3rd Order Reaction Constant

Value of 3rd order reaction constant

Rate=k(B)(A) ²

Value of B =0.02

Value of A =0.05

Rate (mol dm-3 s^-1) = 1.28x10⁴

Or

Rate = k[B][A] ²

so, 1.28x10^-4 = k X 0.02 X 0.05²

k = (1.28X10^-4)/0.00005 = 2.56 mol^-2dm⁶s^-1

If the original figure was 1.28x10⁴

then the answer is 2.56x10⁸ (same units) 

Units of Reaction Rate Constant

Rate is the change in the concentration with time. Therefore, concentration units divided by time units express the rate of reaction. Concentration is expressed in moles/litre and time in seconds.

The units for the rate of reaction are mol litre^-1s^-1as:

Rate = concentration /time = moll^-1/s = mol L^-1s^-1

The units for rate constant are different for different orders.

(1)   For zero order reaction

Rate is expressed as: Rate = k [A]⁰ or k

mol L^-1/s =k

 K = mol L^-1s^-1

 Therefore, the units of rate constant for zero order reaction are mol L^-1s.

 (2) Units of rate constant for first order reaction

Rate is expressed as: Rate = k [A]

Mol L^-1/s = k (mol L^-1)

K =s^-1

 Therefore, the units of rate constant for first order reaction are s^-1.

 (3) Units of rate constant for second order reaction

Rate is expressed as: Rate = k [A]²

Mol L^-1/s = k (mol L^-1)2

K = 1/mol L^-1s= L mol^-1s^-1

 Therefore, the units of rate constant for second order reaction are Lmol^-1s^-1.

 (4) Units of rate constant for third order reaction

Rate is expressed as: Rate = k [A]³

Mol L^-1/s = k (mol L^-1)3

K = 1/mol²L^-2s= L²mol^-2s^-1

 Therefore, the units of rate constant for third order reaction are L² mol^-2s^-1.

Units of rate constants for Gaseous reactions

When the reactants are in gaseous state then the concentration is expressed in terms of pressure in the units of atmosphere. So the rate has the units of atm per second.

Thus units of rate constant would be;

(i) Zero order reaction: atms^-1

(ii) First order reaction: s^-1

(iii) Second order reaction: atm^-1s^-1

(iv) Third order reaction: atm^-2s^-1so on