Equivalence points?

[anbuitachi]

I'm kind of confused about Equivalence points. I'm using the EK Chem book and it says at the equivalence points there are equal equivalents of acid and base. By this do they mean before dissociation or after (H+ and OH-)...

If it's before dissociation, why does the equivalence point exist since some acid/bases dissociate a lot more than others...

Thanks

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[PhilIvey]

I'm kind of confused about Equivalence points. I'm using the EK Chem book and it says at the equivalence points there are equal equivalents of acid and base. By this do they mean before dissociation or after (H+ and OH-)...

If it's before dissociation, why does the equivalence point exist since some acid/bases dissociate a lot more than others...

Thanks

It depends on what you mean. For simplicity, lets look at the strong base/strong acid. If I 25 ml of .1M HCl and I titrate with 25ml of .1M NaOH, at the equivalence point the equivalents of H+= OH- and so you have ONLY the H+ from H20 present yielding a neutral pH.

Now, if we have a strong acid/ weak base, then AFTER the equivalence, then the we will get some dissociation which makes the pH greater than 7 if it's a weak acid titrated by a strong base or less than 7 if it's a weak base titrated by a strong acid.

To illustrate, lets say I have 25ml of CH3COOH. After I add 25 ml of NaOH then the equivalents are the same. I've completely removed the hydrogens. The Problem is that CH3COO- is a weak base. So, it will abstract H+ from either H20 or H3O+ thus there will be you will get some CH3COOH formed thus no longer being equivalent.

So, when they say the equivalents of acid and base are equal, this is just after enough has been added to completely react and BEFORE any dissociation occurs due to weak acids or bases. HTH.

 

[anbuitachi]

oh ok thanks!