Friday, April 21, 2017

equilibrium - Does the number of H+ ions in solution go up on dilution of a weak acid?


In my textbook, a footnote says:



In case of weak acids, on dilution the total number of HX+ ions in solution increases because dissociation of the weak acid increases



This didn't make sense to me however. Consider the general dissociation reaction of a weak acid:


HA(aq)+HX2O HX3OX+(aq)+AX(aq)


Now, the Chatelier's principle tells us that if we add more reactants (HX2O), the reaction will proceed more to the right, and so the dissociation of the acid will increase, but the part about the total number of HX+ ions in water being higher was still unclear to me.


Delving into the math a bit, we can see that



Ka=[HX3OX+][AX][HA]


Since the concentrations of the two products are equal,


[HX3OX+]=[HA]Ka


Now, let us assume that we have a 1 M solution of the acid in question dissolved in 1 litre of water, plugging this into the equation, we have a Ka molar solution of H+ ions. Since only 1 litre of solvent is there, we have Ka moles of ions.


Now, say we dump 1 litre of water into this solution. The concentration of our acid will be halved. Plugging this into the equation, we get the concentration of HX3OX+ ions as approx. 0.7Ka M. We have two litres of solvent, so we have 1.4Ka moles of ions, which is considerably higher. So, the numbers all work out, but what's the intuition behind it?




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