Question

7. (eight marks) Consider the chemical reaction below
4 NH3(g) + 5 O2(g)  4 NO(g) + 6 H2O(g)

Initially, 3.5 moles of NH3 are placed in a 4.0 L reaction chamber. After 3.0 minutes only 1.6 moles of NH3 remain.

a) Calculate the average rate of reaction with respect to NH3.
b) Calculate the rate at which H2O is being formed.

Answer 1

First find concentrations:

- [NH3]initial = 3.5 mol / 4.0 L = 0.875 M
- [NH3]final = 1.6 mol / 4.0 L = 0.400 M
- Δ[NH3] = 0.400 − 0.875 = −0.475 M over Δt = 3.0 min

a) Rate of disappearance of NH3 = −Δ[NH3]/Δt = −(−0.475)/3.0 = 0.158 M min−1 (or −0.158 M min−1 if you keep the sign for a decrease).
If you define the reaction rate r using the stoichiometric coefficient (r = −(1/4)Δ[NH3]/Δt), then r = 0.0396 M min−1.

b) Rate of formation of H2O: d[H2O]/dt = 6·r = 6 × 0.0396 = 0.238 M min−1.

(Units: mol L−1 min−1)