Le chatelier in acid

[yjj8817]

I just watched wikipremed on acid and base and they said that if you add acid, because it disturbs water's equilibrium (h2o <-> h3o+ + oh-) the oh- concentration will decrease to return to the Kw. As the Oh- decreases and reaction goes to left, shouldn't the h3o+ concentration also decrease? What happens to h3o+ concentration as the reaction returns to Kw? Thanks!

---

Members don't see this ad.

 

[Cawolf]

By adding acid you are essentially increasing the hydronium concentration. The qualitative decrease in hydroxide ion occurs through a reaction with hydronium to form water.

So you are correct that the hydronium concentration also decreases, but by adding acid, the hydronium concentration was above the equilibrium value.

This will result in a net gain of hydronium, measured by a decrease in the pH.

 

[yjj8817]

By adding acid you are essentially increasing the hydronium concentration. The qualitative decrease in hydroxide ion occurs through a reaction with hydronium to form water.

So you are correct that the hydronium concentration also decreases, but by adding acid, the hydronium concentration was above the equilibrium value.

This will result in a net gain of hydronium, measured by a decrease in the pH.

Thank you! So are u saying that since the acid concentration is higher than equilibrium, it won't be affected that much by base reacting with acid to reestablish equilibrium?

 

[Mad Jack]

I read this as Le Chatlier on Acid.

 

[Cawolf]

I am not sure what "that much" means exactly, but one can always try an example to understand things better.

At 25 C, we know Kw = 10^-14.

So a neutral water will have [OH-] = 10^-7 and [H3O+] = 10^-7 with a pH = 7

We then add some acid to increase the concentration of hydrodium tenfold so [H3o+] = 10^-6

At this point Q > K so the reaction will run to the left to produce liquid water.

We can do an ICE table and solve for the change to reach equilibrium.

If you do the math you will see that at equilbrium [OH-] = 1.1 e-8 and [H3o+] = 9.11 e-7

The pH is now 6.04.

So even though they react stoichiometrically to produce water, the added hydronium concentration does not shift back to as low as it was when neutral.