Question

Nitrogen monoxide, NO, reacts with hydrogen, H₂, according to the following equation.

2 NO + 2 H₂ → N₂ + 2 H₂O

What would the rate law be if the mechanism for this reaction were as follows? (Rate expressions take the general form: rate = k . [A]^a . [B]^b.)

2 NO + H₂ → N₂ + H₂O₂ (slow)

H₂O₂ + H₂ → 2 H₂O (fast)

Answer 1

Answer :

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Given equation,

2 NO + 2 H₂ → N₂ + 2 H₂O

The rate law in general is written as --

rate = k . [A]^a . [B]^b

where ,[ A]= [NO] and [B] = [H2]

In a multistep reaction process, The rate law is determined from the slowest step. Slowest step of the multistep process decides the rate of the chemical reaction

IN A MULTISTEP MECHANISM , THE RATE OF A CHEMCIAL REACTION IS DEPENDEDNT ON THE SLOW STEP ONLY.

Of the given two steps of the mechanism of the given chemical process --

2 NO + H₂ → N₂ + H₂O₂ (slow)

H₂O₂ + H₂ → 2 H₂O (fast)

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2 NO + 2 H₂ → N₂ + 2 H₂O [ addition of two steps yields the main reaction]

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the slow step i.e 2 NO + H₂ → N₂ + H₂O₂ , determines the rate of the reaction.

The coefficeints of NO is 2 =(a) and coefficient of H₂ is 1= (b) and same are written in the powers of terms in the rate law,

So the rate law is written as,

The overall order of the reaction is 2+1 = 3 ---- ( sum of powers of the concentration terms in the rate law)