Chemistry
In Class Activity 10-2
Unit:
Acids and Bases
Reading: Chapter 1
Working
Mode: Pairs. Solve the problems on a separate sheet of notebook paper.
Each student must submit his own work.
Include the name of your partner.
Learning
Objective: Write equilibrium constant expressions for the dissociation of
weak acids.
Background:
Weak acids and bases don’t dissociate or ionize completely.
This incomplete ionization is measured by a ratio of the equilibrium
product concentrations over the equilibrium reactant concentrations.
For example: The weak acid,
HF only partially ionizes in water. This
partial ionization is indicated by the double arrow on the dissociation
equation: HF(aq) + H2O(l)
ßà F-(aq) + H3O+(aq), and by the Ka
value of HF which is much less than 1, Ka = 6.8 x 10-4.
Task
1: From the
class discussion and notes, write the Ka expression for HF.
Question
2: Write the
explanation for why [H2O] does not appear in the Ka
expression.
Question
3: In a sentence or two describe what the value of HF’s Ka
indicates about the relative amounts of products and reactants in the above
chemical equation.
Problem
4: Write the
chemical equations for the following dissociations of these weak acids.
Ka
values
(a)
hypochlorous
acid: HClO
3.0 x 10-8
(b)
acetic
acid: HC2H3O2
1.8 x 10-5
(c)
formic
acid: HCO2H
1.8 x 10-4
(d)
hydrogen
carbonate ion: HCO3-
5.6 x 10-11
Problem
5: Write the Ka expressions for the acid dissociation
of each of the weak acids in problem 4.
Problem
6: Rank the
acids in problem # 4 based on their Ka values from strongest to
weakest. Explain your ranking.
