I\'m having a hard time figuring out which way the reaction will shift if I add

Question

I'm having a hard time figuring out which way the reaction will shift if I add something. I know about Le Chatelier's Principle, but this seems fuzzy to me. Any help would be greatly appreciated. Thanks.

1.) Consider a solution that is 0.10 M Acetic Acid (HC2H3O2) and 0.10 M Sodium Acetate (NaC2H3O2).
HC2H3O2 + H2O <---> C2H3O2- + H3O+
a.) What happens if you add HCl (acid)?
b.) What happens if you add NaOH (base)?

2.) NH3 + H2O <---> NH4+ + OH-
If you add HCl, which way will it shift?

I know the answers, but I need an explanation. Please provide the answers along with a detialed explanation. Thank you again.

Explanation / Answer

HC2H3O2 + H2O <---> C2H3O2- + H3O+

(a) If you add HCl , this means that amount of H3O+ will increase in the solution. According to the Lechatelier principle, if you make any change to a equilibrium reaction, the reaction will try to minimize that change by shifting the position of the equilibrium.

If excess, H3O+ is present , equilibrium will shift towards left hand side (reverse direction)

(b) If a base is added, it will react with H+ to convert it to water. As H+ is removed from the reaction, it will try to produce more and more H3O+. So, the reaction equilibrium will shift towards right hand side (forward direction)

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As as above, adding H+ will remove OH- as water. Equilibrium will shift towards forward direction

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