Question

Consider the reaction of peroxydisulfate ion (S2O2−8S2O82−) with iodide ion (I−I−) in aqueous solution:
S2O2−8(aq)+3I−(aq)→2SO2−4(aq)+I−3(aq)S2O82−(aq)+3I−(aq)→2SO42−(aq)+I3−(aq).
At a particular temperature the rate of disappearance of S2O2−8S2O82− varies with reactant concentrations in the following manner:

Experiment	S2O2−8(M)S2O82−(M)	I−(M)I−(M)	Initial Rate (M/s)(M/s)
1	0.018	0.036	2.6×10−62.6×10−6
2	0.027	0.036	3.9×10−63.9×10−6
3	0.036	0.054	7.8×10−67.8×10−6
4	0.050	0.072	1.4×10−5

What is the average value of the rate constant for the disappearance of S2O2−8S2O82− based on the four sets of data?

How is the rate of disappearance of S2O2−8S2O82− related to the rate of disappearance of I−?

What is the rate of disappearance of I−I− when [S2O2−8]=[S2O82−]= 2.8×10⁻² MM and [I−]=[I−]= 3.5×10⁻² MM ?

Express your answer using two significant figures.

Answer 1

Part a

The given reaction

S2O8 2- (aq) + 3I- (aq) = 2SO4 2- (aq) + I3- (aq)

Let S2O8 2- = [A]

I- = [B]

Let rate law

r = k [A]m [B]n

From trial 1 and 2

r1/r2 = [A1]m [B1]n / [A2]m [B2]n

2.6/3.9 = [0.018]m [0.036]n / [0.027]m [0.036]n

0.666 = 0.666^m

m = 1

From trial 1 and 3

r1/r3 = [A1]m [B1]n / [A3]m [B3]n

2.6/7.8 = [0.018] [0.036]n / [0.036] [0.054]n

0.333 = 0.5 * 0.666^n

n = 1

Rate law

r = k [S2O8 2-] [I-]

From trial 1

2.6*10^-6 = k (0.018*0.036)

k = 4.0*10^-3 M-1 s-1

Part b

Relation between rate of disappearance and rate of appearance

rS2O8 2- / (-1) = rI- / (-3) = rSO4 2- / (+2) = rI3-​​​​​​​ / (+1)

rS2O8 2- / (-1) = rI-​​​​​​​ / (-3)

rS2O8 2- = rI-​​​​​​​ / 3

Part c

Rate of disappearance of I-

rI-​​​​​​​ = 3 * rS2O8 2-

= 3 * k [S2O8 2-] [I-]

= 3 * 4.0*10^-3 M-1 s-1 * 2.8*10^-2M * 3.5*10^-2 M

= 1.2*10^-5 M/s