Rate of reaction

  • #1

Homework Statement


Suppose there is a chemical reaction : N2 + 3H2 = 2NH3
The following things are written in my book :
rate of disappearance of H2 = - Δ

/ Δt
rate of reaction wrt H2 = - 1/3 * Δ

/ Δt
My question is that why can't we write rate of reaction = rate of disappearance of H2 ? In other words, can I write rate of disappearance of H2 = -1/3 * Δ

/ Δt and vice versa?

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The Attempt at a Solution

 

Answers and Replies

  • #2
haruspex
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why can't we write rate of reaction = rate of disappearance of H2 ?
They are defining one reaction as one occurrence of N2+3H2→2NH3. Each such takes 3 instances of H2, so the rate of loss of H2 is three times the reaction rate.
 
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  • #3
mjc123
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You cannot write "rate of disappearance of H2 = -1/3 Δ[H2]/Δt" because that simply isn't true. By definition, the rate of disappearance of H2 is -Δ[H2]/Δt.
(Strictly, that should be the rate of change of [H2]. The rate of disappearance would be positive, and equal to minus the rate of change.)
It is the "rate of reaction" that is subject to ambiguous definition. You could define the "reaction" as:
N2 + 3H2 → 2NH3 in which case rate of reaction = rate of consumption of N2 = 1/3 rate of consumption of H2
1/2N2 + 3/2H2 → NH3 in which case rate of reaction = rate of production of NH3 = 2/3 rate of consumption of H2
1/3N2 + H2 → 2/3NH3 in which case rate of reaction = rate of consumption of H2 = 3* rate of consumption of N2
That is why I don't like "rate of reaction", and always like to do kinetics in terms of the rate of change of a specified reagent. But you need to know this stuff to do exams these days.
 
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