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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
Note 
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