Equilibria of Weak Acids

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Equilibria ofWeak Acids BasesPart I
Introduction to Equilibrium Law Ionization
Constants

• C. Yau
• Spring 2009

Based on Brady Senese 5/eChap. 16 Sec 1
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Example 16.1 p.644
Nitrous acid, HNO3, is a weak acid that's formed
in the stomach when nitrite food preservatives
encounter stomach acid. There has been some
concern that this acid may form carcinogenic
products by reacting with proteins. Write the
chemical equation for the equilibrium ionization
of HNO2 in water and the appropriate Ka
expression. Do Pract Exer 1 2 p. 644
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• p. 645

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Example 16.2 p.645
A certain acid was found to have a pKa equal to
4.88. Is this acid stronger or weaker than acetic
acid? What is the value of Ka for the acid?
Example 16.3 p.646 Hydrazine, N2H4 is
a weak base. It is a poisonous substance that's
sometimes formed when a chlorine bleach, which
contains hypochlorite ion, is added to an aqueous
soln of ammonia. Write the equation for the
reaction of hydrazine with water and write its
expression for its Kb.
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Example 16.4 p.647
Solutions of chlorine bleach such as Clorox
contain the hypochlorite ion, OCl- which is a
weak base. Write the chemical equation for the
reaction of OCl- with water and the appropriate
expression for Kb for this anion. Do Pract Exer
3, 4, p. 645 and 5, 6 p. 647
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Summary

• HA H2O H3O A-
• pKa - log Ka
• B H2O BH OH-
• pKb - log Kb
• Ka x Kb 10-14 pKa pKb 14

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Henderson-Hasselbalch Eqn
If we can get A- HA, pH pKa.
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How Ka can be determined experimentally

• Prepare a solution of the weak acid and divide it
  into two halves by volume.
• Neutralize one half with a strong base (such as
  NaOH).
• Add the un-neutralized half to the neutralized
  solution and we have "half-neutralized" solution.
• Measure the pH and calculate Ka.

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• For example, to determine the ionization constant
  of barbituric acid
• 1) Prepare 100 mL of 0.100 M barbituric acid.
• 2) Pour 50 mL of this solution into a flask and
  titrate it to the end point with an aqueous
  solution of NaOH.
• 3) Add the other 50 mL (un-neutralized)
  barbituric acid solution and measure the pH.
• If the pH is 4.01, what is the Ka of barbituric
  acid?