Page 259 - Theory and Problems of BEGINNING CHEMISTRY

248 ACID-BASE THEORY [CHAP. 17

EXAMPLE 17.4. What is the difference between the reaction of HC 2 H 3 O 2 with H 2 O and with OH ?
−
Ans. The first reaction takes place to a slight extent; HC 2 H 3 O 2 is a weak acid. The second reaction goes almost 100%.
−
Even weak acids react almost completely with OH .
The acidity of a solution is determined by the hydronium ion concentration of the solution. The greater the
+
+
[H 3 O ], the more acidic the solution; the lower the [H 3 O ], the more basic the solution. Other substances, for
−
example, OH , affect the acidity of a solution by affecting the concentration of H 3 O . The presence in water of
+
OH in greater concentration than H 3 O makes the solution basic. If the relative concentrations are reversed,
−
+
the solution is acidic.
EXAMPLE 17.5. Explain why KC 2 H 3 O 2 tests basic in water solution.
−
+
Ans. The K does not react with water at all. The C 2 H 3 O 2 reacts with water to a slight extent:
−
−→
C 2 H 3 O 2 + H 2 O ←− HC 2 H 3 O 2 + OH −
−
The excess OH makes the solution basic.
17.3. ACID-BASE EQUILIBRIUM
Equilibrium constants can be written for the ionization of weak acids and weak bases, just as for any other
equilibria. For the equation

−
−→
HC 2 H 3 O 2 + H 2 O ←− C 2 H 3 O 2 + H 3 O +
we would originally (Chap. 16) write
[C 2 H 3 O 2 ][H 3 O ]
+
−
K =
[HC 2 H 3 O 2 ][H 2 O]
However, in dilute aqueous solution, the concentration of H 2 O is practically constant, and its concentration is
conventionally built into the value of the equilibrium constant. The new constant, variously called K a or K i for
acids (K b or K i bases), does not have the water concentration term in the denominator:
[C 2 H 3 O 2 ][H 3 O ]
+
−
K a =
[HC 2 H 3 O 2 ]
EXAMPLE 17.6. Calculate the value of K a for HC 2 H 3 O 2 in 0.150 M solution if the H 3 O concentration of the solution
+
is found to be 1.64 × 10 −3 M.
Ans. HC 2 H 3 O 2 + H 2 O ←− C 2 H 3 O 2 − + H 3 O +
−→
Initial 0.150 0 0
Change
Equilibrium 1.64 × 10 −3
−→ − +
HC 2 H 3 O 2 + H 2 O ←− C 2 H 3 O 2 + H 3 O
Initial 0.150 0 0
Change −1.64 × 10 −3 1.64 × 10 −3 1.64 × 10 −3
Equilibrium 1.64 × 10 −3
−→ − +
HC 2 H 3 O 2 + H 2 O ←− C 2 H 3 O 2 + H 3 O
Initial 0.150 0 0
Change −1.64 × 10 −3 1.64 × 10 −3 1.64 × 10 −3
Equilibrium 0.148 1.64 × 10 −3 1.64 × 10 −3