For the reaction 2H$_2$O$_2$ $\xrightarrow{\text{I}^-,\ \text{alkaline medium}}$ 2H$_2$O + O$_2$, the proposed mechanism is: (1) H$_2$O$_2$ + I$^-$ → H$_2$O + IO$^-$ (slow) (2) H$_2$O$_2$ + IO$^-$ → H$_2$O + I$^-$ + O$_2$ (fast) (i) Write the rate law for the reaction. (ii) Write the overall order of the reaction. (iii) Out of steps (1) and (2), which one is the rate-determining step?

CBSE Class 12 · 2019 · Chemical KineticsShort Answer

For the reaction 2H

_{2}

_{2}

I^{-}, alkaline medium

_{2}

O + O

_{2}

, the proposed mechanism is:
(1) H

_{2}

_{2}

+ I

^{-}

→ H

_{2}

O + IO

^{-}

(slow)
(2) H

_{2}

_{2}

+ IO

^{-}

→ H

_{2}

O + I

^{-}

+ O

_{2}

(fast)
(i) Write the rate law for the reaction.
(ii) Write the overall order of the reaction.
(iii) Out of steps (1) and (2), which one is the rate-determining step?

Answer: (i) Rate = k[H₂O₂][I⁻] (ii) 2 (iii) Step (1), the slow step.

Solution

(i) The slow step controls the rate:

Rate = k [H_{2} O_{2}] [I^{-}]

(ii) Overall order

= 1 + 1 = 2

.
(iii) Step (1) — the slow step is the rate-determining step.

CBSE Class 12 Board · 2019 · Subjective

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