective concentration, i.e activity of X.
a[X] = activity coefficient of X*[X]
But activity coefficient depends on the radius of the ion, valency, and the total ionic strength of solution.

As the ionic strength increases, the activity coefficient decreases (I think there is a formula for the relation between them). So Ka will change due to this change.
Reply 5
When the ionic strength rises activity coefficients go down. Thus larger concentrations of ions are required to reach the equilibrium, so Ka calculated using concentrations will rise as a function of the ionic strength.

But that's putting things on the head instead of feets. If Ka is defined and calculated using activities from the start it is constant and doesn't change with the ionic strength. AFAIK published values of Ka are calculated using activities (or interpolated for the zero ionic strength, which yields to the same - zero ionic strength means all activity coefficients equal 1).

Best,
Borek
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Reply 6
Yeah thanks - I worked it out eventually. It was just the way the question was worded that confused me. It was pKa's measured using concentrations they were asking about - but I thought they were asking about the official pKa - which, as you said, shouldn't change.

Thanks to both of you for your help - will rep you both when I get round to it!