The relationships between energies of
the activated state, and products are in the drawing for the NO2
+ CO reaction shown opposite(fig 12).
The activated state, which the molecules
must reach before either the forward or the reverse reaction is
possible, is 31.6 kcal mole-1 higher in
energy (enthalpy) than the NO2 and CO
reactants, and 85.7 kcal mole-1 higher
in energy than the NO and CO2 products.
The difference between the forward and
is the thermodynamic enthalpy of reaction (DH0=
-54.1 kcal mole-1). In the collision
theory of reaction, the activation energy is simply a barrier
that the colliding molecules must surmount before they can react.
theory proposes that this activated state is an intermediate
arrangement that the molecules go through, which has a real physical
existence although it cannot be isolated and studied at leisure.
In the NO2
+ CO reaction, this transition state, or activated
complex, would be one in which the O to be transferred
has not yet been completely lost by NO, nor completely acquired
by CO. If the two molecules approached end-on, the activated complex
would be something like
The dots indicate attractions between
atoms that are longer than normal covalent bonds, and quantum mechanical
calculations suggest that they might be some 30% longer.