From the rate expression for the following reactions, determine their order of reaction and dimensions of the rate constants. (i) 3NO(g) → N 2 O (g) Rate = k[NO] 2 (ii) H 2 O 2 (aq) + 3I – (aq) + 2H + → 2H 2 O(l) + I – 3 Rate = k[H 2 O 2 ][I – ] (iii) CH 3 CHO(g) → CH 4 (g) + CO(g) Rate = k[CH 3 CHO] 3/2 (iv) C 2 H 5 Cl (g) → C 2 H 4 (g) + HCl (g) Rate = k[C 2 H 5 Cl] Order is sum of all powers of the concentrations of the reactant in “rate law expression”

Question

Philoid · Accepted Answer

(i) 3NO(g) → N₂O(g)

Rate = k[NO]²

Here rate law is dependent on concentration of NO only.And power raised to concentration of NO is 2. Hence order of reaction is 2.

To find dimensions of rate constant (K)- We know rate of chemical reaction is measured in . So, unit of K will be

i.e. [concentration]^-1[ time]^-1 . If concentration is in mol L^-1 and time is in sec. then dimensions of k will be : mol^-1 L sec^-1

(ii) H₂O₂(aq) + 3I^-(aq) + 2H⁺ → 2 H₂O(l) + I₃^-

Rate = k[H₂O₂][I^-]

Here rate law is dependent on concentration of H₂O₂ and I^-. And power raised to concentration of H₂O₂ is 1, and power raised to concentration of I^- is 1. order is sum of powers hence, order of given reaction is 2.

To find dimensions of rate constant (K)- We know rate of chemical reaction is measured in [concentration/time]. So, unit of K will be [Concentration]/[time][concentration]² i.e. [concentration]^-1[ time]^-1. If concentration is in mol L^-1 and time is in sec then dimensions of k will be mol^-1 L sec^-1

(iii) CH₃CHO(g) → CH₄(g) + CO(g)

Rate = k[CH₃CHO]^3/2

Here rate law is dependent on concentration of CH₃CHO.And power raised to CH₃CHO is 3/2. Hence order of reaction is 3/2.

To find dimensions of rate constant (K)- We know rate of chemical reaction is measured in [concentration/time]. So, unit of K will be [Concentration]/[time][concentration]^3/2 i.e. [concentration]^-1/2[time]^-1.

If concentration is in mol L^-1 and time is in sec. Then dimensions of K will be mol^-1/2L^1/2 sec^-1

(iv) C₂H₅Cl (g) → C₂H₄(g) + HCl(g)

Rate = k[C₂H₅Cl]

Here rate law is dependent on concentration of C₂H₅Cl. And power raised to C₂H₅Cl is 1. Hence order of reaction is 1.

To find dimensions of rate constant (K)- We know rate of chemical reaction is measured in [concentration/time]. So, unit of K will be [Concentration]/[time][concentration]¹. i.e. [time]^-1

If concentration is in mol L^-1 and time is in sec. Then dimensions of K will be sec^-1.