Iodine Clock Experiment

CHEM 251Week of November 29th, 2010

Alexis Patanarut

Purpose

1. To study the effects of temperature and catalysts on the rate of reaction

2. To calculate reaction order coefficients and the rate constant k using the method of initial rates– Experiments 1, 2 and 3 will be used to final the 3 k

values, then the average k value– Experiment 4 will be used to show you how a

catalyst affects the reaction rate– Experiment 5 will be used to show you how

temperature affects the reaction rate

Background

• The name 'iodine clock' is derived from the fact that, by carefully adjusting the reactants involved in the reaction, the iodine clock reaction can come to completion with the accuracy of a clock– However, this would require glassware of high

precision and meticulous attention to detail– No worries; we won't be putting you through this

Background, con’t

• For today, you will be using the iodine clock reaction to find the reaction order coefficients and rate constant– Last time, you determined the reaction order and

rate constant of the crystal violet dye reaction by graphical means

– This week, you will be determining those values quantitatively; you will also be seeing how temperature and catalysts affect the reaction rate

Background, con’t(I) S2O8 + 2I --> 2SO4 + I2

(II) I2 + 2S2O3 --> 2I + S4O6

• The actual iodine clock reaction between the iodine and the persulfate is VERY fast, so much so that you will most likely miss the endpoint

• The reaction is therefore slowed down with the addition of thiosulfate– The thiosulfate will consume all of the iodine produced in

the persulfate reaction– Once all of the thiosulfate is used up, the excess iodine

forming will react with the starch in the solution to produce the blue color indicative of the endpoint

– This is how to endpoint is made measurable by time

The mathematics – finding [S2O8] and [I]• Remember that you are making dilutions of

both compounds, since you are combinding reagents together into a larger volume (10 mL total)

• Use C1V1 = C2V2 to find the final concentration (C2) of [S2O8] and [1]

The mathematics – finding the rate of reaction• You will use the mathematical rate expression for this

(1) Rate = -[S2O8]/timeWhere [S2O8] = 1/2[S2O3] <-- the thiosulfate reagentAnd [S2O3] = initial [S2O3] concentration

• Therefore, you can find [S2O8] by taking the initial concentration of [S2O3] and dividing it in half.– Take this value and divide it by the amount of time

required to see the blue reaction endpoint, and you can find the reaction rate

The mathematics – finding x and y

• Use the method of initial rates • Example

Trial [A] (mol/L) [B] (mol/L) Rate (M/sec)

1 0.1 0.1 1.0 x 10^-4

2 0.1 0.2 1.0 x 10^-4

3 0.3 0.1 3.0 x 10^-4

The mathematics – finding x and y, con’trate 2/rate 1 = {k[0.1]x[0.2]y)/(k[0.1]x[0.1]y)(1 x 10^-4)/(1 x 10^-4) = k(0.2/0.1)y

1 = k(0.2/0.1)y

y = log(1)/log(2)y = 0

rate 3/rate 1 = {k[0.3]x[0.1]y)/(k[0.1]x[0.1]y)(3 x 10^-4)/(1 x 10^-4) = k(0.3/0.1)x

3 = k(0.3/0.1)x

x = log(3)/log(3)x = 1

because they are the same value, B or 0.1 cancels out from the equation

because they are the same value, A or 0.1 cancels out from the equation

The mathematics – finding the rate constant k• Once x and y are found, the rate constant k

can be found by merely plugging in the right numbers and doing a straight calculation

• Once the k for each experiment is found (for experiments 1, 2 and 3), take the average of the three k values to find the average k.

• Use this average k value as a means of comparison for the k value you calculate for experiments 4 and 5

The experiment1. For experiments #1, 2 and 3, combine the amounts of the first five

reagents in a large test tube. Only add the sixth reagent (the potassium persulfate) when you are ready to begin timing the reaction to the nearest second. Time how long it takes for you to see blue appear in the solution.

2. For experiment #4, add the first five reagents using the same amounts as experiment #1. However, this time, you will also add ONE drop from the "mystery reagent" bottle. Then add the persulfate reagent as before and record the time you see the blue color.

3. For experiment #5, add the first five reagents using the same amounts as experiment #1. In addition, put the required amount of persulfate solution into a small test tube. Put both test tubes into a water bath for at least 30 seconds, then take the temperature of the five-reagent mixture. Add the persulfate to the five-reagent mixture and see how long it takes for the blue to appear. Keep an eye on it, or you may miss this.

The experiment - amountsExperiment 0.20 M

NaI (mL)0.20 MNaCl (mL)

0.010 MNa2S2O3(mL)

2% starch 0.20 MK2SO4(mL)

0.20 MK2S2O8(mL)

1 2.0 2.0 2.0 20 drops 2.0 2.0

2 2.0 2.0 2.0 20 drops 0.0 4.0

3 4.0 0.0 2.0 20 drops 2.0 2.0

4 + 1 drop of “mystery” reagent

2.0 2.0 2.0 20 drops 2.0 2.0

5Heat mixtures for 30 sec

2.0 2.0 2.0 20 drops 2.0 2.0

Experimental notes

• Be sure to rinse your dropper with water between adding individual reagents to avoid contamination of your reagent

• Have about 20 mL of each reagent in LABELED beakers at your lab bench. This will prevent you from having to run back and forth constantly to get chemicals

• After adding the persulfate, stopper the test tube and agitate the solution for 10 seconds to ensure mixing

• The blue color will appear suddenly. You are to record the exact length of time it took for the blue to appear for each trial

In your lab notebookCalculate the following values:1. [S2O8] and [I]2. Reaction rate3. Rate constant k

Answer the following questions:1. What do you think the mystery reagent is? How did adding

it affect your reaction rate?2. What is the effect of temperature on the reaction rate?3. What was the order of the reaction?4. Discuss in 2-3 sentences factors that could have

contributed to errors in your experiment.