Contents
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
Leave a Reply