Introduction :
The
experimental observations of chemical equilibrium tell us that most of
the chemical reactions when carried out in closed vessels do not go to
completion. Under these a conditions, a reaction starts by itself or by
initiation, continues for some time at diminishing rates and ultimately
appears to stop. The reactants may still be present but they do not
appear to change into products any more. What happens in such case is
that the products of the reaction start reacting at the same rate as the
reactants. In other words, the rate of the back reaction becomes equal
to the rate of the forward reaction.
Characteristic features of chemical equilibrium
Thus,
in a given time as much of the products are formed as react back to
give the reactants. The composition of the reaction mixture at a given
temperature is the same irrespective of the initial state of the system,
i.e., irrespective of the fact whether we start with the reactants or
the products. The reaction in such conditions is said to be in a state
of equilibrium.
The
attainment of equilibrium can be recognized by noting constancy of
observable properties such as pressure, concentration, density or color
whichever may be suitable in a given case.
The
relationship between the quantities of the reacting substances and the
products formed can be worked out readily with the help of the law of
mass action.
The Laws of Mass Action of Chemical equilibrium
The
laws of mass action states that the driving force of a chemical
reaction is proportional to the active masses of the reacting
substances. Assuming that the driving force determines the reaction
rate, the law may be stated as follows: The rate at which a substance
reacts is proportional to its active mass and the rate of the chemical
reaction is directly proportional to the product of the active masses of
the reacting substance.
Consider a general reversible chemical reaction
aA + bB ↔ mM + nN
According to the law of mass action, assuming that active masses are equivalent to molar concentrations,
The rate of the forward reaction, rf α [A]a [B]b = Kf[A]a [B]b
The rate of the reverse reaction, rr α [M]m[N]n = Kr[M]m [N]n
Where
Kf and Kr are proportionally constants and square brackets represent
the molar concentrations of the entities enclosed. At equilibrium, the
rate of the frontward reaction is equal to the rate of the reverse
reaction, that is, Kf[A]a [B]b = Kr[M]m [N]n
Kf /Kr = Keq = [M]m [N]n / [A]a [B]b
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