Kinetics : Factors Affecting Reaction Rates
|A Maxwell-Boltzmann diagram shows the distribution
of molecules at differing
kinetic energy levels in a gas sample. At higher temperature, a larger fraction of molecules
has kinetic energy equal to or greater than the activation energy (Ea) for the reaction
than at lower temperature. In other words, a greater proportion of molecules have enough
kinetic energy to participate in the reaction.
CatalystsThe presence of a catalyst increases the reaction rate.
Catalyst - A substance that increases the rate of a reaction but is not consumed in the reaction. It does so by lowering the activation energy (Ea) - The energy level that must be overcome by the reactants in a chemical reaction in order for the reaction to occur) of a reaction. Possible ways of lowering the Ea of a reaction:Increases the frequency of collisions between the reactant molecules.
Changes the relative orientation of the reactant molecules.
Donates electron density to the reactant molecules.
Reduces the intramolecular bonding within the reactant molecules.
Provides an alternate pathway or mechanism for the reaction.
For equilibrium reactions, both the forward and reverse reaction rates are affected by the catalyst.
i.e. the Ea for both directions is decreased.
Therefore, the equilibrium constant (Kc or Kp) - (The product of the concentrations (or partial pressures) of the products of a reaction divided by the product of the
concentrations (or partial pressures) of the reactants.) is notchanged by the presence of a catalyst.
The relative concentrations of the reactants and products is not changed.