Part A. Rate of Reaction at Normal
Pressure
1. Fill a 16 x 150 mm test tube 1/2 full of water.
The water should be at about room temperature.
2. Break an Alka-Seltzer tablet in half and drop
the pieces into the test tube.
3. 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!
1. Fill the 16 x 150 mm test tube 1/2 full of
water. The water should be about room temperature.
2. Break an Alka-Seltzer tablet in half. Drop
the pieces into the test tube.
3. 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.
4. 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.
5. 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.
6. 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.
Procedure
1. Reaction With Stirring
a. Add 100 ml of water to 400 ml beaker. The
water temperature should be at or near room temperature.
b. Add 40 ml of vinegar solution to the beaker.
Mix the vinegar and water.
c. Measure out 1/2 teaspoon of baking soda.
d. Set the 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.
e. 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
a. Add 100 ml of water to 400 ml beaker.
b. Add 40 ml of vinegar solution.
c. Set the beaker on a dark surface.
d. Add 1/2 teaspoon of baking soda; no stirring!
e. Measure the time required for all of the banking
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?
a. Involve the students in a discussion in which
they exchange ideas and opinions related to the
question.
b. Ask each student to make a prediction as to
whether stirring will or will not effect the rate
at which reactants in solution combine to produce
products.
Procedure to Test Predictions:
a. Add 200 ml of water to a 400 ml beaker.
b. Add 20 ml of vinegar solution to the 400 ml
beaker.
c. Add 50 ml of water to a 100 ml beaker. Dissolve
as much baking soda in the water as possible.
d. Pour the saturated baking soda solution into
the 400 ml beaker containing the vinegar. DO NOT
STIR THE MIXTURE.
e. Measure the time that elapses before the vigorous
bubbling action stops.
f. After a minute or two, stir the solution.
Observe and record the effect that the stirring
has on the reaction.
Clean-Up
All solutions can be safely washed down the drain.
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