Ok, so here\'s the question: For the reaction between H2 and I2, a proposed mech

Question

Ok, so here's the question:

For the reaction between H2 and I2, a proposed mechanism is  

step 1:   I2 ?2 I (fast equilibrium)

step 2:   2 I + H2? 2 HI (slow)

What rate law do you expect to be observed for the overall reaction?

My book gives me this solution:

       From step 2,     

      rate = k2[I]2[H2]     

      From step 1, k1f[I2] = k1r[I]2     

      [I]2 = (k1f/k1r)[I2]     

      Substituting for [I]2 in the rate law, we have     

      rate = k2(k1f/k1r)[I2][H       2] = k[I2][H2]     

The Problem is...

I don't really understand this explaination at all. Specifically the last part, where it says "Substituting for [I]2 in the rate law...", and then the subsequent equation. Visually I can't see how they got what they got. So I need a simple, but detailed, worded explaination please. Thank you!!

Explanation / Answer

step 2: 2 I + H2? 2 HI (slow)

slow step of the reaction is used determine Rate law

but 2I is not exactly the reactent it is intermediate

so as per the calculations given [I]^2 = (k1f/k1r)[I2]

substituting this value inrate eq.

rate = k2[I]2[H2]

rate = k2(k1f/k1r)[I2][H 2] = k[I2][H2]   (k2(k1f/k1r) = consatnt or rate constant

so that we can obtain rate = K2[I2][H2]

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