Auto-ionization equilibrium of water shifted with acid-base addition? [closed]








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Adding acids or bases to water, so that either pH or pOH decreases independently of the other, will that affect the auto-ionization of water? For example, pure water + auto-ionized state, with some base added to remove some protons, will it auto-ionize a bit more (create more H+ and OH-) or a bit less (remove some H+ and OH-)?




It depends how you write the reaction.



> 
> For example, pure water + auto-ionized state, with some base added to remove some protons, will it auto-ionize a bit more (create more H+ and OH-) or a bit less (remove some H+ and OH-)?
> 
> 
> 


If we call the base $\ce{B}$ and the conjugate acid $\ce{BH+}$, you could write:


$$\ce{B + H3O+ <=> BH+ + H2O}$$


This would mean that you just lost some hydronium ions, and water will ionize a bit more to attain equilibrium with its ions.


Or you could write it like this:


$$\ce{B + H2O <=> BH+ + OH-}$$


This would mean that you just gained some hydroxide ions, and water will ionize a bit less to attain equilibrium with its ions.


**What about adding acid?**


Same story:


$$\ce{AH + H2O <=> A- + H3O+}$$


or


$$\ce{AH + OH- <=> A- + H2O}$$


**Which one is correct?**


Either one is fine. You will arrive at the same equilibrium concentrations if you consider one of the acid/base reactions and water auto-dissociation. Depending on what is the major species and what is the minor species, you can choose one or the other. Or if you can't decide, you can be creative and write:


$$\ce{AH + 1/2 OH- <=> A- + 1/2 H3O+}$$


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