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Rates and Equilibrium

Rate of the chemical reaction is defined as the rate of change of the reactant or the product.  The rate of change of reactant is the change in concentration of the reactant with time.  When the rate of the reaction is fast it means that the reactant is quickly decomposed into product.  Some reaction takes place slowly some reaction takes place fast.  The rate of the reaction depends on various factor like concentration of the reactant, temperature, pressure if the reactant are the gases, catalyst etc.,  The  rate law defines the rate of the reaction as directly proportional to the concentration of the reactant raised to some power.  The power is not the stiochiometric coefficient in the reactant it is a experimentally found out quantity and is called as the order of the reaction. 

Chemical equilibrium and rate of the reaction

A chemical reaction in equilibrium involves the opposing reactions. One of the reactions produces the products and is known as the forward reaction while the other produces the reactants from products and is known as the reverse reaction. Chemical equilibrium is dynamic in nature. At equilibrium, reactant and product molecules are both present in the reaction mixture in definite amounts. The equilibrium concentrations of the reactants and products do not change under constant temperature, pressure and catalysts etc. Consider a general equilibrium reaction at constant temperature represented by
aA + bB  <-------------->cC + dD

k f is rate constant of the forward reaction
k r is the rate constant of the reverse reaction

According to law of mass action, the rate of forward reaction is
Rf = kf [A]a [B]b
and the rate of reverse reaction is
Rr = kr [C]c [D]d
where kf and kr are the rate constants of the forward and reverse reactions
At equilibrium Rf= Rr

∴ kf[A]a [B]b = kr [C]c [D]d

Kf/Kr  = Kc =equilibrium constant

∴K = [C]c[D]d/[A]a[B]b

This equation of Kc is also known as equilibrium law equation. Kc is the equilibrium constant expressed in terms of molar concentrations of reactants and products.