|Weak acid K|
Acid Dissociation Constants
When an uncharged weak acid is added to water, a heterogeneous equilibrium forms in which aqueous acid molecules, HA(aq), react with liquid water to form aqueous hydronium ions and aqueous anions, A-(aq). The latter are produced when the acid molecules lose H+ ions to water.
In writing an equilibrium constant expression for this heterogeneous equilibrium, we leave out the concentration of the liquid water. The equilibrium constant for this expression is called the acid dissociation constant, Ka.
When the equilibrium in question occurs in solution, the chemical formulas enclosed in brackets in the equilibrium constant expression represent the molarities of the substances (moles of solute per liter of solution).
Remember that H+ can be used to represent H3O+, thus simplifying our depiction of the reaction between a weak acid and water and its acid dissociation constant expression:
For example, acetic acid is a weak acid, because when it is added to water, it reacts with the water in a reversible fashion to form hydronium and acetate ions.
EXAMPLE 1 - Writing an Acid Dissociation Constant: Write the equation for the reaction between the weak acid nitrous acid and water, and write the expression for its acid dissociation constant.
Exercise 1- Writing an Acid Dissociation Constant: Write the equation for the reaction between the weak acid hydrofluoric acid and water, and write the expression for its acid dissociation constant.
The table below lists acid dissociation constants for some common weak acids. These Ka values can be used to describe the relative strength of the acids. A stronger acid will generate more hydronium ions in solution. A larger Ka indicates a greater ratio of ions (including hydronium ions) to uncharged acid. Therefore, a larger Ka indicates a stronger acid. For example, the larger Ka for chlorous acid (1.2 x 10-2) compared to acetic acid (1.8 x 10-5) tells us that chlorous acid is stronger than acetic acid.
Acid Dissociation Constants, Ka, for Common Weak Acids
Exercise 2 - Comparing the Strengths of Weak Acids: Which is the stronger acid, formic acid (an irritant found in ant bites) or phenol (formerly used as an antiseptic in hospitals)?
The following study sheet describes one procedure for calculating the pH of solutions of weak acids. If you take other chemistry courses, you will find that there are variations on this procedure for some weak acid solutions.
Study Sheet - Calculating pH for Weak Acid Solutions
Tip-off - You are given the concentration of a weak acid solution and asked to calculate its pH.
General Steps -
EXAMPLE 2 - pH Calculations for Weak Acid Solutions: Vinegar is a dilute water solution of acetic acid with small amounts of other components. Calculate the pH of bottled vinegar that is 0.667 M HC2H3O2, assuming that none of the other components affect the acidity of the solution.
Exercise 3 - pH Calculations for Weak Acid Solutions: Hydrofluoric acid is used to make chlorofluorocarbons (CFCs), to etch glass, and in processing uranium for nuclear power plants. Calculate the pH of a 1.5 M HF solution.