SCH4U Unit 3- Activity 15- Lab Rate Factors Instructions LAB Factors Affecting the Rate of a Reaction   Introduction:   In Part I of this experiment you investigate the general relationship between the concentration of a reactant and the time required for a reaction to occur

SCH4U Unit 3- Activity 15- Lab Rate Factors Instructions

LAB

Factors Affecting the Rate of a Reaction

Introduction:

In Part I of this experiment you investigate the general relationship between the concentration of a reactant and the time required for a reaction to occur. In Part II you examine the general rela­tionship between the temperature and the time required to complete a given reaction. In these reactions you hold all factors constant except the one being studied.

The reaction which you will study in Parts I and II is known as the iodine "clock" reaction. This type of reaction involves several steps. The slowest step is called the rate-determining step. In this reaction an excess of iodate ions (IO3-) react with an acidified sodium sulfite solution. The first step in the reaction is the slow, rate-determining step.

3HSO3-(aq) + IO3-(aq)   + 3H2O    3SO42-(aq) +I-(aq) + 3H3O+                                                                                                     Eq. 1

The I- ions formed react with excess  IO3- ions to form free iodine.

IO3-(aq) + 5I-(aq) + 6H3O+     3I2(s) + 9H2O                                                Eq. 2

The I2 liberated in Eq. 2, however, reacts rapidly with HSO3-

I2(aq) + HSO3-(aq) +   4H2O  2 I-(aq) + SO42-(aq) + 3H3O+

Thus the I2 concentration never builds up until all of the HSO3- has been consumed. Then the blue color appears. The blue coloration is from the interaction of the molecular iodine with a starch suspension.

In Part I you hold the concentration of  the HSO3-  ions and the temperature constant, and vary the concentration of the IO3-  ions. You measure the elapsed time from the instant of mix­ing to the sudden appearance of the blue color.

In Part II you hold the concentrations of all solu­tions constant and vary the temperature. You use your data to plot graphs and draw some general conclusions concerning the effects of concentra­tion and temperature on the rate of a reaction.

Purpose:

- To determine the effect of concentration on the rate of a reaction.

- To determine the effect of temperature on the rate of a reaction.

Materials:

250 mL beaker                                                                      100 mL graduated cylinder

6 test tubes                                                                            test tube rack

solution A containing sulfurous acid (H2SO3)  and starch

solution B containing 0.02 mol/L potassium iodate (KIO3) 

stop watch                                                                             hot plate

thermometer                                                                         hot water bath (600 mL beaker)

Procedure:

Part I Effect of Concentration on Rate

1.   Measure 60 mL (or your group's volume) of the KlO3 solution (solution B) in a dry graduated cylinder and pour the solution into a 250 mL beaker. Add 10 mL (or your group's volume) of water and mix. Pour 30 mL (or your group's volume) of the HSO3- starch solution (solution A) into a second beaker. Place the beaker containing solution A on a piece of white paper. Add solution B rapidly to the beaker containing A and swirl the contents for a few seconds. Use a stop watch to record the time  which elapses from the instant of mixing (first contact) to the sudden appearance of the blue color. Record in Table I

2.   Rinse the reaction beaker and repeat the procedure using your given combinations of volumes shown in Table I two more times. Be sure to use the same graduated cylinders for measuring each of the solutions that were used in the original procedure.

Table I Effect of Concentration on Rate (Group Results)

Part II Effect of Temperature on Rate

3.   Measure 10 mL of solution A into each of three dry test tubes.  Place all three test tubes in a 300 mL water bath at your given temperature.

4.   Now measure 10 mL of solution B into each of three separate clean dry test tubes.  Place all three test tubes in a 300 mL water bath at your given temperature.

5.   After 5 minutes in the water bath, take one pair of test tubes and add 10 mL of solution A to 10 mL of solution B. Stir and record in Table II the time required for the appearance of the blue color.

6.   Repeat procedure 5 two more times. (No need to wait an additional 5 minutes)

7.   Record you data in the table designated for class results.

Table II  Effect of Temperature on Rate (10 mL A + 10 mL B)

Table III Effect of Temperature on Rate (Group Results)

Discussion Questions:

1.   Using the dilutions called for in Part I calculate the IO3- ion concentration of each run after mixing but before the reaction takes place.

      The stock KIO3 solution is 0.0l87 mol/L.

2.   For each fractional part of the reaction (that took place per second) find the speed of reaction by calculating the values of reciprocal time (1/s) to the nearest thousandth for each set of conditions.  This fraction is proportional to the rate of the reaction.  Make a general statement regarding time versus rate.

3.   Construct a graph by plotting molar concentrations of IO3- ion on the y-axis

      (vertical axis) and the reciprocal times (rates) on the x-axis (horizontal axis).

4.   What generalization can be derived from the graph? Express this generalization mathematically in terms of an equation.

5.   Make a general statement regarding the effect of temperature on the time required for a reaction to occur.  How does the temperature affect the rate of the reactions which you observed in Part II?  Plot a graph to represent temperature vs rate.