Question

Rate Laws (Section)

Consider the reaction of peroxydisulfate ion (S2O8 2-) with iodide ion (I - ) in aqueous solution:

S2O82 - (aq) + 3 I - (aq)→2 SO42 - (aq) + I3- (aq)

At a particular temperature the initial rate of disappearance of S2O8 2 - varies with reactant concentrations in the following manner:

Experiment	[S2O82‑] (M)	[I_] (M)	Initial Rate (M/S)
1	0.018	0.036	2.6 × 10-6
2	0.027	0.036	3.9 × 10-6
3	0.036	0.054	7.8 × 10-6
4	0.050	0.072	1.4 × 10-5

(a) Determine the rate law for the reaction and state the units of the rate constant. (b) What is the average value of the rate constant for the disappearance of S2O8 2 - based on the four sets of data? (c) How is the rate of disappearance of S2O8 2 - related to the rate of disappearance of I - ? (d) What is the rate of disappearance of I - when [S2O8 2-] = 0.025 M and [I - ] = 0.050 M ?

Answer 1

(a)

Consider the reaction between with is given as follows:

Consider the following data for rate of disappearance of and are given as follows:

Calculate x,y as follows:

Calculate y as follows:

Hence, write the rate expression as follows:

Overall order of the reaction is 1+1=2

Therefore, overall order of the reaction is 2.

Units of rate constants are,

Hence, units of rate constant are .

(b)

Calculate the average value of rate constant as follows:

Experiment 1:

Hence, rate constant for this experiment is .

Experiment 2:

Hence, rate constant for this experiment is .

Experiment 3:

Hence, rate constant for this experiment is .

Experiment 4:

Hence, rate constant for this experiment is .

Calculate the average value of rate constant as shown below:

Hence, average value of rate constant is .

(c)

The relation between rate of disappearance of and rate of disappearance of is given as follows:

Negative sign for , because of disappearance. Reactants are disappearing to form products.

(d)

Calculate the rate of disappearance of when is 0.025 M and is 0.050 M.

Hence,

Since the given data in the problem is given in terms of .

Hence,

Hence,

Hence, rate of disappearance is .