Experiment 3: Reaction Kinetics: The Iodide Persulphate Reaction

Question 1

What is the reaction used in this experiment? What is the simplified form of this reaction?

Answer 1

(NH4)2S2O8 + 2KI -> I2 + (NH4)2SO4 + K2SO4
S2O8 2- + 2I- -> I2 + 2SO4 2-

Question 2

What is the clock reaction used in this experiment? What is the simplified form of this reaction? What is the theory behind using this reaction as a clock reaction?

Answer 2

I2 + 2Na2S2O3 -> 2NaI + Na2S4O6
I2 + 2S2O3 2- -> 2I- + S4O6 2-
A clock reaction is used to study the rate of reaction of another reaction. In this reaction, as long as thiosulphate is present to react, the iodine produced will immediately be used up. When the thiosulphate has been used up, a violet colour will appear (due to iodine), and starch further enhances this by forming a blue complex with the iodine. Since the amount of thiosulphate added is known, the amount of iodine would also be known. So rate of reaction will also be known.

Question 3

What is the rate law for the reaction of I- with S2O8 2-? What does this rate law mean?

Answer 3

-Δ[S2O8 2-]/Δt = k[S2O8 2-]^m [I-]^n
m and n are the orders of reaction with respect to S2O8 2- and I- respectively.
The rate of loss of persulphate ion per unit of time is proportional to the concentrations of both the persulphate and iodide ions.

Question 4

Does the rate constant change? How does ionic strength affect this? How is ionic strength kept constant?

Answer 4

It is expected to remain constant, however, it is strongly dependent on ionic strength, increasing with ionic strength. When reactant ions are lowered, electrolytes such as (NH4)2SO4 or KNO3 are added.

Question 5

How is ionic strength calculated?

Answer 5

µ = 1/2 Σ Ci Zi^2
Ci is the molar concentration of the ith (ion) species and Zi is its ionic charge.

Question 6

What error has occurred if the solution turns yellow (not blue) several minutes after mixing?

Answer 6

Starch was not added

Question 7

What error has occurred if the solution turns blue immediately upon mixing?

Answer 7

Sodium thiosulfate was not added

Question 8

Describe what we are measuring and what each color shows?

Answer 8

-the initial rate of oxidation of iodide ion by per sulphate ion will be measured in this experiment to determine the rate law for this reaction
- when thiosluphate is present the solution is colorless but when all the thiosulphate has been used up by reaction with iodine a violet color will appear

Question 9

Why is starch added

Answer 9

-starch is added because it forms a blue complex with free iodine and facilitates a more accurate measure of the time at which the color first appears
-the rate of loss of persulphate ion per unit of time is proportional to the concentrations of both the persulphate ion and iodide ions
-rate constant, K, is not constant because it’s an ionic reaction that involves two negative ions and has a rate that is strongly dependent upon ionic strength, increasing with increasing ionic strength
-to keep the ion strength constant an electrolyte like KNO3 or (NH)2SO4 is added whenever the concentration of the reactant ions is being lowered

Question 10

How is the rate determined experimentally from this reaction?

Answer 10

-the time elapsed from the moment of mixing the reaction solutions to the appearance of the blue colour is inversely proportional to the rate of reaction
rate=change in persulfate/ change in time
=k [persulfate]^n [iodine ion]^m
-the change in persulfate represents the amount consumed between start of the reaction and the color change and it is equal to half the concentration of the initial amount of thiosulphate
-the iodine ion concentration remains constant

Question 11

what is ionic strength

Answer 11

a measure of the concentration of charges in a solution