FSc Notes Chemistry Part 1 Chapter 8 Chemical Equilibrium Lecture 3
Law of Mass Action & Equilibrium Constant Expression:
In 1864, two Norwegian chemist, Goldberg and Peter, presented a law for the rate of a chemical reaction which is known as law of Mass Action.
This law states that the rate of a chemical reaction is
directly proportional to the product of the concentration (or active masses) of reading substances.
The term active mass represents the concentration (in mole*
dm-3) of reactants and products. Let we have a reversible reaction, i.e
A+B----------> kr C+D
kr
The equilibrium concentration of A, B, C and D are
represented in square brackets i.e (A), (B), (C) and (D) are expressed in moles xdm-3.
According to law of Mass Action, the rate of forward
reaction, is proportional to the product of molar concentration of A and B.
Rate of forward reaction (rf) * (A) (B)
Or rf = kf (A) (B) --------I
Here Kf is the constant of proportionality and is known as
“rate constant for forward reaction”
Similarly the rate of reverse reaction is,
Rate of reverse reaction (*r)----- (C) (D)
Or Rr = Kr (C) (D)-------II
Here Kr is the constant of proportionality and is known as
“rate constant for reverse reaction”
As the reaction , Under consideration is a reversible
reaction, therefore at equilibrium Rf = Rr.
Kc is known as equilibrium constant in terms of concentration of reactants and products.
Equilibrium Content Expression. For Heterogeneous Equilibrium:
A heterogeneous equilibrium is an equilibrium in which the
reactants and products are in more than one physical state. While writing the equilibrium constant expression for a
heterogeneous equilibrium, the concentration for pure solids and liquids are neglected i.e.
they are not indicated, It is because of the fact, that , the concentration of a
pure solid or pure liquid is constant at a constant temperature and does not depend upon the quantity
of the substance.
For example the molar concentration of copper (at 20 c) is
the same, whether we have 1g or 1 ton of copper.
3Fe (s) + 4H2O(g)---------------> Fe3O4(s) + 4H2(g)
The equilibrium constant expression for the above system is
,
Kc = Concentration of Fe & Fe3 O4 are neglected because they are in
their pure solid state and their concentrations do not undergo appreciable change.
Significance or Applications of Kc:
Kc ( equilibrium constant ) has the following applications,
- Prediction of Direction of Reaction :
- Prediction of extent of reaction.
- Calculation of equilibrium concentration.
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