CHEMICAL KINETICS

Order of a Reaction

Order of a Reaction

It is the sum of powers of the concentration of the reactants Rate = k [A]x [B]y order = x + y

Zero Order Reactions

Rate = k[R]0 k = [R]0−[R]/t unit of k is mol L-1s-1 E.g. The decomposition of gaseous ammonia on a hot platinum surface The thermal decomposition of HI on gold surface

First Order Reaction

Rate = k[R] k = 2.303/t log [R]0/[R] unit of k is s-1 E.g. All natural and artificial radioactive decay of unstable nuclei, decomposition of N2O5 and N2O

Half-Life of a Reaction

It is the time in which the concentration of a reactant becomes half of its initial concentration.

It is denoted by t1/2.

For zero order reaction t1/2 ∝ [R]0 or ko

concentration t1/2 = [R]0/2k km

For first order

t1/2 is independent of [R]0 and is equal to 0.693/k

Half-Life of a Reaction

It is the number of collisions per second per unit volume Rate of reaction = ZAB e−Ea /RT

ZAB - collision frequency of reactants

e−Ea /RT - fraction of molecules with energies equal or more than Ea (activation energy)

Arrhenius Equation

It explains the temperature dependence of the rate of a reaction

k = A e-Ea /RT

A - Arrhenius factor or the frequency factor

Ea - activation energy in joules/ mole (J mol –1)

Pseudo First Order Reaction

Reactions that become first order under certain conditions

E.g. Acid hydrolysis of ethyl acetate

Acid catalysed inversion of cane sugar

Effective Collisions

Collisions in which molecules collide with sufficient kinetic energy

or threshold energy

and proper orientation

Rate = PZab e–Ea/RT P - probability or steric factor

Half-Life of a Reaction

It is the energy required by reactant molecules to form the intermediate or activated complex (C)

ΔH = Activation energy of forward reaction – Activation energy of backward reaction