eq. point vs. half eq. point

[TotalDomination]

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Ok I read the other threads on this but still confused.

My understanding:

Equivalence point is when the solution is neutralized. So, moles of bases added is exactly equal to the moles of acid originally present in solution. This is reversed if acid was being added to base.

Half equivalence point occurs when half the volume of base at the equivalence point has been added. This is the point where the concentration of undissociated acid is equal to the conc. of A- (conj. base). This is also the point where pka=ph.

But does pka=ph only if it is a buffer system (weak acid+conj base or weak base+ conj acid)? or is it for all acid base titrations? a little help please?

 

[AureatEagle]

That is correct. The Henderson-Hasselbalch equation from which that identity arises is only an estimation that is fairly accurate when applied to buffer systems. An easy way of thinking about this is that for a strong acid, log(A-/HA) would approach infinity due to all of HA being dissociated.

 

[yoyohomieg5432]

the solution is only neutralized for the titration of STRONG acids and strong bases. because in these titrations, water is formed and the conjugate bases (i.e Cl- for HCL are so weak they don't really affect pH).

in titrations of weak acid with strong base the equivalence point will be basic because you've added enough strong base to react with all of the weak acid to form H+ and A-. since HA is a weak acid, the conjugate base will be stronger, meaning it will affect the pH. i.e.

CH3COOH + OH- ----> CH3COO- + H20. this goes to completion since enough OH- added to interact w/ allCH3COOH. Then the solution contains CH3COO- which is a base, hence basic pH. the reaction becomes CH3COO- + h20 ---> CH3COOH + OH-. (again demonstrating the basic pH).

as for your second question, i believe it's for all acid/base titrations that are weak acids (or weak bases). Strong acids don't make buffers because they completely dissociate and will not want to reform the associated reactant (I.e. H+ and Cl- will not want to go back to HCl).

buffers would happen for all weak acids because if you're titrating with a strong base it will deprotonate the acid and form the conjugate base. at pH=pKa the buffering capability wlil be the best

 

[Mindbuffer]

the solution is only neutralized for the titration of STRONG acids and strong bases. because in these titrations, water is formed and the conjugate bases (i.e Cl- for HCL are so weak they don't really affect pH).

If I remember correctly, neutralization does not necessarily imply that the solution must be pH neutral. It's correct that a strong acid + strong base titration will result in an equivalence point with pH = 7, but neutralization occurs for all acid base titrations at the equivalence point. This is when [H+] = [OH-].