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2602 Factors Affecting Reaction Rates

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    440619
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    1.0 INTRODUCTION

    1.1 Objectives

    After completing this experiment, the student will be able to study the effects of concentration, temperature, and catalysts on a reaction rate.

    1.2 Background

    This experiment will allow you to study the effects of concentration, temperature,

    and catalysts on a reaction rate. The reaction whose rate you will study is the reduction of permanganate ion by oxalic acid. The overall balanced equation is:

    2 MnO4-(aq) + 5 H2C2O4 (aq) + 6 H+(aq) ­čí¬ 2 Mn2+(aq) + 10 CO2(aq) + 8 H2O

    Measuring reaction rate entails measuring some unique property of one of the

    products or reactants. In this experiment, the unique property you will look for is the

    decolorization of the permanganate solution. The change in color from pink to colorless can be used as a signal to measure the rate of the reaction. The time from mixing the reactants to the disappearance of the pink color can easily be measured. This

    length of time is the time required by the reaction to reduce MnO4- to Mn2+.

    As the experiment will show, the time required varies with the concentrations of the

    reactants, with temperature, and with the presence of a catalyst.

    To determine the effect of concentration, the reaction will be carried out with the reactants at several different initial concentrations. This will be done by varying the amount of either sulfuric acid (source of H+) or oxalic acid added to the reaction mixture. The total volume will be kept at 45 mL in each case by adding appropriate amount of laboratory water.

    To determine the effect of temperature, the reaction will be performed at a specific concentration of reactants (held constant) and the temperature will be varied. A small change in temperature can cause a large change in reaction rate. For a typical reaction near room temperature, a 10°C increase in temperature doubles the rate

    of reaction. This is because, in a typical reaction, very few of the collisions between

    reactant ions or molecules take place with sufficient energy to cause reaction. The

    energy necessary for a collision to result in reaction is called the energy of activation, Ea,

    of that reaction. A small increase in temperature can have a considerable effect on the

    number of molecules that have this required amount of energy to undergo reaction when

    collision occurs.

    A catalyst is a substance that can increase the rate of reaction by changing the mechanism of the reaction. Usually, a catalyzed mechanism will have a lower activation energy than an uncatalyzed mechanism. At any given temperature, there will be more molecules having sufficient energy to react via the catalyzed mechanism than via the uncatalyzed route. Therefore, the rate of the catalyzed reaction will be faster.

    In this experiment, the catalyst will be copper metal. Both the catalyzed and the uncatalyzed reactions will be performed at several lengths of a copper wire while keeping the initial concentrations of the reactants and the reaction temperature constant. In this way, the effect of the presence and amount of catalyst on the reaction rate can be determined.

    2.0 SAFETY PRECAUTIONS AND WASTE DISPOSAL

    !!Wear your safety goggles!!

    All solutions from this experiment should be poured into the INORGANIC WASTE container in the fume hood.

    Copper wires must be separated from the solution, dried, and disposed of in the SOLID WASTE container in the fume hood.

    3.0 CHEMICALS AND SolutionS

    3.0 CHEMICALS AND SolutionS

    Chemical

    Concentration

    Approximate Amount

    Notes

    Laboratory water

    Pure

    150 mL

    Be sure to use laboratory water

    KMnO4(aq)

    0.0014 M

    50 mL

     

    H2C2O4(aq)

    0.20 M

    50 mL

     

    H2SO4(aq)

    2.0 M

    50 mL

     

    Copper wire

    Various lengths

    10 in

     

    4.0 GLASSWARE AND APPARATUS

    50 mL beakers (4) (or any size 250 mL)

    Droppers (plastic or glass with rubber bulb)

    400 mL beaker (for water bath)

    Thermometer

    Test tubes (medium)

    Hot plate

    5.0 PROCEDURE

    This experiment should be performed by two people working together. During each run, one person can watch the reaction mixture and the other can watch the clock or timer.

    5.1 Effect of H+ Concentration

    1. Using the four clean and dry 50-mL beakers (or any size 250 mL available), obtain about 50 mL of each of the four reagents needed to prepare the mixtures listed in the table below. Label each beaker appropriately. Assign one dropper for each reagent as well.
    1. Prepare each reaction mixture by first mixing KMnO4, H2SO4, and laboratory water in a clean and dry test tube. Then, quickly add H2C2O4 to the test tube, mix by flicking, and start the timer. Record the time when the pink color has completely disappeared. Monitor the reaction for about 10 minutes. It will be helpful to use a white paper as background to see the decolorization process. Enter the time on your data recording sheet.

    Reaction #

    KMnO4

    H2O

    H2SO4

    H2C2O4

    1A

    15 drops

    25 drops

    5 drops

    0

    2A

    15 drops

    20 drops

    5 drops

    5 drops

    3A

    15 drops

    10 drops

    15 drops

    5 drops

    4A

    15 drops

    0

    25 drops

    5 drops

    5.2 Effect of H2C2O4 Concentration

    Repeat step 2 in Part 5.1 above using the reaction mixtures below. Be sure to use clean and dry test tubes.

    Reaction #

    KMnO4

    H2O

    H2SO4

    H2C2O4

    1B

    15 drops

    25 drops

    0

    5 drops

    2B

    15 drops

    10 drops

    15 drops

    5 drops

    3B

    15 drops

    5 drops

    15 drops

    10 drops

    4B

    15 drops

    0

    15 drops

    15 drops

    5.3 Effect of Temperature

    1. Set up a hot-water bath in a half-filled (with tap water) 400 mL beaker placed on a hot plate. Heat the water bath until the temperature of the water is between 40oC and 50oC. Try to maintain this temperature interval by heating or letting the water bath to cool. The exact temperature at which the reaction is run is not crucial.
    1. In this part of the experiment, the concentrations of reactants will be the same for both runs but the temperature will be higher than room temperature.
    1. Prepare each reaction mixture by first mixing KMnO4, H2SO4, and laboratory water in a clean and dry test tube. Place this test tube into the water bath for several minutes to allow the solution to reach the bath temperature. Then, remove the test tube from the bath and quickly add H2C2O4, mix by flicking, and return the test tube to the bath. Start the timer. Record the time when the pink color has completely disappeared.

    Run #

    KMnO4

    H2O

    H2SO4

    H2C2O4

    Temp

    1C

    15 drops

    10 drops

    15 drops

    5 drops

    40oC50oC

    2C

    15 drops

    10 drops

    15 drops

    5 drops

    40oC50oC

    5.4 Effect of Catalyst

    1. In this part of the experiment, the concentrations of reactants will be the same for all runs and the reaction will be kept at room temperature.
    1. Repeat step 2 in Part 5.1 above. Be sure to use clean and dry test tubes. Copper wire will be added as indicated in the table below. The exact length of the copper wire is not crucial. You may coil the wire to allow for maximum contact with the solution.

    Run #

    KMnO4

    H2O

    H2SO4

    H2C2O4

    Cu Length

    1D

    15 drops

    10 drops

    15 drops

    5 drops

    0

    2D

    15 drops

    10 drops

    15 drops

    5 drops

    1 – 3 in.

    3D

    15 drops

    10 drops

    15 drops

    5 drops

    4 – 6 in.














    6.0 DATA RECORDING SHEET

    Last Name

    First Name

     

    Partner Name(s)

    Date

    6.1 Effect of H+ Concentration

    Reaction #

    Notes/Observations

    Reaction Time

    1A

       

    2A

       

    3A

       

    4A

       

    6.2 Effect of H2C2O4 Concentration

    Reaction #

    Notes/Observations

    Reaction Time

    1B

       

    2B

       

    3B

       

    4B

       

    6.3 Effect of Temperature

    Reaction #

    Notes/Observations

    Reaction Time

    1C

       

    2C

       

    6.4 Effect of Catalyst

    Reaction #

    Notes/Observations

    Reaction Time

    1D

       

    2D

       

    3D

       

    7.0 POST-LAB QUESTIONS

    1. What is the effect of the concentration of H+ (from sulfuric acid) on the rate of the reaction? What is the effect of the concentration of H2C2O4 on the rate of the reaction?







    1. Did you observe any similarities or differences between the effect of H+ and H2C2O4 concentrations? Explain your answer fully.








    1. What are the effects of the temperature and the presence of Cu metal on the reaction rate? Provide a physical explanation to each of these observations. What is occurring on the molecular level?









    1. Which of the factors you determined experimentally has the most profound effect on the reaction rate? Explain your choice.

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