Thermodynamics. For a reaction to be practical, the equilibrium must favor products and the reaction rate must be fast enough to form them in a reasonable time. These two conditions depend on thermodynamics and kinetics respectively.
The size of Keq expresses whether the starting materials or products predominate once equilibrium is reached.
between ∆G° and Keq
G° is related to the equilibrium constant Keq by the following equation:
These equations can be used for any process with two states in equilibrium. As an example, monosubstituted cyclohexanes exist as two different chair conformations that rapidly interconvert at room temperature, with the conformation having the substituent in the roomier equatorial position favored.
This equation indicates that the total energy change is due to two factors: the change in bonding energy and the change in disorder.
In most other reactions that are not carried out at high temperature, the entropy term (TS°) is small compared to the enthalpy term (H0), and therefore it is usually neglected.
The energy of activation is the minimum amount of energy needed to break the bonds in the reactants.
Comparing ∆H° and Ea in two
Complete energy diagram for
the two-step conversion of
The effect of a catalyst
on a reaction