Acid Equilibrium
Objective
lesson 1
chapter 17
You will be able to define an equilibrium
system involving acids and bases based upon
the concentration of the ions and molecules.
Recall from Chemistry 20
strong acids...
-dissociate completely in solution to form hydrogen ions
HA ➔ H+(aq) + A-(aq)
OR
HA(aq) + H2O(l) ➔ H3O+(aq) + A-(aq)
(no equilibrium)
There are only 6 strong acids:
General Formula for Acids and Bases
B = base or A-
HA = acid
HA + H O ⇌ H3O+ + A2
HA donates a proton (H+) to water to become a hydronium ion
and the conjugate base.
B + H2O ⇌ OH- + BH+
base accepts a proton from water to become a hydroxide ion
and the conjugate acid.
weak acids...
-partially dissociate in water
General Equation
HA
H+(aq) + A-(aq)
OR
HA
+H O
(aq)
2 (l)
H O+
+A
3 (aq)
(aq)
(equilibrium favors the reactants)
Examples
hydrocyanic acid
hydrogen sulfite ion
-weak acids differ in their ability to ionize or dissociate
in solution
-a solution of weak acid contains mostly acid molecules
(undissociated) and a small fraction of hydrogen ions
and acid anions
Acid strength
-refers to how the acid ionizes in solution
-strong acids dissociate 100%
-weak acids vary in their ability to produce hydrogen
ions
Acid Concentration
-refers to the moles of solute per litre of solution
Example:
An unknown acid with a concentration of 0.200 mol/L
has a pH = 2.500. Is the acid strong or weak?
Empirical Differences Between Weak and Strong Acids
-weak acids react at slower rates that strong acids
-pH of weak acids are closer to 7 than strong acids of
equal concentration
-weak acids are weak electrolytes (conductivity)
Theoretical Differences Between Weak and Strong
Acids
-strong acids are 100% dissociated in solution
-weak acids have less than 100% dissociation
Communicating Acid Strength
Ka values
Ka = an equilibrium constant for a weak acid. It refers
to the equilibrium with water
HA(aq) + H2O(l)
H3O+(aq) + A-(aq)
Ka =
Example: Write the Ka expression for nitrous acid.
Ka values are listed in the data book for common acids
-the larger the Ka value, the stronger the acid
Example
Calculating Ka given the pH of a solution
You measure the pH of a 0.25 mol/L solution of a weak acid to be
3.48. What is the Ka of the unknown weak acid?
Example
Using Ka to calculate the pH of a solution (Use an ICE box)
What is the pH of a 0.10 mol/L solution of acetic acid?
Note:An approximation may be used instead of the quadratic
formula when the concentration of H3O+ produced is less than 5%
of the original acid concentration (or the concentration of the acid is
1 000 times greater than the Ka).
Example
What is the pH of a 0.0075 mol/L solution of hypochlorous
acid?
Assignment
Read p. 691-2
Do p. 692 #11,13
p. 693 #14,17
p. 699 #24,27