Question

Define and explain Law of Mass Action.

Answer 1

:Hint: Law of mass action states that the rate of chemical reaction is proportional to the product of the masses of the reactants, with each mass raised to a power equal to the coefficient that occurs in the chemical equation. The rate of reaction is directly proportional to the concentration of reactants.

Complete answer:
Law of Mass action is used to predict behavior of solutions. This law specifies that for a chemical reaction which is in equilibrium, the ratio between the concentration of reactants and products remains constant. The initial formulation of this law consists of the equilibrium and kinetic aspects. The composition of the reactants at equilibrium and the rate equations for the elementary reactions play an important role. These aspects are based on the research done by Norwegian scientists Cato Guldberg and Peter Waage in 1864. Both of them formulated this law according to the effects of concentration, temperature and mass on chemical reaction rates. This law deals with equilibrium and gives an expression for equilibrium constant.
According to law of mass action the total amount of the product obtained increases with time in a logarithmic manner. In a chemical reaction- $a A +b B\to c C + d D$
According to law of mass action the equilibrium constant is given as: ${{K}_{c}}=\dfrac{{{C}^{c}}{{D}^{d}}}{{{A}^{a}}{{B}^{b}}}$
Law of mass action is a general description of the equilibrium condition. For a reversible reaction at equilibrium and at constant temperature the ratio of concentration of products and reactants is equal to a constant known as equilibrium constant. In the above given expression a, b, c and d are stoichiometric constants. Stoichiometric constants are the number appearing before the symbol for each compound in the equation for a chemical reaction. To apply this law to ions and molecules in aqueous solution, their molar concentrations must be replaced by their activities which are basically just the effective concentration.

Note:
The ratio for mass action must be always taken for products over reactants. Pure solids and pure liquids are not included in the mass law. In the expression ${{K}_{c}}=\dfrac{{{C}^{c}}{{D}^{d}}}{{{A}^{a}}{{B}^{b}}}$ the lowercase letters represents the coefficients of the reactants and products. The law of mass action can be applied to semiconductors by giving a relationship between holes and free electrons when the semiconductor is in thermal equilibrium.