POLYPROTIC ACIDS AND POLYBASIC SPECIES
Polyprotic Acid
·
An acid that can release more than one proton
·
2 Types:
·
diprotic: 2 protons eg: H2SO4, H2CO3, HOOCCOOH, H2CO3, H2S,
H2SO3
·
triprotic: 3 protons eg: H3PO4, H3BO3
Polybasic Species
·
A base that can accept more than one proton
·
2 Types:
·
dibasic: SO42-, CO32-, S2-, OOCCOO2-, HPO42-, SO32-, S2O32·
tribasic: PO43-, BO33Reactions of Polyprotic acids and Polybasic species:
A.
Reactions of Polyprotic Acids With Water
·
As a general rule, the first hydrogen (proton) can be removed much more
easily than the second, which in turn can be removed more easily than the
second, which in turn can be removed more easily than the third (if any).
Examples:
1.
Assume all solutions prepared are 0.10 mol/L
Write the equations for the step-wise ionization of sulfuric acid in water.
Step 1:
H2SO4 (aq) + H2O (l)  H3O+ (aq) + HSO4-(aq)
100% ionization
Ka = 1.0 x 103
Step 2:
HSO4-(aq) + H2O (l)  H3O+ (aq) + SO42- (aq)
27% ionization
Ka = 1.3 x 10-2
2.
Write the equations for the step-wise ionization of carbonic acid in water.
Step 1:
H2CO3 + H2O (l)  H3O+ (aq) + HCO3-(aq)
2.1 x 10-1% ionization
Ka = 1.3 x 10-7
Step 2:
HCO3-(aq) + H2O (l)  H3O+ (aq) + CO32-(aq)
2.2 x 10-3% ionization
Ka = 1.3 x 10-11
3.
Write the equations for the step-wise ionization of phosphoric acid in water.
Step 1:
Step 2:
Step 3:
H3PO4 (aq) + H2O (l)  H3O+ (aq) + H2PO4-(aq)
H2PO4-(aq) + H2O (l)  H3O+ (aq) + HPO42-(aq)
HPO42-(aq) + H2O (l)  H3O+ (aq) + PO43-(aq)
B.
Reactions of Polyprotic Acids and Polybasic Species with Acids and
Bases Stronger Than Water
General Rules:
1.
Only one proton is donated/accepted at a time, therefore reactions of
polyprotic acids or polybasic species involve more than one step.
2.
The other chemical reactant involved (monobasic species) is present “in
excess” or is “continuously added” to ensure complete reaction of all protons
available.
3.
All of the strongest acid or base reacts first before the next strongest acid or
base can react.
4.
If all reaction steps are quantitative, then an overall reaction can be written.
5.
Follow the Bronsted-Lowry rules for writing acid/base reactions.
Example 1:
If a solution containing sodium hydroxide is continuously added to the oxalic acid in
rhubarb leaves, write the steps and overall reaction for this reaction.
Step 1:
OH- (aq) + HOOCCOOH(aq)  H2O (l) + HOOCCOO-(aq)
Step 2:
HOOCCOO-(aq) + OH- (aq)  H2O (l) + OOCCOO2-(aq)
____________________________________________________
Overall:
2OH- (aq) + HOOCCOOH(aq)  2H2O (l) + OOCCOO2-(aq)
Example 2:
Sulfuric acid reacts with an excess of sodium hydroxide.
Step 1:
H2SO4 (aq)+ OH- (aq)  HSO4-(aq)+ H2O (l)
Step 2:
HSO4-(aq) + OH- (aq)  H2O (l) + SO42-(aq)
_______________________________________________
Overall:
H2SO4 (aq) + 2OH- (aq) +  2H2O (l) + SO42-(aq)
Example 3:
If a solution of sodium carbonate is reacted with a strong acid by the continuous
addition of HNO3(aq) , write the steps involved in this reaction and write the overall
reaction.
Step 1:
H3O+(aq) + CO32-(aq)  H2O (l) + HCO3- (aq)
Step 2:
H3O+(aq) + HCO3- (aq)  H2O (l) + H2CO3 (aq)
_______________________________________________
Overall:
2H3O+(aq) + CO32-(aq)  2H2O (l) + H2CO3 (aq)
Titrations of Polyprotic acids and Polybasic species:
Example:
The Titration of Sodium Carbonate with Hydrochloric Acid
Step 1:
Na+, CO32-, H3O+, Cl-SB
SA
H3O+(aq) + CO32-(aq)  HCO3-(aa) + H2O(l)
Step 2:
HCO3-(aa) + H3O+(aq)  H2CO3(aa) + H2O(l)
Overall:
2H3O+(aq) + CO32-(aq)  H2CO3(aa) + 2H2O(l)
Titration Curve
Polyprotic and Polybasic Acid–Base Titrations (Selecting the Best Indicator)
Polyprotic Acids and Polybasic Species Worksheet
1.
2.
5.
For each of the following write the steps involved in the reaction and the
overall reaction:
a)
Nitric acid is continuously added to a solution of lithium thiosulfate.
b)
An excess of potassium hydroxide solution is added to a solution of
sulfurous acid.
c)
Lithium hydroxide is continuously added to citric acid,
C3H4OH(COOH)3(aq). Write citric acid as H3Ct(aq) for convenience.
Use the titration curve below to answer the questions that follow:
a.
Does the buret contain the acid or the base?
b.
Is the sample reacted an acid of a base?
c.
How many endpoints are present? Estimate each pH at endpoint.
d.
Choose the best indicator for each endpoint.
Sketch a pH curve for the addition of sodium hydroxide to a sulfuric acid
solution. Include reaction equations.