Transition State Theory
Consider the reaction : aA  bB  cC  dD
AB
A+B
C+D
To produce new species (product) not just A and B come in contact with each other ,
but physically join together forming an intermediate species AB , In reality this may
happen when the reactants have sufficient energy and correct orientation for joining to
occur .
AB called transition state and it is reorganization of transition state which leads
directly to the products C and D .The sufficient energy is called activation
Theory tell us that the transition state must be higher free energy than the sum of free
energies of reactants A and B.
From thermodynamics : G  G   RT ln J
For the above reaction J 
[C ]c [ D] d
[ A] a [ B]b
Where J is non equilibrium value corresponding to the non equilibrium free energy
change ∆G, if the system reaches a point where is no change of free energy (∆G=0)
then J=K and
G    RT ln K
In nature most metals are found in a combined state known as are ore (may be oxides ,
sulfides , carbonates etc) , in order to separate a metal such as iron from its ores e.g.
iron oxide it is necessary to supply amount of energy by heating in blast furnace to
about 1600○C, therefore metals ( iron in our case) in uncombined condition are
usually high energy state
According to thermodynamics laws there is a strong tendency for high energy state to
transfer into low energy state , the figure show the progress of a typical metal atom
from being combined in ore , separated as metal atom and recombined as a corrosion
products
From above we can say


The metal tend to be more stability
∆G between metal and its corrosion products represents only the tendency of
metal to corrode but tell us nothing about corrosion rate , and this is because of
the energy barrier between metal and corrosion products , metal atom must
surmount this barrier before it can corrode and energy must be supplied for
this to occur (it`s come during separation process)
Energy barrier called free energy of activation ∆G*.
Corrosion rate =Kcorr [reactant]
Kcorr =A exp[-∆G*/RT]
A is constant
R gas constant
T temperature(○K)
O2 and H2 have a strong tendency to form H2O but no measurable reaction occurs in
a mixture of gases until a spark is introduced
There are many form of energy but the driving force for corrosion comes from
chemical energy , this is partly derived from energy stored within chemical bonds of
substance called internal energy and metal gain this energy during the separation
process for metal from its ore.
Why gold not corrode?
The theory give two explanation
1-Aording to thermodynamics to make gold corrode its need to supply energy because
of positive value of free energy ( see below example)
1
Mg  H 2 O  O2  Mg (OH ) 2
2
G   596 KJ / mol
1
Cu  H 2 O  O2  Cu(OH ) 2
2
G   119 KJ / mol
Au 
3
3
H 2 O  O2  Au(OH ) 3
2
4
G   66 KJ / mol
For the first two reactions , they are occur spontaneously (negative free energy
change) while the third reaction is need to supply energy to occur because of positive
value of free energy change
2-the free energy of activation is too great , so the rate of reaction is very slow