The reaction between aqueous thiosulfate ions and iodine is represented by the macroscopic equation
2 S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2 I⁻.
Stopped-flow UV–visible spectroscopy detects a transient species with a peak absorbance at 360 nm that is assigned to S₂O₃I⁻. Two mechanistic proposals are summarized below.
Mechanism A
Step 1: S₂O₃²⁻ + I₂ → S₂O₃I⁻ + I⁻ (fast)
Step 2: S₂O₃I⁻ + S₂O₃²⁻ → S₄O₆²⁻ + I⁻ (slow)
Mechanism B
Step 1: S₂O₃²⁻ + I₂ → S₄O₆²⁻ + 2 I⁻ (slow, concerted)
Assume all reactions occur in dilute aqueous solution at 25 °C.
(a) Using the definition of a reaction intermediate, explain why S₂O₃I⁻ qualifies (or does not qualify) as an intermediate in each mechanism.
(b) For Mechanism A, write the rate law expected if Step 2 is rate-determining. Express the rate in terms of reactant concentrations only.
(c) Describe how detection of S₂O₃I⁻ at 360 nm supports or refutes each mechanism. Include in your answer a discussion of the relative time scales implied by the observed transient peak.
(d) Identify one additional experimental technique (other than UV–vis spectroscopy) that could corroborate the presence of S₂O₃I⁻ in solution. Briefly state the type of data that would constitute positive evidence.