The Rate of Gas Evolution from the Reaction of Alka-Seltzer Tablets with Water

Whenever two chemicals react, the reacting molecules or ions must collide in order for the reaction to take place. If the reactants are forced to collide more frequently then the reaction should be faster. Two factors which can influence the frequency of collision are temperature and concentration. Increasing the temperature of the reactants causes them to move faster and thus collide more frequently. If the concentration of a reactant is increased, then there are more opportunities for collisions and so the reaction rate increases.

The purpose of this experiment is to measure the effect of temperature and concentration on the rate of a reaction, the formation of carbon dioxide from an Alka-Seltzer (or similar) tablet plopped into water. Alka-Seltzer tablets contain aspirin, sodium bicarbonate (NaHCO₃) and citric acid. Recall carbonate (e.g., Na₂CO₃) and hydrogen carbonate compounds (e.g., NaHCO₃)react with acids to form carbon dioxide gas.

HCO₃^-1 + H₃O⁺ ® 2H₂O(l) + CO₂(g)

and CO₃^-2 + 2 H₃O⁺ ® 3H₂O(l) + CO₂(g)

In an Alka-Seltzer tablet the bicarbonate and acid are solids and so the H⁺ and CO₃^-2 ions are not free to move, collide and react. When plopped into water, the citric acid and sodium bicarbonate dissolve , freeing the ions to react. This results in the formation of the carbon dioxide gas and water as the reaction products. By measuring how fast the carbon dioxide is formed, we are measuring the rate of the reaction.

Special Equipment: Things for each group to use and return on the same day.

Procedure:
The volume of CO₂ gas formed is directly proportional to the masses of CO₂ formed in the reaction and the masses of NaHCO₃ or Na₂CO₃ decomposed. In this method the volume of the CO₂ formed at 10 second time intervals is measured, using the apparatus shown below, to determine the rate of its formation.  This is then used to study the effect of temperature and tablet area on the reaction rate. 

Half-fill a copper bath with room temperature tap water. Measure and record the temperature of the water. Fill the larger Erlenmeyer flask with water and half fill the graduated cylinder with water.  Attach the stopper to the large flask as shown above, leaving the smaller flask's stopper free.  Place the other hose into the graduated cylinder so that the opening remains below the surface of the water.  

Using a pipet bulb, blow air into the free stopper tube to force water from the large Erlenmeyer flask into the graduated cylinder to create a siphon. There should be no air in the hose that goes from the large flask to the graduated cylinder.  If a siphon is successfully established, the water level  inside the large flask can be raised or lowered by moving the graduated cylinder above or below the level of the flask.  Check to be sure your set-up is siphoning.

Allow enough water to siphon into the large flask to nearly fill it, then pinch the water filled tubing with a pinch clamp to prevent the water from siphoning over.  Empty the water in the graduated cylinder and reposition the hose into it.  

Carefully place a vial of water taken from the water bath and an Alka-Seltzer tablet into the dry 250 mL flask (smaller flask). Put the stopper tightly into the flask, and remove the pinch clamp from the hose. A small amount of water will drain into the cylinder but should not siphon from the flask. (If it does, there is a leak which must be fixed). Hold the 250 mL flask in the water bath and swirl to start the reaction. Holding the cylinder below the bench level, record the volume of water displaced every 10 seconds until it is unchanged for 20 seconds. The most accurate results are obtained by marking the level of the water in the cylinder directly onto a strip of tape, affixed to the cylinder, with a pen, every 10 seconds. When gas evolution is complete, read and record the volume measurements in your notebook using the markings on the cylinder and replace the tape for the next trial.  Record the reaction temperature.

Repeat the experiment using the same temperature. Also do two trials each for a low temperature reaction and a high temperature reaction using cold water or hot water in the water bath and reaction vial. Be certain to measure and record all of your reaction temperatures. 

How does the rate of the reaction and total volume of CO₂ evolved depend on the size of tablet? Test this by cutting a tablet in half and measuring the rate of the reaction at room temperature with each half-tablet.  Be certain the tablets have the same approximate size and are in one piece, not crumbled.

The tables of times and volumes along with the identification of reaction conditions will be in your Results section. Use Excel to make plots of CO₂ volume vs. time for your results. Be certain to title each graph and label the axes. Several trials can be plotted on one graph using different colors for each (label!) but use the maximum area possible on your graph paper. Normally a straight line can be drawn from the graph origin (0 sec, 0 mL) through the initial points in each trial. Use a TRENDLINE to determine the equation (slope) of this line.  .This slope is a measure of the rate of evolution of CO₂. It is expressed in units of ml CO₂ per second and can be calculated by dividing the number of milliliters of CO₂ collected by the time it took to collect this gas, although Excel  TRENDLINE is easier and more accurate. Be sure you select only those initial points as an added series to get the linear equation.  An example is shown below for a reaction which has a rate (slope) of 2.4 ml/sec.

For your Conclusion tabulate the average rate of the reaction at each of the three temperatures for the whole tablet, and for the reaction with the smaller (half) tablet. How do temperature and tablet size effect the rate of the reaction and the amount of CO₂ evolved?

Go here for a prepared data sheet to print.

                                                         Summary of Experiments

last edited by jb on 1/20/06