Question

8. Which of the following reactions cannot proceed through a single elementary step in view of

the experimental rate law given?

(a) NO(g) + O3(g) ⟶ NO2(g) + O2(g) rate = k[NO][O3]

(b) 2HI(g) ⟶ H2(g) + I2(g) rate = k[HI]2

(c) C4H8(g) ⟶ 2C2H4(g) rate = k[C4H8]

(d) 2NO(g) + H2(g) ⟶ N2(g) + H2O(g) rate = k[NO][H2]

(e) 2NO(g) + Cl2(g) ⟶ 2NOCl(g) rate = k[NO]2[Cl2]

The worksheet says that the answer is D, but I don’t get the reasoning for it. Please explain. Thank you!

Answer 1

Consider the reaction : aA (g) + bB (g) cC (g) + dD (g)

where A (g) and B (g) are reactant species and C (g) and D (g) are product species.

a, b, c and d are stoichiometric coefficients of A, B, C and D respectively

If this reaction is elementary, then rate law is given as : Rate = k[A]^a[B]^b

Concentration of reactant A, [A] , is raised to power equal to its stoichiometric coefficient 'a'

Similarly, Concentration of reactant B, [B] , is raised to power equal to its stoichiometric coefficient 'b'

All the given reactions except (d) obeys this.

If reaction (d) was elementary, then rate law would be : rate = k[NO]²[H₂]   (notice the power of NO)

But, the rate given is : rate = k[NO][H₂]

Since the given rate law does not match the rate law of elementary reaction, therefore, reaction (d) cannot proceed through a single elementary step