Kinetics is the study of the rate of
reaction and the factors that affect it.
Collision theory
In order for a reaction to occur, the molecules must collide in the
correct orientation with the minimum energy needed for the
transition from the reactants to the products (the activation
energy). Only a very few collisions meet these requirements and
result in a reaction.
Reaction Coordinate Diagrams
Reaction coordinate diagrams show the energy of the reactants, the
activation energy up to the activated complex, or transition state
(the in-between state between the reactants and the products), and
the energy of the products. The overall energy change of the
reaction is also shown.
Factors affecting rate
Increasing the temperature increases the number of collisions, and
also the number of collisions with the needed energy. Therefore,
increasing temperature increases the rate of reaction. Increasing
the concentration or the surface area also increases the number of
collisions, therefore increasing the chance that a successful
collision will occur�which increases rate. Adding a catalyst, a
species that increases the rate of reaction without being used up in
the reaction, also increases the rate.
Equilibrium
Dynamic equilibrium is when the rate of the forward and reverse
reactions are equal. The reaction will appear to have stopped, as
the concentrations of each species won�t change, but the reaction
continues to proceed in both directions.
Equilibrium constants
Equilibrium constant expressions are the ratio of the concentrations
of products to reactants using their coefficients from the balanced
equation as exponents. Pure solids and liquids are not included in
the equilibrium constant expression. Equilibrium concentrations are
plugged into the expression to solve for the equilibrium constant.
A large equilibrium constant means that the reaction �lies to the
right� with more products than reactants at equilibrium. A small
constant means that the reaction �lies to the left� with more
reactants the products present at equilibrium.
Reaction Quotients
The reaction quotient is the value when concentrations at any time
are plugged into the equilibrium constant expression. If the
reaction quotient equals the equilibrium constant, then the system
is at equilibrium. If the reaction quotient is too large, the
reaction will proceed to the left to produce more reactants (and
reduce products). The opposite will happen if the reaction quotient
is too small.
Le Chatelier�s Principle
Le Chatelier�s principle says that if a system at equilibrium is
stressed or changed, the system will shift to reach equilibrium
again. Whatever you do to the system, it will un-do it. For
example, if you add more reactants, the reaction will proceed to the
right to get rid of the extra reactants and make more products.
