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QUANTITATIVE STUDY OF REACTION RATES

Text References:    6.3  How Rates are Measured
                                 6.5  Dependence of Reaction Rate on Temperature and Other Factors
                                 6.7  Catalysis



INTRODUCTION:

Factors that can affect the rate of a reaction include temperature, the presence of a catalyst, and the concentration of reactants.  In this experiment, you will conduct some simple reactions to investigate, quantitatively, how these factors can influence the outcome of a reaction.

In Part One, the reaction examined is that between sodium thiosulfate and hydrochloric acid:
                        Na2S2O3 + 2 HCl --> 2 NaCl  + H2O + S + SO2
The concentration of the thiosulfate will be changed by adding water and the appearance of the opaque colloidal sulfur will be used to determine the rate of the reaction.

In Part Two, the reaction examined is the reduction of potassium permanganate by glucose:
                        MnO4- + glucose --> Mn2+ + CO2 + H2O
The reaction rate is determined by timing how long it takes for the purple colour of the permanganate to disappear at different temperatures.

In Part Three, the reaction is between sodium thiosulfate and iron(III) nitrate:
                        Fe3+ + S2O32- --> purple complex --> colourless
The rate of the reaction is measured by timing how long it takes for the mixture to become colourless.  Different compounds are then added to test for their catalytic activity in this reaction.



OBJECTIVES:

PRE-LAB:

1.    In your lab notebook, write the title of the lab and the date you will be doing the lab at the top of a new page.  On the Table of Contents, write the title and include its page number.
2.    Read the procedures well so that you will be familiar with what is to be done.  You could also assign different tasks to each member of the group in order to maximize your efficiency during the lab.
3.    Prepare a data table as follows:

Part 1:    Effect of Change of Concentration
Concentration of sodium thiosulfate
Time until cross disappears(s)
20%
                                             
40%
                                
60%
                              
80%
                             
100%
                                 

4.    Prepare a data table as follows:

Part 2:  Effect of Temperature
Temperature of Solution (oC)
Time (s)
                                    
                                   
                                       
                                              
                                               

5.    Prepare a data table as follows:

Part 3:  Effect of Adding A Catalyst
Catalyst
Time(s)
none           
nickel(II) sulfate              
iron(II) sulfate                  
copper(II) sulfate                
cobalt(II) chloride                      



APPARATUS AND MATERIALS:
 
 Erlenmeyer flask, 150 mL  graduated cylinder, 50 mL beaker, 250 mL
burette, 50 mL burette clamp thermometer
retort stand triple beam balance timer
5 test tubes hotplate sheet of white paper with a cross marked on it
test tube rack iron(III) nitrate, Fe(NO3)3,0.10 mol/L
hydrochloric acid, HCl, 2.0 mol/L sulfuric acid, H2SO4, 2.0 mol/L nickel(II) sulfate, NiSO4, 0.10 mol/L
iron(II) sulfate, FeSO4, 0.10 mol/L copper(II) sulfate, CuSO4, 0.10 mol/L cobalt(II) chloride, CoCl2, 0.10 mol/L
glucose, C6H12O6, 0.10 mol/L potasium permanganate, KMnO4, 0.10 mol/L sodium thiosulfate, Na2S2O3, 0.25 mol/L


SAFETY:

PROCEDURE:

Part One:  Effect of Change in Concentration
1.    Place 10 mL of sodium thiosulfate in a 150 mL Erlenmeyer flask and add 40 mL of water.
2.    Place the flask on top of a sheet of white paper with a cross marked on it.
3.    Add 5 mL of 2.0 mol/L hydrochloric acid and at the same time start the stop watch.  Swirl the contents of the flask.
4.    Look vertically down the flask at the cross and record the time taken until the cross can no longer be seen through the solution.
5.    Repeat the experiment using 20, 30, 40 and 50 mL of sodium thiosulfate, diluted with water so that a total volume of 50 mL is maintained in each case before addition of the acid.  Make sure that the Erlenmeyer flask is rinsed well between reactions.
6.    Clean the Erlenmeyer flask thoroughly and drain well.

Part Two:  Effect of Temperature
1.    In a 250 mL beaker, place 25 mL of 2.0 mol/L sulfuric acid and 75 mL of water.
2.    From a burette, add 4 mL of potassium permanganate.
3.    Warm the beaker o a hotplate until the temperature of the liquid is about 55 - 60 oC.
4.    Place the beaker on a sheet of white paper and record its exact temperature in your table.   Add 5 mL of glucose solution and start the timer.  Swirl to mix, and record the time it takes for the purple colour to disappear.
5.    Repeat the experiment, heating the beaker to a temperature 5oC below the previous temperature used.  Continue for a total of five different temperatures.  Make sure the beaker is cleaned well between the reactions.
6.    Clean your glassware.

Part Three:  Effect of Adding A Catalyst
1.    Place 10 mL of iron(III) nitrate in a 150 mL Erlenmeyer flask.  Put the flask on a sheet of white paper.
2.    Add 10 mL of sodium thiosulfate and start timing.  Swirl to mix, and record the time it takes for the liquid to become colourless. (This is the reaction with no catalyst.)  Clean and drain the Erlenmeyer flask well.
3.    Repeat step 2, this time adding 5 drops of nickel(II) sulfate.  Continue as before.
4.    Repeat the experiment three more times, using 5 drops of copper(II), iron(II) and cobalt(II) compounds respectively.
5.    Clean all equipment and return it to the appropriate locations.



CALCULATIONS:

Part One:
1.    Plot a graph of your data for the concentration of thiosulfate (vertical axis) versus time (horizontal axis).
2.    Plot the concentration of thiosulfate versus the reciprocal of the time (1/T, i.e. one divided by the time).  The reciprocal of time is a measure of the rate of the reaction.

Part Two:
1.    Plot a graph of temperature (vertical axis) versus time (horizontal axis).



QUESTIONS:

1.    Make a quantitative statement about how the rate of a reaction varies with concentration.
2.    what can be concluded about the rate of a reaction as temperature changes?
3.    Do any of the transition metal compounds catalyse the reaction in Part three?  Which one is the best?
4.    State three ways in which this reaction (Part 3) could be slowed down.

Adapted from:  Chemistry: A Second Course
                        Lab Manual
                        Art Frankel, John Li
                        Addison-Wesley Publishers
                        Don Mills, Ontario, 1989