Neutralization Reaction

[nothing123]

Neutralization reactions occur regardless of the strength of the acid/base involved right? Why can't we titrate a weak acid with a weak base then or vice versa?

Thanks.

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[161927]

Most of the discussions of neutralizations are simplifications and assume equal stoichiometry. Example: reaction between strong acid, strong base, is a classic neutralization. It is assumed that these compounds are equimolar. If you had an excess amount of base, clearly not all of it would react with acid to form a neutral species.

When you're dealing with weak species, the effective concentration of free acid or free base is reduced, because they do not dissociate as much. Predicting the pH after a mixture of weak acid and weak base is not possible without knowledge of the amount of reactants and the starting pH.

If I remember from intro chemistry, there are predictions you can make about titrations with weak or strong species. Since you control the input of one of the reactants, and you assume the solution is abundant in either acid or base (depending on the given information), then you can make a good prediction about the end pH.

 

[BerkReviewTeach]

Neutralization reactions occur regardless of the strength of the acid/base involved right? Why can't we titrate a weak acid with a weak base then or vice versa?

Thanks.

First off, neutralization does not mean to make a pH = 7 solution as the previous response alludes to. That misconception is usually the source of confusion.

The concept of neutralizing something is to add enough of another reactant that you completely consume it. In the case of an acid-base reaction, neutralization occurs when an equal number of moles of a reacting acid are mixed with an equal number of moles of a reacting base. This means that the acid neutralizes the base OR the base neutralizes the acid, depending on which species is added to the other.

In the case of a weak acid and a weak base, they cannot consume one another because they are not strong enough to push the reaction in the forward direction. It's a question of semantics.

 

[kish180]

The main point that is missing here is that when you titrate a weak acid or base, you change that acid/base into its conjugate acid/base. However the conj. base of a weak acid is a weak base, which is the exact reason why when you titrate a weak acid with a strong base, the eq. point has a pH above 7. At the eq. pt, youve added an equimolar amt of weak acid and strong base, which means all of the weak acid is now in its conj. base form, which is a weak base and would raise the pH above 7. The reason why we ignore the fact that the same thing occurs with the strong base (which is converted into a weak conjugate acid) is because the conj. acid of the strong base is so incredibly weak it can be ignored.
Anyway, this explains why you can't titrate a weak acid with a weak base. Although they would neutralize each other, since they both have weak acid/base conjugates, those conj. could also react (essentially the reverse rxn). Thus the titration would not show a clear jump in pH from the eq. pt.

 

[BerkReviewTeach]

The main point that is missing here is that when you titrate a weak acid or base, you change that acid/base into its conjugate acid/base. However the conj. base of a weak acid is a weak base, which is the exact reason why when you titrate a weak acid with a strong base, the eq. point has a pH above 7. At the eq. pt, youve added an equimolar amt of weak acid and strong base, which means all of the weak acid is now in its conj. base form, which is a weak base and would raise the pH above 7. The reason why we ignore the fact that the same thing occurs with the strong base (which is converted into a weak conjugate acid) is because the conj. acid of the strong base is so incredibly weak it can be ignored.

Absolutely right on the mark in terms of titration. The equivalence point is the point where the reagent initially in the flask has been neutralized by the reagent being added.

Anyway, this explains why you can't titrate a weak acid with a weak base. Although they would neutralize each other, since they both have weak acid/base conjugates, those conj. could also react (essentially the reverse rxn). Thus the titration would not show a clear jump in pH from the eq. pt.

They cannot truly neutralize one another as the word neutralize is defiened. To neutralize, you must fully consume the other species. By mixing a weak acid (HA) with a weak base (B-), you will get a solution that is a mixture of HA, A-, HB, and B-, with the equilibrium shifted to the side of the slightly weaker reagents (the weaker of the two acids will be on the same side as the weaker of the two conjugate bases), but because they are all weak reagents, they can each assume some concentration that is measurable. Again, it's semantics, but just as a strong acid is defined as one that fully dissociates, neutralization is defined as fully consumed. It is generally accepted convention in all general chemistry books that a weak acid cannot fully neutralize a weak base.

You are definitely right that there will be a reaction of some sort, but the magnitude will depend on the relative strengths of the species. It could highly favor one side of the equilibrium, but never 100%. A great way to think about this is to start with a diprotic acid at its first equivalence point. The first proton is esentially completely dissociated and the second proton essentially completely bound (again the exact amounts will depend on the relative strengths of those two particular protons). While the addition of a weak acid or a weak base will shift the equilibrium, it cannot take the diprotic acid all the way to its fully protonated state or to its second equivalence point. Only a strong acid or base can take it all the way to one of those two points.