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?
#"HA + OH"^(-) -> "A"^(-) "+ H"_2"O"#
#"A"^(-) "+ H"_2"O" rightleftharpoons "HA + OH"^(-)#
#"B " "+ H"^(+) -> "BH"^(+)#
#"BH"^(+) rightleftharpoons "B + H"^(+)#
#"H"_2"O" rightleftharpoons "H"^(+) "+ OH"^(-)#
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:
(There, that's easier to read.)
Let's eliminate the ones that are just way off.
You said that
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.
I would say (b).
From what I can tell, you're dealing with a weak acid,
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
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
#"HA" + "OH"^(-) -> "A"^(-) + "H"_2"O"#
That happens because the conjugate base
#"A"^(-) + "H"_2"O" rightleftharpoons "HA" + "OH"^(-)#
These hydroxide ions will decrease the solution's
On the other hand, when you titrate a weak base with a strong acid, the pH of the resulting solution will be smaller than
#"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
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
This is why the equivalence point for a weak acid - strong base titration is greater than