BTHS AP Chemistry Experiment 7: Kinetics of Alka-Seltzer™

Name _Date

IntroductionAlka-Seltzer™ is a combination of aspirin (acetylsalicylic acid), sodium hydrogen carbonate (NaHC03), and citric acid,designed to treat pain and simultaneously neutralize excess stomach acid. It is provided in the form of large effervescenttablets, about I inch (25 mm) in diameter, which are placed (two at a time for the usual adult dosage) in a glass of water.As the tablets dissolve, the base (hydrogen carbonate ion) and the acid (citric acid) react vigorously producing carbondioxide gas (hence the "Seltzer"), which also produces enough agitation to dissolve the remaining active ingredients. Thepatient then ingests the resulting solution.

The acetylsalicylic acid acts as an analgesic (reducing pain) and the citric acid reacts with the sodium hydrogencarbonate to torm water, carbon dioxide and sodium citrate solution. The carbon dioxide gas produced can be collected tomeasure the rate of the reaction.

1) Write the net ionic equation here.

Multiple factors can affect reaction rate. Among these are temperature and reactants' concentration, andexperiments varying these can be conducted to determine the rate law for a particular reaction. Since we have multipleweak acids in equilibrium with their conjugate bases, we can assume that the solution acts like a buffer, and therefore [UY]

will be constant for our purposes.

2) Write the chemical equilibrium equation for citric acid and the citrate ion.

Problem What are the rate law, rate constant, and activation energy for the reaction between Alka-Seltzer™ and water?

In this experiment, the concentration of the bicarbonate ion, [HC03-], decreases as the reaction takes place. Thischange can be monitered by converting the volume of the CCh formed to moles ofCOz, allowing us to then,stoichiometrically, convert to moles of bicarbonate ion. Use the moles of the bicarbonate ion (disappearing) over time todetermine the order of reaction with respect to bicarbonate ion. This will give the effective rate law for the reaction.

We will separately observe the effect of excess acid on the solution (if there is sufficient acid to overwhelm thebuffer system) as part of our experiments. We will also determine the rate constant from the rate law data and theactivation energy by conducting the trials at two different temperatures.

Materials and equipment

Alka-Seltzer tabletsDe-ionized water

VinegarMarker or grease pencilPaper towels '

Mortar and pestleErlenmeyer flask, 150mL with side armBuret with ringstand and clampSmall beaker or vialRubber stopper and rubber tubing

Therm9meterpH meterHot plateSeparatory funnelFunnel support

Experimental Procedure1. Each group will receive two Alka-Seltzer™ tablets for use in their experiments. Break the first tablet into small piecesanet select a piece that masses approximately 1.0 - 1.5 grams andrecord the mass. Cmsh the rest of the tablets intopowder so that they can be used uniformly in experiments. Divide the powder into 4 samples that match the mass of theone chunk you saved. (5 trials in total to be conducted.)

2. Set up an apparatus as shown in the diagram. The sidearm flask is attached to the buret on top, and the open buret isattached to the separatory funnel on the bottom. The funnel will serve as a reservoir for water that can be lowered so thatthe volumes within the funnel and the buret are level, keeping internal pressure of the reaction constant and equal toatmospheric pressure ...

Alka Seltzer ~) Make sure to wipe marks off of buret after each trial.in flask

3. Place one measured amount of powder into the Erlenmeyer flask. Save the other samples of the same mass foradditional trials. Fill the vial with solvent and stand it up in the flask. Put the stopper on the flask and shake to start thereaction. Make sure the level of water in the buret and separatorv funnel are the same and record the volume on the buret

every 5 seconds. You can do this with a grease pencil on the buret and then write down the volumes when you arefinished with the reaction.

Attach buret to ring stand

Stabilize funnel but do notattach. This needs to behand-held while the trials

are conducted. Why?

Vial contains solvent

Water enters here. Keep levelof water in funnel and buret

the same. Why?

1. Fill-in data: 2 trials at 2 differents temps, 1 trial w/chunkTrial/mass!

Data and Results

Since the only source ofCOz is BCO)-, which is soluble, we can calculate the number of moles of HC03' by measuring thevolume of COz formed during the reaction at each timepoint, convert these volumes to moles, and plot to determine theinstantaneous rate of reaction at each timepoint. The final volume for COz converts to the final moles of C02 which arethe product of the initial moles of RC03-

Moles RC03- in solution = total moles COz formed - moles COz formed at your time-point. Once you have moles

of RCO)- you can find the molarity by dividing by the volume of the solution.

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-5

Time (sec): ---- ---- ---- ---- ---- --- ---- ---- ----- ---- ---- ----Total timer:

2. [HC03 -] at each timepoint123456789101112ini tall

TempiTempz

3. Prepare plot of [HC03'] versus rate o~disappearance on graph paper and attach.

Questions1. Determine the order with respect to the bicarbonate ion. Write the rate law assuming this ion is the only factor involvedand calculate the rate constant for the reaction at each temperature.Show work.

2.a. How is the rate affected by temperature?

b. Calculate the activation energy using the Arrhenius equation. Show work.

3. Predict the effect lower pH would have on the reaction rate. How could you confirm your prediction using the sameapparatus? What observations would confirm your prediction?

4. What were sources of error for your group and how did they affect your results?

p"poi) CHEMISTRY FREE-RESPONSE QUESTIONS

, A student performed an experiment to investigate the decomposition of sodium thiosulfate, N~SZ03' in acidic~oiution, as represented by the equation above. In each trial the student mixed a different concentration of

sodium thiosulfate with hydrochloric acid at constant temperature and determined the rate of disappearance of

SzOl-(aq). Data from five trials are given below in the table on the left and are plotted in the graph on the right.

0.060InitialInitial Rate of

ConcentrationDisappearance

Trialof SzO/- (aq)of SzO/- (aq)

(M)(M $-1)

1

0.050 0.020

2

0.075 0.030

3

0.088 0.034

4

0.112 0.045

5

0.125 0.051

0.010 I0.05

[

0.07 0.09 0.11 0.13

Initial Concentration of SZ03z- (M)

.i>! Identify the independent variable in the experiment.

(b) Determine the order of the reaction with respect to SZ032-. Justify your answer by using the informationabove.

(c) Determine the value of the rate constant, k, for the reaction. Include units in your answer. Show how youarrived at your answer.

(d) In another trial the student mixed 0.10 M NaZSZ03 with hydrochloric acid. Calculate the amount of time it

would take for the concentration of S203L- to drop to 0.020 M .

(e) On the graph above, sketch the line that shows the results that would be expected if the student repeated thefive trials at a temperature lower than that during the first set of trials.