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Home » Class 12 » Chemistry » Chemical Kinetics » Order of a Reaction

Order of a Reaction

Last Updated on July 3, 2023 By Mrs Shilpi Nagpal

Contents

  • 1 Order of a Reaction
    • 1.1 (a) Reactions of first order
    • 1.2 (b) Reactions of second order
    • 1.3 (c) Reactions of third order
    • 1.4 (d) Reactions of fractional order
    • 1.5 (e) Zero order reaction
  • 2 Units of Reaction Rate Constants
    • 2.1 1) Units of rate constant for zero order reaction
    • 2.2 2) Units of rate constant for first order reaction
    • 2.3 3) Units of rate constant for second order reaction
    • 2.4 4) Units of rate constant for third order reaction
  • 3 Units of Rate Constants for Gaseous Reactions

Order of a Reaction

The order of a reaction is defined as:

the sum of the powers to which the concentration terms are raised in the rate law equation to express the observed rate of the reaction.

The power of the concentration of a particular reactant in the rate law is called the order of the reaction with respect to that reactant.

If the rate of a reaction,

aA + bB + cC ——> Products

is given by the rate law as:

k[A]p [B]q [C]r

Rate = -dx /dt = k[A]p [B]q [C]r

then, the order of the reaction, n, is n = p+q+r

where p, q and r are the orders with respect to individual reactants and overall order of the reaction is sum of these exponents, i.e., p + q +r.

When n =1, the reaction is said to be first order reaction, if n = 2, the reaction is said to be second order reaction and so on.

Some Examples of Reactions of Different Orders

 

(a) Reactions of first order

(1) Decomposition of nitrogen pentoxide (N2O5)

2N2O5 (g) ——–> 2 NO2 (g) + ½ O2 (g) 

Rate = k [N2O5]

(2) Decomposition of ammonium nitrite in aqueous solution

NH4NO2 ———>N2+2H2O

Rate = k[NH4NO2]

(3) Decomposition of H2O2 in the presence of I¯ ions

H2O2 ———-> H2O + ½ O2

Rate = k [H2O2]

(b) Reactions of second order

(1) Decomposition of nitrogen peroxide

2NO2 ——-> 2NO + O2

Rate =k [NO2]2

(2) Reaction between H2 and I2 to give HI

H2 + I2 ——–> 2HI

Rate =k[H2] [I2]

(c) Reactions of third order

(1) Reaction between nitric oxide and oxygen

2NO + O2 ——->  2NO2

Rate = k [NO]2 [O2]

(2) Reaction between nitric oxide and Cl2

2NO +Cl2 ——>2NOCl

Rate = k [NO]2 [Cl2]

 

(d) Reactions of fractional order

(1) Decomposition of acetaldehyde is a fractional order reaction

CH3CHO ——-> CH4 + CO

Rate = k [CH3CHO]3/2

Order  = 3/2 or 1.5

(2) The reaction between hydrogen and bromine to form hydrogen bromide is a fractional order reaction.

H2(g) + Br2(g)——> 2HBr

Rate = k [H2] [Br2]½ 

Order = 1 + ½ = 1½

(e) Zero order reaction

A number of zero order reactions are known in which the rate of the reaction is independent of the concentration of the reactants.

For example: the decomposition of ammonia at the surface of metals like gold, platinum etc., is a zero order reaction.

2NH3 ——–>  N2 + 3H2

The rate of the reaction is independent of the concentration of ammonia, i.e.,

Rate = -dx/dt = k[NH3 ]0

Rate = k

order =0

Units of Reaction Rate Constants

The rate is the change in concentration with time. The rate of reaction is expressed by concentration units divided by time units.

If the concentration are expressed in moles/litre and time in seconds, then the units for rate of a reaction are mol litre-1 s-1 as:

 

1) Units of rate constant for zero order reaction

For zero order reaction, rate may be expressed as

Rate = k [A]° or =k

mol L-1 / s = k

k= mol L-1 s-1

The units of rate constant of zero order reaction are mol L-1 s-1

2) Units of rate constant for first order reaction

For first order reaction, rate may be expressed as:

Rate = k [A]

mol L-1 / s  = k (mol L-1)

k=s-1

The units of rate constant for first order reaction are s-1

3) Units of rate constant for second order reaction

For second order reaction, rate may be expressed as:

Rate = k [A]2

mol L-1 / s  = k (mol L-1)2

k= L mol-1 s-1

The units of rate constant for second  order reaction are L mol-1 s-1

4) Units of rate constant for third order reaction

For third order reaction, rate may be expressed as :

Rate = k [A]3

mol L-1 / s  = k (mol L-1)3

k= L2 mol-2 s-1

The units of rate constant for third order reaction are L2 mol-2 s-1

Units of Rate Constants for Gaseous Reactions

In case of gaseous reactions, the concentrations are pressure in the units of atmosphere. Therefore, the rate has the units of atm per second.

Zero order reaction = atm s-1

First order reaction = s-1 

Second order reaction = atm-1 s-1

Third order reaction = atm-2 s-1

Filed Under: Chemical Kinetics, Chemistry, Class 12

About Mrs Shilpi Nagpal

Author of this website, Mrs. Shilpi Nagpal is MSc (Hons, Chemistry) and BSc (Hons, Chemistry) from Delhi University, B.Ed. (I. P. University) and has many years of experience in teaching. She has started this educational website with the mindset of spreading free education to everyone.

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