le Châtelier's Principle: Gas Pressure

Carbonated beverages such as soda pop contain a saturated solution of carbon dioxide in water.  The gas is under pressure, and in equilibrium with the water in the drink through the following chemical reaction:

H₂O (l)  + CO₂ (g)  H₂CO₃ (aq)

H₂CO₃ (aq) or carbonic acid is a weak acid.

There are two reasons why we enjoy carbonated beverages: the bubbles of CO₂ that are released in our mouth when we drink it cause a "tingling" sensation that we enjoy, and the weak acid makes the drink slightly sour, which is a taste that humans like.  However, we don't like drinking them when they go flat, which they will do if we leave the top off the container.

You will also notice that a drink will go flat (lose its carbonation) if you partially empty a resealable bottle.  Why is that, since the gas cannot escape from a sealed bottle?  When you partially empty a bottle, you increase the headspace (the volume above the liquid).  This headpace is filled with CO₂ (g).  When the headspace is large, the volume available for the gas is larger.  This changes the gas pressure.

Changing the volume will change the gas pressure in the container and the result can be predicted from le Châtelier's principle.  We can see neither the CO₂ gas, nor the H₂CO₃ (aq).   However, we can detect the presence of the carbonic acid by using an acid-base indicator such as methyl red which changes color from red in weak acids (pH 5) to yellow when neutral.  This will allow us to determine the effect of changing the pressure on the equilibrium.

Materials:

• 300 mL of carbonated water
• clear bottles of approximately 2 L, 500 mL, and 100 mL volume.  These bottles must have stoppers that can let them be sealed gas tight.  Empty PET soda bottles are ideal for the larger volume containers.
• methyl red acid-base indicator
• 100 mL graduated cylinder

Procedures:

Wear safety goggles while doing this lab.

1. If your teacher has not already done so, add about 1 mL of methyl red acid-base indicator dye to the carbonated water, and note the color.  Stir very gently to mix the dye (you don't want to lose the carbonation at this point).
2. Measure 100 mL of the carbonated water, add it to each of the bottles, and seal them.
3. Shake the bottles vigorously for about 30 seconds.  This will increase the surface area in contact between the gas and liquid phases, so equilibrium is achieved more rapidly.
4. Observe the color of each solution.
5. Dispose of the solutions down the sink.

Conclusion:

Observe the color of each bottle, and explain the results in terms of le Châtelier's principle.