Question

The reaction of peroxydisulfate ion (S₂O₈^2-) with iodide ion (I ^-) is given below.

S₂O₈^2-(aq) + 3 I ^-
The reaction of peroxydisulfate ion (S2O8^2-) with iodide ion (I -) is given below. The following data are collected at a certain temperature. Determine the rate law. ____________________ Calculate the rate constant. ______________ /(M�s)
2SO²₄^-(aq) +I₃^-

The following data are collected at a certain temperature.

Experiment	[S2O82- ](M)	[I- ](M)	Initial Rate (M/s)
1	0.080	0.034	2.2 X 10-4
2	0.080	0.017	1.1 X 10-4
3	0.16	0.017	2.2 X 10-4

Determine the rate law.

____________________

Calculate the rate constant.
______________ /(M·s)

Answer 1

Compare Exp 2 to 1. When [I^-] is halved, the rate ishalved. ∴ the rate is 1storder with respect to iodide.

Compare Exp 3 to 2. When [S₂O₈^2-]is doubled, the rate is doubled. ∴ the rate is1st order with respect toperoxydisulfate.

Rate =k[I^-][S₂O₈^2-]

Use any of the experiments to determine the rate constant (k)

k = Rate / [I^-] /[S₂O₈^2-] = (2.2 x 10^-4M/s) / (0.034 M)¹ /(0.080 M)¹ = 0.081m^-· s^-

Answer 2

By observing the above data keeping the concentration ofS₂O₈^2- constant & ondecresing the concencentration of I^- to half of theinitial amount , the rate of the reaction is also reduced tohalf

So , the rate of the reaction w.r.t I ^-is   rate α [I^-] ¹

keeping the concentration of I - constant & ondoubling the concencentration S₂O₈^2- , the rate of the reaction is also

doubled.

So , the rate of the reaction w.r.t S₂O₈^2-   is   rate α[S₂O₈^2- ]¹

So the over all rate law of the reaction is givenby       Rate = k[S₂O₈^2- ] ¹ [I^-] ¹

Where k is rate constant

Consider the first case

Experiment	[S2O82- ](M)	[I- ](M)	Initial Rate (M/s)
1	0.080	0.034	2.2 X 10-4

Substitutethe values in Rate = k [S₂O₈^2- ] ¹ [I^-]¹   we have

                    2.2 * 10^-4 = k * 0.08 * 0.034

                                     k = 8.088 * 10^-2