Fifth Form Chemistry Reaction wc 2013-11-18 Iodine Clock Reaction EY 2014 Today you will investigate how concentration affects reaction rate. An oxidizing agent such as hydrogen peroxide H 2 O 2 may be used to oxidize iodide ion in solution to iodine and the rate of this reaction can be monitored in the iodine clock reaction. A solution containing potassium iodide, KI, starch indicator, sodium thiosulphate Na 2 S 2 O 3 , call this mixture solution A, is mixed with a separate solution of acidified hydrogen peroxide, solution B. The mixing of A and B starts the reaction. The hydrogen peroxide oxidizes the iodide ion to iodine which at high enough concentration turns the starch indicator present blue-black. The reaction is: H 2 O 2 (aq) + 2H + (aq) + 2I - (aq) 2H 2 O(l) + I 2 (aq) In order that potassium iodide will not be oxidized prematurely on standing, sodium thiosulphate is present in solution A. The thiosulphate is a reducing agent which even after A and B are mixed prevents the blue-black colour from being formed because it turns the iodine formed back to iodide faster than the peroxide is able to oxidize it to iodine. 2 S 2 O 3 2- (aq) + I 2 (aq) S 4 O 6 2- (aq) + 2I - (aq) thiosulphate ion It is only after the thiosulphate is used up in this process of turning iodine to iodide that there is sufficient iodine formed to be able to produce the blue-black colour with the starch indicator. Method Using a 25cm 3 measuring cylinder, measure out 25cm 3 of solution A and place it in a beaker to it add 25 cm 3 of solution B and simultaneously start the stop-watch. Quickly stir the mixture thoroughly without spilling and as soon as the blue-black colour appears stop the stop-watch. Record the time. Repeat the experiment using the combination of volumes specified in the table below. In each of these subsequent experiments the concentration of hydrogen peroxide is being reduced. Write a report on your lab. Expt. No. Vol.H 2 O 2 /cm 3 Vol. Water/cm 3 Vol.Thio./ cm 3 Time/s 1/time s -1 1 25 0 25 2 20 5 25

Iodine Clock Reaction EY 2014

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Fifth Form Chemistry Reaction wc 2013-11-18

Iodine Clock Reaction EY 2014

Today you will investigate how concentration affects reaction rate. An oxidizing agent such as hydrogen peroxide H2O2 may be used to oxidize iodide ion in solution to iodine and the rate of this reaction can be monitored in the iodine clock reaction.

A solution containing potassium iodide, KI, starch indicator, sodium thiosulphate Na2S2O3, call this mixture solution A, is mixed with a separate solution of acidified hydrogen peroxide, solution B. The mixing of A and B starts the reaction. The hydrogen peroxide oxidizes the iodide ion to iodine which at high enough concentration turns the starch indicator present blue-black. The reaction is:

H2O2(aq) + 2H+ (aq) + 2I- (aq) 2H2O(l) + I2(aq)

In order that potassium iodide will not be oxidized prematurely on standing, sodium thiosulphate is present in solution A. The thiosulphate is a reducing agent which even after A and B are mixed prevents the blue-black colour from being formed because it turns the iodine formed back to iodide faster than the peroxide is able to oxidize it to iodine.

2 S2O32- (aq) + I2(aq) S4O62-(aq) + 2I- (aq) thiosulphate ion

It is only after the thiosulphate is used up in this process of turning iodine to iodide that there is sufficient iodine formed to be able to produce the blue-black colour with the starch indicator.

Method

Using a 25cm3 measuring cylinder, measure out 25cm3 of solution A and place it in a beaker to it add 25 cm3 of solution B and simultaneously start the stop-watch. Quickly stir the mixture thoroughly without spilling and as soon as the blue-black colour appears stop the stop-watch. Record the time. Repeat the experiment using the combination of volumes specified in the table below. In each of these subsequent experiments the concentration of hydrogen peroxide is being reduced. Write a report on your lab.

Expt. No. Vol.H2O2/cm3 Vol. Water/cm3 Vol.Thio./cm3 Time/s 1/time s-1

1 25 0 252 20 5 253 15 10 254 10 15 255 5 20 25 Treatment of Results

1. Draw a graph of volume of hydrogen peroxide vs. 1/time

2. Use the graph to find the time it would take if hydrogen peroxide volumes of (a) 22 cm3 (b) 12 cm3 were used.

3. From your graph what is the relationship between the concentration of hydrogen peroxide and the reaction rate?