half equiv and equiv point

[TimeforDAT]

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Hi all,

I'm getting a bit confused as to the differences. I've read around and am finding definitions that don't really shed any light on the differences between them. What is the difference between a half equivalence and equivalence point?

i know that half equivalence have 50% of an base and 50% of its conjugate acid.

Equivalence point is suppose to be equal amounts of both the acid and the base?

 

[levyusupov]

Hi all,

I'm getting a bit confused as to the differences. I've read around and am finding definitions that don't really shed any light on the differences between them. What is the difference between a half equivalence and equivalence point?

i know that half equivalence have 50% of an base and 50% of its conjugate acid.

Equivalence point is suppose to be equal amounts of both the acid and the base?

at the equivalance point the # of moles if base equal the # of moles of acid

where at the half eq point the pKa value = pH value, which is the desired buffer zone.

 

[TimeforDAT]

at the equivalance point the # of moles if base equal the # of moles of acid

where at the half eq point the pKa value = pH value, which is the desired buffer zone.

hmm...but if at half equivalence when pKa = pH, that is when 50% is acid and 50% is base, so my confusion is here because here there are the same amt of acid and base; but then at equivalence you say there is the same amount of base and acid.

could you perhaps elaborate if possible?

thanks

 

[navajoDDS]

I use the half equivalence point mainly when dealing with a buffered solution. As you said, that occurs when [wk. acid]=[conj. base]. Using the henderson-hasselbach equation pH=pKa+log [conj. base]/[wk. acid], you can see that when [conj. base]=[acid], pH=pKa b/c log1=0. If you are titrating a wk. acid with a str. base, this will occur when the base reacts with enough of the wk. acid to form the same amount of conj. base (of the wk. acid) as there is left over acid. (SORRY IF THIS IS CONFUSING).

The half equiv is also seen on titrations of polyprotic acids/bases. It is the flat part before each equiv. point.

The equiv point is when moles of base added = moles of acid in solution.

Don't mix these up:

half equiv point is equal amounts of a wk. acid and its conj. base (or wk. base and its conj. acid) [usually used when dealing w/ buffers]

equiv point is # of acid equivalents = # of base equivalents added (not its own conj. base) [all titrations]

I don't know if I explained it clearly, but I hope this helps.

 

[navajoDDS]

Here's an example:

you're titrating a solution of xM HC2H3O2 with yM NaOH

When HC2H3O2 reacts with OH, it forms C2H3O2- and H20

C2H3O2- is the conj. base of HC2H3O2. The half equiv point will be when an X amount of NaOH has reacted with HC2H3O2 so that the
# moles of C2H3O2- (conj. base) = # moles of remaining HC2H3O2 (wk acid)

At this point, the pH = pKa od HC2H3O2

To reach the equiv point, keep adding NaOH until it has consumed all of the remaining acid. This happens when moles OH- = moles HC2H3O2. The acid will be neutralized.

 

[TimeforDAT]

Here's an example:

you're titrating a solution of xM HC2H3O2 with yM NaOH

When HC2H3O2 reacts with OH, it forms C2H3O2- and H20

C2H3O2- is the conj. base of HC2H3O2. The half equiv point will be when an X amount of NaOH has reacted with HC2H3O2 so that the
# moles of C2H3O2- (conj. base) = # moles of remaining HC2H3O2 (wk acid)

At this point, the pH = pKa od HC2H3O2

To reach the equiv point, keep adding NaOH until it has consumed all of the remaining acid. This happens when moles OH- = moles HC2H3O2. The acid will be neutralized.

ohhh. ok i get it now. Yeah. that's sort of what I had in mind as to the differences between half eq and eq point. Just wasn't sure how to word it i guess. Thanks👍