1
$\begingroup$

OKay I learned in class that equivalence point is the time of neutralization, where the # of moles of acids and base present is in equivalence/same amount.

Now I was also told that Half equivalence point is when [HA] = [A$^-$] , the concentration of a weak acid = concentration of conjugate base.

Problem: 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 an equivalence point?

Based on what i have been told, they sound the same to me.

Improve this question
$\endgroup$
4
  • 4
    $\begingroup$ Look at the benzoic acid titration curve here: chemistry.stackexchange.com/a/117033/79678 . Halfway to the equivalence point, the concentration of benzoic acid equals that of benzoate ion. The is the middle of the buffer region. At the equivalence point, you just have a solution of sodium benzoate. There are lots of pH questions and answers here, so please use the search function. $\endgroup$ – Ed V Feb 23 '20 at 3:41
  • 2
    $\begingroup$ In short then the difference is striking. $\endgroup$ – Alchimista Feb 23 '20 at 8:55
  • $\begingroup$ Start with exactly 1 M HCl solution. Now add exactly 0.5 moles of solid NaOH. This is the half-equivalence point: half of the total HCl has been neutralized, resulting in a solution that is 0.5 M HCl (aq) and 0.5 M NaCl (aq). Note that chloride ion is the (pathetically weak) conjugate base of HCl (aq). Now add another 0.5 moles (exactly) of solid NaOH. This is the equivalence point: all of the acid and base have exactly neutralized and you have a 1 M NaCl solution with pH 7. Hope this helps. $\endgroup$ – Ed V Feb 23 '20 at 15:07
  • $\begingroup$ I assumed exactly 1 liter of the exactly 1 M HCl solution. Activities and auto-ionization of water are neglected. At the half-equivalence point, the molar concentrations of hydronium and sodium ions are each 0.5 M and the chloride concentration is 1 M. This is charge neutrality. At the equivalence point, the sodium and chloride ion concentrations are each 1M. The hydronium and hydroxide are the values at pH 7, i.e., 100 nM each. $\endgroup$ – Ed V Feb 23 '20 at 15:43
1
$\begingroup$

Now I was also told that Half equivalence point is when [HA] = [A−] , the concentration of a weak acid = concentration of conjugate base.

That is correct. You make the weak acid in situ when you titrate a weak base with a strong acid, or when you titrate a weak acid with a strong base. At the half equivalence point, the pH is roughly equal to the pKa of the weak acid.

What is the difference between a half equivalence and an equivalence point?

The equivalence point is what it always is. The point when you added stoichiometric amounts of titrant to the analyte. In the weak acid / strong base case, it means that you have quantitatively turned the weak acid into weak base, and don't have any excess of strong base (all was neutralized). The pH would be that of the weak base. In the weak base / strong acid case, it means that you have quantitatively turned the weak base into weak acid, and don't have any excess of strong acid (all was neutralized). The pH would be that of the weak acid.

Based on what i have been told, they sound the same to me.

Compared to the equivalence point, you have to add half the titrant to reach the half equivalence point, so they are different.

Improve this answer
$\endgroup$
1
$\begingroup$

The equivalence point is where the amount of moles of acid and base are equal, resulting a solution of only salt and water. If you are titrating an acid against a base, the half equivalence point will be the point at which half the acid has been neutralised by the base. For instance, if you have 1 mole of acid and you add 0.5 mole of base, exactly half of the acid will have been neutralised. The solution remaining will be half salt and half acid.

Improve this answer
$\endgroup$
0

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.