An equimolar mixture of a weak acid and a strong base has a pH > 7 at the equivalence point. Which of the following describes the reaction that occurs at that equivalence point?

a) #"HA + OH"^(-) -> "A"^(-) "+ H"_2"O"#
b) #"A"^(-) "+ H"_2"O" rightleftharpoons "HA + OH"^(-)#
c) #"B " "+ H"^(+) -> "BH"^(+)#
d) #"BH"^(+) rightleftharpoons "B + H"^(+)#
e) #"H"_2"O" rightleftharpoons "H"^(+) "+ OH"^(-)#

2 Answers
Aug 20, 2015

It appears that you are saying an equimolar mixture of a weak acid and a strong base has a pH > 7 at the equivalence point, which, yes, it should.

Looking at the question, you wrote out options:

a) #"HA + OH"^(-) -> "A"^(-) "+ H"_2"O"#
b) #"A"^(-) "+ H"_2"O" rightleftharpoons "HA + OH"^(-)#
c) #"B " "+ H"^(+) -> "BH"^(+)#
d) #"BH"^(+) rightleftharpoons "B + H"^(+)#
e) #"H"_2"O" rightleftharpoons "H"^(+) "+ OH"^(-)#

(There, that's easier to read.)

Let's eliminate the ones that are just way off.

You said that #HA# is like acetic acid, which is a weak acid (#pKa ~~ 4.76# in water), and #B# is like ammonia, which is a weak base (#pKa ~~ 36# in water).

  • a) has a weak acid reacting with a strong base. This looks right, but this is the reaction that occurs UP UNTIL the equivalence point, not right at the equivalence point.

  • b) is correct because the strong base neutralizes the weak acid completely, and then the weak conjugate base associates in water to leave pH > 7.

  • c) implies that the acidic species is strong (hence there is a dissociated #"H"^+#) but is opposite to what the question is asking. Here, pH < 7.

  • d) is the dissociation of a conjugate acid into the original base and a proton. It doesn't specify the solvent, and this is closest to the reaction that occurs at the equivalence point for a strong acid reacting with a weak base. Here, pH < 7.

  • e) is just the autoionization of water and is not relevant because this reaction is presumed suppressed.

Aug 20, 2015

Answer:

I would say (b).

Explanation:

From what I can tell, you're dealing with a weak acid, #"HA"#, and a weak base, #"B"#, which you titrate with a strong base, symbolized by #"OH"^(-)"#, and a strong acid, symbolized by #"H"^(+)"#, respectively.

I think that the problem wants to see if you understand why the equivalence points for a weak acid - strong base titration and for a weak base - strong acid titration do not take place at #pH_"sol" = 7#.

If we go by this logic, then reactions (a) and (b) are part of the same process. The same can be said for reactions (c) and (d).

When a weak acid is titrated with a strong base, the pH of the solution at the equivalence point will indeed by greater than #7#.

#"HA" + "OH"^(-) -> "A"^(-) + "H"_2"O"#

That happens because the conjugate base #"A"^(-)# will react with water to reform some of the weak acid and produce hydroxide ions .

#"A"^(-) + "H"_2"O" rightleftharpoons "HA" + "OH"^(-)#

These hydroxide ions will decrease the solution's #pOH#, which is another way of saying that they will increase the solution's pH.

On the other hand, when you titrate a weak base with a strong acid, the pH of the resulting solution will be smaller than #7#.

#"B" + "H"^(+) -> "BH"^(+)#

The conjugate acid will then react with water to reform some of the weak base and produce hydronium ions in the process.

#"BH"^(+) + "H"_2"O" rightleftharpoons "B" + "H"_3"O"^(+)#

These hydronium ions will decrease the solution's pH.

Reaction (e) is simply the self-ionization of water, which is what determines what a neutral solution is. At room temperatur, this self-ionization equilibrium is what gives water a pH equal to #7#.

So, if I can pick two reactions as the answer, I'd go with (a) + (b), but if I can only pick one, I'd go with (b).

Once the acid is completely comsumed by the neautralization reaction, the conjugate base #"A"^(-)# will react with water and reform some of that acid, at the same time increasing the concentration of hydroxide ions in solution.

This is why the equivalence point for a weak acid - strong base titration is greater than #7#.