Question

The Haber reaction for the manufacture of ammonia is:

N₂ + 3H₂ → 2NH₃

Without doing any experiments, which of the following can you say MUST be true?

The activation energy is positive.
Disappearance rate of N₂ = 3 (Disappearance rate of H₂).
Disappearance rate of H₂ = 3 (Disappearance rate of N₂).
The reaction is not an elementary reaction.
The reaction is first order in N₂.
Reaction rate = -Δ[N₂]/Δt.
Δ[H₂]/Δt will have a positive value.

Answer 1

Given reaction:

N₂ + 3H₂ → 2NH₃

1) The activation energy is positive: TRUE

The activation energy is overcome at high temperature and pressure to yield ammonia.

2) Disappearance rate of N₂ = 3 (Disappearance rate of H₂): FALSE

As per the reaction stoichiometry: -d[N2]/dt = -1/3d[H2]/dt

therefore: Disappearance rate of N₂ = 1/3(Disappearance rate of H₂)

3) Disappearance rate of H₂ = 3 (Disappearance rate of N₂): TRUE

As per the reaction stoichiometry: -d[N2]/dt = -1/3d[H2]/dt

therefore: Disappearance rate of H₂ = 3(Disappearance rate of N₂)

4) The reaction is not an elementary reaction: TRUE

Synthesis of NH3 does not occur via direct interaction of N2 and H2. Instead, it involves a number of intermediate steps.

5) The reaction is first order in N₂ : FALSE

Kinetic data is required to determine the order.

6) Reaction rate = -Δ[N₂]/Δt : TRUE

As per reaction stoichiometry, the above is true for the rate of disapprearance of N2.

7) Δ[H₂]/Δt will have a positive value : FALSE

It will be negative as H2 is a reactant and it is consumed during the course of the reaction.