1.   What is the balanced net ionic equation for the main reaction between KI and (NH4)2S2O8?

2KI(aq)+(NH4)2S2O8(aq)→2K(SO4)2(aq)+SO4(NH4)2(aq)+2I(aq)

What is/are the spectator ion(s)?

K, I, NH4, S2O8

Is this reaction fast or slow?

Fast

  1.   What is the net ionic equation for the clock reaction?

Na2S2O3KIKNO3(aq)+NH4S2O8N42SO4(aq)

  • What is/are the spectator ion(s)?
  • Is this reaction fast or slow? Fast
  • What reagent is added that will change color? What is it reacting with? What color will it turn?
  • The reagent added that makes it change color is starch. The starch combines with the 2I, turning it a dark purple.
  • Which reactant must be used up before the indicator changes color?

Part 1: Determination of Rate Law

  1.   What is the generic form of the rate law being investigated?                                                                                                                                   
  2.   You are given suggested volumes of reagents for performing 5 runs, adjusting the volume of the two main reagents (the I- or the S2O8) to discover the effect of the concentration on rate.
  • Which of the main reagents was in the beaker?                                                                                  Which of the main reagents was in the test tube?                                                
  • Which reagent was added to keep a constant volume and constant concentration of K+?                                                                                                                                       
  • Which reagent was added to keep a constant volume and constant concentration of NH4+?                                                                                                                                  
  1.   FILL IN THE EXPERIMENTAL VOLUMES (IN DROPS) FOR RUNS 1-5 INTO TABLE 1.

Table 1: Experimental Volumes for Iodine Clock Reactions for Part 1

What was the TOTAL volume (in drops) used for each run (beaker + test tube)?      

1-48 drops       4-48 drops

2-48 drops       5-48 drops

3-58 drops                  

  1.   FILL IN YOUR EXPERIMENTAL VALUES FOR TIME, AVERAGE TIME, AND TEMPERATURE INTO TABLE 2 BELOW. (WHITE COLUMNS)

Did you modify any of the volumes or accidentally add more drops of one or more reagents during your multiple trials?  If so, describe any differences to your procedural steps. (The changes in volume must be accounted for in your calculations!)

  1.   Pick one run from your data set above. Upon the instant the solutions are mixed, what is the 2-concentration (in M) of I- and S2O8

Run #:? (Show your work and indicate which run’s data you are using)

Run #:

[I-]1:

[S2O8]1:

V1 (in drops): V2 (in drops):

V1 (in drops):

V2 (in drops):

2-

[I-]2  =                         

[S2O82-]   =                   

  1.   CALCULATE THE REST OF THE CONCENTRATIONS OF IODIDE AND PERSULFATE IONS AND RECORD THESE VALUES INTO TABLE 2 BELOW. (YELLOW COLUMNS)
  2.   Notice that the same number of drops of sodium thiosulfate, Na2S2O3, is used for each trial. Why was this done?

Upon the instant the solutions are mixed, what is the concentration (in M) of thiosulfate for all of the trials? (Show your work)

  1. You will need to determine the rate of reaction to solve for rate constant and the orders.

Which reactant’s concentration can you calculate an exact change over a given time?                                                                                                                                            Write the equation for calculating the initial rates of each of these reactions.                                                                                                                                    

The indicator changed color when all the thiosulfate ions, S2O32-, were used up. Upon the instant the 2-solutions were mixed, you calculated a molarity of S2O3 (from Q 9). Since the concentration of 2-thiosulfate changed from this value to zero this is the value of D[S2O3 point of the color change for each of the runs? (Show your work)

]. What is the value of D[I2] at the

2-                        –                                               2-

Consider the balanced reaction for the clock step:  I2(aq) + 2 S2O3

(aq) → 2 I (aq) + S4O6

(aq)

Pick one run from your data set in Table 2 and calculate the initial rate for that reaction using this value of I2 concentration and the average time. Show your work and indicate which run’s data you are using.

  1. CALCULATE THE REST OF THE RATES AND RECORD THESE VALUES INTO TABLE 2

BELOW. (ORANGE COLUMN)

  1.  Study the concentration data in Table 2 (yellow columns) and choose two runs that would allow you to determine how the rate of reaction depends on the change in concentration of iodide. Calculate the order for iodide. Show your work and clearly indicate which two runs you are comparing. Remember to round to the nearest whole number.

The order with respect to iodide is                                

  1. Study the concentration data in Table 2 (yellow columns) and choose two runs that would allow you to determine how the rate of reaction depends on the change in concentration of persulfate. Calculate the order for persulfate. Show your work and clearly indicate which two runs you are comparing. Remember to round to the nearest whole number.

The order with respect to persulfate is                           

  1. Now that you have the reactant orders and rates of reaction, calculate the rate constant, k (with appropriate units!) for one of the runs below. Show your work and indicate which run’s data is being used.    Rate = k[I-]x [S2O82-]y
  2. CALCULATE THE REST OF THE RATE CONSTANTS AND RECORD THESE VALUES INTO TABLE 2 BELOW. (PURPLE COLUMN) FILL IN THE UNITS IN THE COLUMN HEADER.

The average rate constant, k, for this reaction at _         oC is                           

  1. Briefly discuss the precision of your rate constant values and if the rate results you got agree with what you might predict. Briefly discuss sources of error.