Class 12 Chemistry Chemical Kinetics Order of a reaction

Order of a reaction

The addition of power of the concentration of reactant in a rate law expression gives the order of reaction.

Let A + 2B --> C + D be a chemical reaction.

From rate law R = k [A]x [B]y

Now Order of reaction is defined as addition of the order of all the reactants participating in a chemical reaction.

order w.r.t. A = x

Order w.r.t. B = y

Overall order of the given reaction = (x + y).

Units of order of reaction:

k = rate / [A]n = (mol.L-1s-1/molL-1) n

For 1st order reaction , n=1

k = rate / [A]1 = (mol.L-1s-1/molL-1) 1= s-1

For 2nd order reaction, n=2

k = rate / [A]2 = (mol.L-1s-1/molL-1) 2= L.mol-1s-1

PROBLEM.   From the rate expression for the following reactions, determine their order of reaction and the dimensions of the rate constants.

(i) 3 NO(g) → N2O(g)  Rate = k[NO]2

(ii) H2O2 (aq) + 3 I - (aq) + 2 H+→ 2 H2O (l) +  I3-  Rate = k[H2O2][I - ]

(iii) CH3CHO(g)  → CH4(g) + CO(g)  Rate = k [CH3CHO]3/2

(iv) C2H5Cl(g) → C2H4(g) + HCl(g)  Rate = k [C2H5Cl]

SOLUTION.   (i) Rate = k[NO]2

Order of the reaction = 2

Dimension of k = Rate / [NO]2

= mol L-1 s-1 / (mol L-1)2

= mol L-1 s-1 / mol2 L-2

= L mol-1 s-1

(ii) Rate = k[H2O2][I - ]

Order of the reaction = 2

Dimension of k = Rate  / [H2O2][I - ]

= mol L-1 s-1  / (mol L-1) (mol L-1)

= L mol-1 s-1

(iii) Rate =k [CH3CHO]3/2

Order of reaction = 3/2

Dimension of k = Rate / [CH3CHO]3/2

=  mol L-1 s-1   / (mol L-1)3/2

= mol L-1 s-1   / mol3/2  L-3/2

= L½ mol-½  s-1

(iv) Rate = k [C2H5Cl]

Order of the reaction = 1

Dimension of k = Rate /  [C2H5Cl]

= mol L-1 s-1   / mol L-1

= s-1

PROBLEM.   For a reaction, A + B → Product; the rate law is given by,  r  = k [A]½ [B]2. What is the order of the reaction?

SOLUTION.   The order of the reaction = 1/2 + 2

= 2 1/2

= 2.5

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