# Rate of reaction

## 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

## The Attempt at a Solution

haruspex
Homework Helper
Gold Member
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.

baldbrain
mjc123
Homework Helper
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.