Which of the following mechanisms can account for your observed results. For eac

Question

Which of the following mechanisms can account for your observed results. For each mechanism, justify why it can or cannot be a reasonable mechanism. Note that more than one possible mechanism can be reasonable. Likewise, maybe none of the proposed mechanisms are reasonable.

Observed result -- Rate = k [I-] [S2O82-]

[I-] was first order and [S2O82-] was first order making the overall reaction a 2nd order reaction

Mechanism A

I1-(aq) + S2O82-(aq) SO42-(aq) + ISO41-(aq)     Slow

ISO41-(aq) + I1-(aq) I2(aq) + SO42-(aq)     Fast

Mechanism B

I1-(aq) + I1-(aq) + S2O82-(aq) I2S2O84-(aq)     Slow

I2S2O84-(aq) I2(aq) + SO42-(aq) + SO42-(aq)     Fast

Mechanism C

I1-(aq) + I1-(aq) I22-(aq)     Fast

I22-(aq) + S2O82-(aq) ISO42-(aq) + ISO42-(aq)     Slow

ISO42-(aq) + ISO42-(aq) I2(aq) + SO42-(aq) + SO42-(aq)     Fast

Mechanism D

I1-(aq) + S2O82-(aq) IS2O83-(aq)     Slow

IS2O83-(aq) ISO41-(aq) + SO42-(aq)     Fast

I1-(aq) + ISO41-(aq) I2(aq) + SO42-(aq)     Fast

Explanation / Answer

Given rate is Rate = k [I-] [S2O82-]

So order with respect to I- is 1 & with respect to S2O82- is 1

So in the slowest step(which is the rate determining step) involves one mole of I- and one mole of S2O82-

so the correct mechanisms are A or D

Mechanism A

I1-(aq) + S2O82-(aq) SO42-(aq) + ISO41-(aq)     Slow

ISO41-(aq) + I1-(aq) I2(aq) + SO42-(aq)     Fast

In this the slowest step involves one mole of I- and one mole of S2O82- this is correct mechanism

Mechanism B

I1-(aq) + I1-(aq) + S2O82-(aq) I2S2O84-(aq)     Slow

I2S2O84-(aq) I2(aq) + SO42-(aq) + SO42-(aq)     Fast

In this the slowest step involves two moles of I- and one mole of S2O82- this is not correct mechanism

Mechanism C

I1-(aq) + I1-(aq) I22-(aq)     Fast

I22-(aq) + S2O82-(aq) ISO42-(aq) + ISO42-(aq)     Slow

ISO42-(aq) + ISO42-(aq) I2(aq) + SO42-(aq) + SO42-(aq)     Fast

In this the slowest step involves one mole of S2O82- this is not correct mechanism

Mechanism D

I1-(aq) + S2O82-(aq) IS2O83-(aq)     Slow

IS2O83-(aq) ISO41-(aq) + SO42-(aq)     Fast

I1-(aq) + ISO41-(aq) I2(aq) + SO42-(aq)     Fast

In this the slowest step involves one mole of I- and one mole of S2O82- this is also correct mechanism

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