Experiment 10 Factors Affecting Reaction Rates by uxx99201

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									  St. Catharine College                Chemistry 103                Experiment 10

Experiment 10
Factors Affecting Reaction Rates

Introduction

        In chemical reactions where there is more than one reactant, reactions are
dependent on the reactants colliding in exactly the proper orientation and at sufficient
force to allow the reaction to take place. Thus, we should be able to increase the rate of
the reaction for slow reactions by:

       •       increasing the number of collisions in a given period of time
       •       increasing the force of the collisions
       •       providing an alternative mechanism by which the reaction will happen and
                       thereby decrease the energy of activation (a catalyst).

As a consequence, we can do several things to a reaction to affect the rate of that
reaction:

       •       change the concentration of reactants,
       •       change the size of the reactant particles,
       •       increase or decrease the temperature in the reaction vessel,
       •       change the degree of agitation of the contents of the reaction vessel,
       •       and introduce a catalyst to the reaction vessel.

        An increase in temperature will speed up a reaction while a decrease in
temperature will slow down reaction times. The increase in temperature will increase the
speed of the motion of the molecules, both increasing the number and the force of each
collision. This increases the number of productive collisions, increasing the reaction rate.
Cooling will produce the opposite effect.

        An increase in the concentration of reactants will also increase reaction rate.
More particles in a given space will increase the total number of collisions. This will
result in an increased number of productive collisions, which will increase the rate of the
reaction. Decreasing the concentration, then, slows down a reaction.

        Since only the surface molecules in a solid piece of matter can collide with
surrounding reactants, if we provide more surface area on the solid, we will increase the
possibility of collisions. Smaller pieces accomplish this goal and, thus, will increase the
possibility of productive collisions, increasing the reaction rate.

       Stirring the contents of the reaction vessel will insure that reactant environments
are constantly changing, increasing the number of productive collisions over a period of
time. If the pot is not stirred the reactants in close proximity will react and need to be


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  St. Catharine College                Chemistry 103                 Experiment 10

replaced by the slower process of diffusion before there are more reactant molecules
available to continue the reaction.


        A catalyst increases reaction rates, most often by providing an alternative
mechanism which decreases the activation energy. It provides an intermediary on which
reactant particles can interact at a more favorable energy level. This increases the
likelihood that each collision will be a productive one. Thus the activation energy is
reduced. Catalysts can also hold reactants in a particular alignment so that incoming
reactants are more likely to hit the held reactant at an angle right for reaction. Thus, each
collision has a greater possibility of being a productive one. Either of these two
mechanisms will speed up the rate of reaction. Either scenario results in the overall
reaction proceeding more rapidly and/or more completely. If the end result is a more
complete reaction, a catalyst will also increase yield. A catalyst recycles automatically.
Thus, it takes part in an intermediary reaction but ends up the same as it began.

        In this experiment, to investigate these concepts we will use the reaction between
the calcium carbonate in calcium carbonate antacid tablets such as Tums® and
hydrochloric acid. Since this reaction produces bubbles, it is easy to tell when the
reaction is complete. Only part of the Tums® is calcium carbonate. Many of the
additional ingredients are not soluble in water and do not react with acid. Read the label
to determine what these ingredients might be.




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     St. Catharine College               Chemistry 103             Experiment 10

Pre-lab questions:

1.      List the factors that affect reaction rates.




2.     Calcium carbonate reacts with hydrochloric acid (HCl) to produce calcium
chloride, carbon dioxide, and water. Write the balanced equation for this reaction.




3.      In Part A:
         What condition(s) are you changing ? ______________________________

         What condition(s) are you keeping constant? ____________________________


4.      In Part B:
         What condition(s) are you changing ? ______________________________

         What condition(s) are you keeping constant? ____________________________



5.      In Part C:
         What condition(s) are you changing ? ______________________________

         What condition(s) are you keeping constant? ____________________________




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  St. Catharine College               Chemistry 103               Experiment 10

Procedure

       Make sure you record the start time (when the Tums ® is added to the acid),
and the finish time (the time the tablet stops bubbling), and calculate and report
elapsed time. Alternatively, you may use a stop watch if you have one and record
only the elapsed time. Measure all times to the nearest second.

       Use only clean, dry 50 or 100 mL beakers to carry out the reactions. Use the
same size beaker for all of the reactions.



Part A.       Determining the effect of concentration on reaction rate.

You may start all of the following four reactions at the same time.

                    ®
1.     Add 1 Tums tablet to 20 mL 1 M HCl at room temperature. Measure the time it
takes until bubbling stops. Do not stir.

                    ®
2.     Add 1 Tums tablet to 20 mL 3 M HCl at room temperature. Measure the time it
takes until bubbling stops. Do not stir.

                    ®
3.     Add 1 Tums tablet to 20 mL 6 M HCl at room temperature. Measure the time it
takes until bubbling stops. Do not stir.

                     ®
4.      Add 1 Tums tablet to 20 mL 0.1 M HCl at room temperature. Do not time this
reaction but note differences between what you observe in this reaction compared to the
other reactions.
_____________________________________________________________________

_____________________________________________________________________




                Sample           Start Time       End Time    Elapsed Time




                1 M HCl


                3 M HCl


                6 M HCl
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 St. Catharine College                Chemistry 103                Experiment 10


       a. How does your data illustrate the effect of concentration on reaction rate?




      b. Is your data consistent with what you think should have happened? Why or
why not.




       c. Which is the limiting reagent in each case?

                                  ®
              0.1 M HCl or Tums

                                ®
              1 M HCl or Tums

                                ®
              3 M HCl or Tums

                                ®
              6 M HCl or Tums




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  St. Catharine College                Chemistry 103               Experiment 10

Part B.        Identifying the effect of temperature on reaction rates.

        As the first step for this section, prepare a hot water bath — a 400 mL beaker on a
hot plate about 2/3 full of tap water with a beaker containing your acid for the experiment
suspended in it and thermometer suspended in the acid. Monitor the temperature
periodically and stabilize it at 60°- 75° C. Record the exact temperature at which you
conduct the experiment.

        Also set up an ice bath in a styrofoam bucket - ice and water mixture with beaker
containing acid suspended in it and thermometer in the acid. Allow the temperature to
stabilize as low as it will go and record the exact tempeerature at which you conduct the
experiment.




Figure 1. Ice bath/hot water bath set-up
                   ®
Reaction for Tums with 1 M HCl at different temperatures.

                    ®
1.     Add 1 Tums tablet to 20 mL 1 M HCl at room temperature. Measure the time it
takes until bubbling stops. Do not stir.

                    ®
2.     Add 1 Tums tablet to 20 mL 1 M HCl in the hot water bath. Measure the time it
takes until bubbling stops. Do not stir.

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     St. Catharine College             Chemistry 103                 Experiment 10

                    ®
3.      Add 1 Tums tablet to 20 mL 1 M HCl in the ice bath. Measure the time it takes
until bubbling stops. Do not stir.




           Sample           Temp, °C    Start Time       End Time         Elapsed Time




           Hot Water Bath


           Ice Bath


           Room Temp

        a. How does your data illustrate the effect of temperature on reaction rate?



      b. Is your data consistent with what you think should have happened? Why or
why not.




Part C.        Identifying the effect of piece size and stirring on solubility times

1.      Add 20 mL 1 M HCl to each of four small beakers.

                                           ®
2.     Into beaker 1 put one, whole Tums       tablet. Measure the time it takes until
bubbling stops. Do not stir.

                              ®
3.     Break another Tums tablet into about 10 chunks and add to beaker 2. Measure
the time it takes until bubbling stops. Do not stir.

                           ®
4.    Crush a third Tums tablet using a mortar and pestle and put in beaker 3.
Measure the time it takes until bubbling stops. Do not stir.




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 St. Catharine College                 Chemistry 103                  Experiment 10

                            ®
5.      Break a fourth Tums tablet as in Step 3. Try to make the chunks about the same
in size and number as used in step 3. Put these pieces in beaker 4 and stir the solution
with a glass stirring rod. Measure the time it takes until bubbling stops.


                Tablet             Start Time      End Time       Elapsed Time
                Condition
                Whole

                Chunks

                Crushed

                Chunks, stirred



       a. How does your data illustrate the effect of stirring on reaction rate?



       b. How does your data illustrate the effect of particle size on reaction rate?



       c. Which seems to effect the rate more?


          In this particular experiment, why do think that is true?



      d. Is your data consistent with what you think should have happened? Why or
why not.




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     St. Catharine College             Chemistry 103                Experiment 10

Part D.        Data comparison of control samples

1.     In each part of the experiment, one set of data was collected for a whole tablet at
room temperature. Find the data and transfer it to the table.

           Sample                                Elapsed Time
           Whole tablet/1 M HCl/Room Temp


           Part A


           Part B


           Part C


2.      a. What was the range of the data?        ____________________

        b. In a “perfect world” what would have been the range of this data?
         ____________________

        c. Why are controls such as this run? ____________________________

        ___________________________________________________________

        ___________________________________________________________

       Hint: If you are having trouble with the above question, think about this. If you
were applying for a job that required you to take a drug test, would you want the testing
done in a lab that ran controls or one that did not run controls?




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     St. Catharine College              Chemistry 103                 Experiment 10




Post Lab Questions

1.      Looking at the entire experiment to this point, which variable had the greatest
effect on reaction rate, concentration of one reactant, temperature, stirring, or piece size?
(To determine this look at the time difference between the shortest value in the set and
the longest.)


        Which variable had the least effect on reaction rate?




2.      If you have heart burn and want your Tums® to be effective quickly,

        a. why do the directions say “chew”?




        b. What two actions does your stomach provide?




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