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^-] [S₂O₈^2-]

[I^-] was first order and [S₂O₈^2-] was first order making the overall reaction a 2nd order reaction

Mechanism A

I^1-(aq) + S₂O₈^2-(aq) → SO₄^2-(aq) + ISO₄^1-(aq)     Slow

ISO₄^1-(aq) + I^1-(aq) → I₂(aq) + SO₄^2-(aq)     Fast

Mechanism B

I^1-(aq) + I^1-(aq) + S₂O₈^2-(aq) → I₂S₂O₈^4-(aq)     Slow

I₂S₂O₈^4-(aq) → I₂(aq) + SO₄^2-(aq) + SO₄^2-(aq)     Fast

Mechanism C

I^1-(aq) + I^1-(aq) → I₂^2-(aq)     Fast

I₂^2-(aq) + S₂O₈^2-(aq) → ISO₄^2-(aq) + ISO₄^2-(aq)     Slow

ISO₄^2-(aq) + ISO₄^2-(aq) → I₂(aq) + SO₄^2-(aq) + SO₄^2-(aq)     Fast

Mechanism D

I^1-(aq) + S₂O₈^2-(aq) → IS₂O₈^3-(aq)     Slow

IS₂O₈^3-(aq) → ISO₄^1-(aq) + SO₄^2-(aq)     Fast

I^1-(aq) + ISO₄^1-(aq) → I₂(aq) + SO₄^2-(aq)     Fast

Answer 1

Given rate is Rate = k [I^-] [S₂O₈^2-]

So order with respect to I^- is 1 & with respect to S₂O₈^2- is 1

So in the slowest step(which is the rate determining step) involves one mole of I^- and one mole of S₂O₈^2-

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 S₂O₈^2- 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 S₂O₈^2- 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 S₂O₈^2- 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 S₂O₈^2- this is also correct mechanism