Chemistry 12 Name: _____________________________
Acid/Base Equilibriums: Investigating Buffer Solutions
A buffer solution is a solution of special composition, which is able to resist changes in pH that occur if
small amounts of acid or base are added. Many important chemical reactions, especially enzyme
reactions, will occur only over small ranges in pH. Investigating such reactions requires the use of buffer
solutions to maintain the pH in that range. One such physiological buffering system is the bicarbonate
buffering system, an important buffer system in the acid-base homeostasis of living things,
including humans. As a buffer, it tends to maintain a relatively constant plasma pH and
counteract any force that would alter it. In this system, carbon dioxide (CO2) combines with water
to form carbonic acid (H2CO3), which in turn rapidly dissociates to form the hydrogen ion and
bicarbonate (HCO3- ) as shown in the reactions below.
CO2 + H2O Û H2CO3 Û HCO3- + H+
Any disturbance of the system will be compensated by a shift in the chemical equilibrium
according to Le Chatelier's principle.
In this activity you will investigate the effect of a buffering system on the rate of a known
chemical reaction – CaCO3(s) + H+ ® Ca2+(aq) + CO2(g) + H2O(l), (this is not an equilibrium).
Using Le Chatelier’s principle you will be asked to explain the observed differences in reaction
rate.
Procedure
Test Tube A (18mm x 150mm) Test Tube B (18mm x 150mm) Test Tube C (18mm x 150mm)
Add 10.00 mL 3M CH3COOH Add 10.00 mL 3M CH3COOH and Add 10.00 mL 3M CH3COOH and
1.0 g of NaCH3COO (dissolve) 2.0 g of NaCH3COO (dissolve)
Mark a 25 mm x 200 mm test tube Mark a 25 mm x 200 mm test tube Mark a 25 mm x 200 mm test tube
with a line 10 cm from the bottom with a line 10 cm from the bottom with a line 10 cm from the bottom
Place 0.5 g of CaCO3 in the bottom Place 0.5 g of CaCO3 in the bottom Place 0.5 g of CaCO3 in the bottom
of the 25 mm x 200 mm test tube of the 25 mm x 200 mm test tube of the 25 mm x 200 mm test tube
Add the contents of Test Tube A to Add the contents of Test Tube B to Add the contents of Test Tube C to
the 25 mm x 200 mm test tube the 25 mm x 200 mm test tube the 25 mm x 200 mm test tube
and shake quickly. Record the and shake quickly. Record the and shake quickly. Record the
time it takes for the foam to o rise time it takes for the foam to o rise time it takes for the foam to o rise
to the 10 cm mark. to the 10 cm mark. to the 10 cm mark.
Unit Four: Aqueous Equilibriums (Buffer Solutions) Adapted from Essential Experiments for Chemistry
Chemistry 12 Name: _____________________________
Data Collection
Solid Reagent Solution Added Time (s) for Foam to Rise (10 cm)
CaCO3 CH3COOH
CaCO3 CH3COOH + 1.0 g NaCH3COO
CaCO3 CH3COOH + 2.0 g NaCH3COO
Analysis (The Effect of adding a Common Ion to a Weak Acid)
1) Write a balanced chemical equation to represent the ionization equilibrium for acetic acid.
2) Sodium acetate is a soluble salt. Write a balanced chemical reaction that represents the dissociation
of sodium acetate in an aqueous environment.
3) Using LeChatelier’s Principle explain the effect of adding sodium acetate to test tubes B and C. Use
the balanced chemical equations from questions 1 and 2 to support your explanation.
4) Rank the three test tubes, A, B and C, from slowest to fastest based on that time it took for the foam
to rise to the 10 cm mark.
5) Using the net ionic equation for the reaction between CaCO3 and H+ presented earlier and tour
answer to question 3 explain your ranking from question 4.
Unit Four: Aqueous Equilibriums (Buffer Solutions) Adapted from Essential Experiments for Chemistry
Chemistry 12 Name: _____________________________
6) The balanced chemical equations used in question 3 represent the formation of a buffer solution that
is able to resist large changes in pH upon the addition of small amounts of acids and bases. Using the
acetic acid ionization equilibrium, (you must use balanced chemical equations), explain how the
buffer solution will neutralize the addition of
a. A small amount of H+
b. A small amount of OH-
Follow-up Question
Blood must be maintained at a fairly constant pH of between 7.3 and 7.4. If you breathe too deeply and
rapidly, a condition known as hyperventilation may occur in which some muscles may become
temporarily paralyzed because the pH of the blood becomes too high. Explain why the pH changes in this
way and why exhaling into a paper bag and then breathing the exhaled air again can correct the
condition.
Unit Four: Aqueous Equilibriums (Buffer Solutions) Adapted from Essential Experiments for Chemistry
