Neutralization with Polyprotic Acids

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NEUTRALIZATION REACTIONS
FOR POLYPROTIC AND
POLYBASIC SPECIES
POLYPROTIC ACIDS
The titration curves we have considered so far
have a single defined equivalence point, because
the acids are monoprotic.
 They have a single proton (H+) that may
dissociate.
 Polyprotic acids have more than one proton
that dissociates.


Oxalic acid (H2C2O4) is diprotic


It has two protons that can dissociate.
Phosphoric acid (H3PO4) is triprotic

It has three protons that can dissociate.
POLYPROTIC ACIDS
Polyprotic acids do not donate all their H+
at the same time when they dissociate.
 The dissociation of a polyprotic acid occurs as a
series of separate steps.
 For example:
H3PO4(aq) + H2O(l)
H2PO4-(aq) + H3O+(aq)

Ka = 6.9 x 10-3
H2PO4-(aq) + H2O(l)
HPO42-(aq) + H3O+(aq)
Ka = 6.2 x 10-8
HPO42-(aq) + H2O(l)
Ka = 4.8 x 10-13
PO43-(aq) + H3O+(aq)
POLYPROTIC ACIDS



Notice that each of these dissociation steps is an
equilibrium with its own Ka value.
Notice also how these acid dissociation values change
with each successive dissociation (H3PO4 is a much
stronger acid than H2PO4-).
In other words, for phosphoric acid:



Ka1 > Ka2 > Ka3
Remember that the mole ratio for neutralization
reactions of this and other polyprotic acids depends on
the number of protons donated to the base.
When there are more than one proton that may
dissociate, the proton will be donated one at a
time.
POLYBASIC COMPOUNDS
Basic compounds and ions that can accept
two or more protons are polybasic.
 For example, the sulfide ion, S2- is dibasic,
because it can accept up to two protons.

WRITING EQUATIONS FOR A POLYBASIC
ION

Problem

The borate ion, BO33-, is tribasic. Write balanced,
stepwise net ionic equations for a solution of sodium
borate titrated with hydrochloric acid, and write the
overall neutralization reaction for the titration.
WRITING EQUATIONS FOR A POLYBASIC
ION

Step 1
For all bases, remember that the metal ion is a
spectator. Therefore, from a solution of sodium
borate, only the borate ion is included in the first
stepwise equation. You know that any strong acid
will dissociate completely in water, so you can write
it as H+.
Stepwise equation 1: BO33-(aq) + H+(aq)
HBO32-(aq)
Stepwise equation 2: HBO32-(aq) + H+(aq)
H2BO3-(aq)
Stepwise equation 3: H2BO3-(aq) + H+(aq)
H3BO3(aq)

WRITING EQUATIONS FOR A POLYBASIC
ION

Step 2
To write the overall equation, notice that there are
three protons on the left side of the equation: these
add. All borate-related conjugate bases cancel,
leaving only the BO33- reactant and the H3BO3
product.
Overall equation: BO33-(aq) + 3H+(aq)
H3BO3(aq)

TITRATION CURVES FOR POLYPROTIC AND
POLYBASIC SPECIES
Sulfurous acid is a weak diprotic acid (Ka1 = 1.4 x
10-2; Ka2 = 6.3 x 10-8).
 What you will see in a titration curve for a
diprotic acid is separate equivalence points for
the removal of each mole of H+.
 You can think of the acid as neutralizing in two
steps, one for each dissociation.

TITRATION CURVE FOR SULFUROUS ACID

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