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. Carbonate (e.g., Na₂CO₃) and bicarbonate compounds react with acids to form carbon dioxide.

HCO₃^-1 + H⁺ ------------> H₂O(l) + CO₂(g)

and CO₃^-2 + 2 H⁺ -----------> H₂O(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.

Procedure:
The volume of CO₂ formed is directly proportional to the mass of CO₂ formed and the mass of NaHCO₃ decomposed. The volume of the CO₂ formed at 10 second time intervals is measured using the apparatus shown below.

Half-fill a copper bath with room temperature tap water. Measure and record the temperature of the water. Blow into the tube to force water from the large flask into the beaker to create a siphon. Pinch the tubing to prevent the water from siphoning over. Carefully place a vial of water taken from the water bath and an Alka-Seltzer tablet into the dry 250 ml flask. Put the stopper tightly into the flask, transfer the end of the tube to the empty graduated cylinder, and unpinch the tube. 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 on the cylinder every 10 seconds. When gas evolution is over, read and record the volume measurements in your notebook using the markings on the cylinder.

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 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.

The tables of times and volumes along with the identification of reaction conditions will be in your Results section. Use graph paper 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. The slope of this line is a measure of the rate of evolution of CO₂. It is expressed in units of ml CO₂ per second and calculated by dividing the number of milliliters of CO₂ collected by the time it took to collect this gas. An example is shown below 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?

Click here for a prepared data sheet to print.

                                                         Summary of Trial Runs

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