# 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) $\text{HA + OH"^(-) -> "A"^(-) "+ H"_2"O}$ b) ${\text{A"^(-) "+ H"_2"O" rightleftharpoons "HA + OH}}^{-}$ c) ${\text{B " "+ H"^(+) -> "BH}}^{+}$ d) ${\text{BH"^(+) rightleftharpoons "B + H}}^{+}$ e) ${\text{H"_2"O" rightleftharpoons "H"^(+) "+ OH}}^{-}$

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) $\text{HA + OH"^(-) -> "A"^(-) "+ H"_2"O}$
b) ${\text{A"^(-) "+ H"_2"O" rightleftharpoons "HA + OH}}^{-}$
c) ${\text{B " "+ H"^(+) -> "BH}}^{+}$
d) ${\text{BH"^(+) rightleftharpoons "B + H}}^{+}$
e) ${\text{H"_2"O" rightleftharpoons "H"^(+) "+ OH}}^{-}$

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

You said that $H A$ is like acetic acid, which is a weak acid ($p K a \approx 4.76$ in water), and $B$ is like ammonia, which is a weak base ($p K a \approx 36$ 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 ${\text{H}}^{+}$) 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

I would say (b).

#### Explanation:

From what I can tell, you're dealing with a weak acid, $\text{HA}$, and a weak base, $\text{B}$, which you titrate with a strong base, symbolized by $\text{OH"^(-)}$, and a strong acid, symbolized by $\text{H"^(+)}$, 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 $p {H}_{\text{sol}} = 7$.

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 $7$.

$\text{HA" + "OH"^(-) -> "A"^(-) + "H"_2"O}$

That happens because the conjugate base ${\text{A}}^{-}$ will react with water to reform some of the weak acid and produce hydroxide ions .

${\text{A"^(-) + "H"_2"O" rightleftharpoons "HA" + "OH}}^{-}$

These hydroxide ions will decrease the solution's $p O H$, 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 $7$.

${\text{B" + "H"^(+) -> "BH}}^{+}$

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

${\text{BH"^(+) + "H"_2"O" rightleftharpoons "B" + "H"_3"O}}^{+}$

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 ${\text{A}}^{-}$ 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 $7$.