If any of the products or reactants involved in a chemical reaction are gases, the rate of reaction will decrease as pressure on the system is increased.
When water reacts with Alka-Seltzer, one of the products of the reaction is carbon dioxide gas.
» 400 mL/13.5 oz beaker
» Safety goggles
» Test tube 16 x 150 mm
» Cork Stopper, #4
» 1 package original effervescent Alka-Seltzer formula
» Baking Soda
Part A. Rate of Reaction at Normal Pressure
Fill a 16 x 150 mm test tube 1/2 full of water. The water should be at about room temperature.
Break an Alka-Seltzer tablet in half and drop the pieces into the test tube.
Measure the time required for the reaction between the Alka-Seltzer and water to be completed. Record the time.
Part B. Rate of Reaction Under Increased Pressure
PUT ON YOUR SAFETY GOGGLES!
Fill the 16 x 150 mm test tube 1/2 full of water. The water should be at about room temperature.
Break an Alka-Seltzer tablet in half. Drop the pieces into the test tube.
Immediately insert the cork stopper in the end of the test tube to slow down the escape of carbon dioxide gas. The pressure being exerted upon the reactants inside the test tube is equal to the pressure you feel being exerted against the cork.
Note: Do not try to stop all of the gas from escaping! If you try to do so, the test tube will break. Just slow down the escaping gas as pressure builds up inside of the test tube. That pressure acts upon the surface of the liquid. The liquid moves in a direction that will relieve the pressure. The only direction, without breaking the test tube, is upward and out. Avoid a total "blow-out" of the liquid from the tube by releasing enough pressure on the cork from time to time to let gas and liquid "squeeze" its way past the cork, in controlled amounts. This is a struggle; you against the gas pressure.
When gas bubbles are no longer visible in the liquid contained in the test tube, consider the reaction to be over. Observe and record the time of the reaction.
Release the pressure on the cork. Note that the reaction starts again. Increase pressure on the cork and the reaction stops.
Note that there is still a small amount of Alka-Seltzer left in the bottom of the test tube. You have now reached the point when you are able to exert enough pressure to be equal to the pressure being exerted by the gas inside the test tube. As a result, the reaction stops. When you release your pressure, the reaction is able to start again.
Remove the cork. Observe and record what happens to the remaining Alka-Seltzer.
Part C. Effect of Mechanical Mixing
Ideas to be Developed
The rate of reaction can be increased if the movement of the REACTANTS is increased as a result of mechanical mixing (stirring).In this investigation, baking soda, NaHCO3, and vinegar, HCH3O2, will be the reactants. The rate of reaction will be determined by measuring the time required for a specific amount of NaHCO3, to be used up, first with stirring and then without stirring.
1. Reaction With Stirring
Add 3.4 oz./100 mL of water to 400 mL/13.5 oz. beaker. The water temperature should be at or near room temperature.
Add 1.35 oz./40 mL of vinegar solution to the beaker. Mix the vinegar and water.
Measure out 1/2 teaspoon of baking soda.
Set the 13.5 oz./400 mL beaker on a dark surface so as to provide good contrast with the white NaHCO3. Add the 1/2 teaspoon of baking soda to vinegar solution and stir continuously.
Measure the time required for all of the baking soda to be “used up” in the reaction with vinegar. Record the time.
2. Reaction Without Stirring
Add 100 mL/3.4 oz of water to 400 mL/13.5 oz. beaker.
Add 1.35 oz./40 mL of vinegar solution.
Set the beaker on a dark surface.
Add 1/2 teaspoon of baking soda; no stirring!
Measure the time required for all of the baking soda to react. Record the time.
3. Effect of Stirring on Rate of Reaction Where Reactants are Already in Solution
Introduction: Observable evidence from investigation 1 and 2 shows that stirring increases Rate of Reaction when at least one of the reactants involved starts out as a solid.
Questions to be investigated:
Does stirring have any effect on the Rate of Reaction if the reacting substances are already in solution?
Do you think stirring will or will not affect the rate at which reactants in solution combine to produce products?
Procedure to Test Predictions:
Add 6.8 oz./200 mL of water to a 13.5 oz./400 mL beaker.
Add 0.7 oz./20 mL of vinegar solution to the 13.5 oz./400 mL beaker.
Add 1.7 oz./50 mL of water to a 3.4 oz./100 mL beaker. Dissolve as much baking soda in the water as possible.
Pour the saturated baking soda solution into the 13.5 oz./400 mL beaker containing the vinegar. DO NOT STIR THE MIXTURE.
Measure the time that elapses before the vigorous bubbling action stops.
After a minute or two, stir the solution. Observe and record the effect that the stirring has on the reaction.
This investigation showed slowing down the rate at which the gas is allowed to escape from the reaction chamber will control pressure inside the reaction chamber. The effect that an increase in pressure has on the Rate of Reaction was measured.