Diluting weak acids

I'm so confused about what happens when you dilute weak acids.

HA + H₂O ->/<- H₃O⁺ + A-
I have in my notes (which I took from a mark scheme) that diluting a weak acid shifts the above equilibrium to the right and so the acid dissociates more, increasing the H+ concentration and therefore decreasing the pH.

But I just did a past paper (Edexcel CH04 January 2012)
The bubble bath ‘Colour Change Matey’ has amongst its ingredients the weak acid
benzoic acid, as well as the indicator bromocresol green. When it is added to bath
water, its colour changes from yellow to blue.

Use the data booklet and value for pH of 0.0025moldm^-3 of benzoic acid why the bubble bath changes colour when it is diluted by being added to the bath
water.

For reference:
pH of benzoic acid: 3.4
Bromocresol green: pKIn 4.7, yellow in acidic, blue in alkali. pH range is 3.8—5.4.

Mark scheme:
- (pH) range (of indicator) 3.8 to 5.4 OR pKin= 4.7
- Bubble bath is (initially yellow since) pH less than 3.8 / is 3.4
-Adding of water/dilution (of acid) causes pH to rise/means [H+] decreases
-Hence pH rises to ≥ 5.4 so blue/changes colour

So this question is saying that diluting it decreases [H+] and therefore increases the pH?

:s-smilie:

Thanks

Reply 1

Pigster

20

Diluting doesn't affect [H2O] by much - pure water is about 55 mol dm-3, adding a little weak acid won't change this much, so the equilibrium might shift slightly to the right, but not far.

Far more importantly, [H+] will decrease since you're diluting it, hence pH will rise.

Reply 2

Borek

4

Equilibrium shifts to the right, but pH goes up.

While higher fraction of the acid is dissociated, this is countered by dilution of the dissociation products.

See how pH goes up when the acetic acid is diluted from 1 M to 10^-6 M:

Reply 3

blossomx

OP

20

Original post by Pigster

Diluting doesn't affect [H2O] by much - pure water is about 55 mol dm-3, adding a little weak acid won't change this much, so the equilibrium might shift slightly to the right, but not far.

Far more importantly, [H+] will decrease since you're diluting it, hence pH will rise.

Original post by Borek

Equilibrium shifts to the right, but pH goes up.

While higher fraction of the acid is dissociated, this is countered by dilution of the dissociation products.

See how pH goes up when the acetic acid is diluted from 1 M to 10^-6 M:

I see! Thank you! :smile:

Diluting weak acids

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