Law of Mass Action

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Statement of Law of Mass Action:

  • In 1864, Norwegian chemists Cato Guldberg and Peter Wage put forward the law of mass action
  • Law of mass action states that  “The rate of a chemical reaction is directly proportional to the product of active masses of reactants, at given temperature at that instant”.
  • Where active mass is molar concentration per unit volume of that substance.  It is denoted by enclosing the symbol or the formula of that substance in the square bracket and expressed in mol dm-3.

Explanation of Law of Mass Action:

  • Consider a hypothetical reaction A + B →  Products.
  • According to the law of mass action the rate of reaction R ∝ [A] [B].
  • For any general reaction aA + bB → Products.
  • According to the law of mass action the rate  of reaction R ∝ [A]a [B]b

Equilibrium mixture:

  • The mixture of reactants and products formed at the chemical equilibrium state is called equilibrium mixture.

Equilibrium Concentrations:

  • The concentrations of the reactants and products in a chemical equilibrium state for a reversible reaction are called equilibrium concentrations.

The Expression for Equilibrium Constant in Terms of Concentrations:

Consider a hypothetical reversible reaction A + B  ⇌  C + D

According to the law of mass action the rate of the forward reaction

Rf ∝ [A] [B]

 ∴  R =  Kf [A] [B]



Where Kf  = rate constant for the forward reaction

According to the law of mass action the rate of the backward reaction

Rb ∝ [C] [D]

 ∴  Rb =  Kb [C] [D]



Where Kb = rate constant for the backward reaction

Now, for a chemical equilibrium,

R= Rb

K[A] [B] = Kb [C] [D]

Law of mass action 01



Where KC is known as equilibrium constant and the equation is called “mass law equation”.

Consider a general reversible reaction

aA + bB + cC + …   ⇌  lL  + mM  + nN + ….,    Then,

Law of mass action 02

  • The equilibrium constant ‘ KC‘ is defined as the ratio of the product of the equilibrium concentrations of the products to that of the reactants with each concentration term raised to the power equal to the stoichiometric coefficients of the substance in the balanced chemical equation.
  • Concentration is expressed in terms of mol dm-3 or mol L-1 or M.
  • The equilibrium constant is denoted by Kc.
  • More is the numerical value of ‘K’ then more is the concentration of products in comparison with reactants & vis-a-vis.

Concentration Quotient of a Chemical Reaction:

  • At a given temperature, the ratio of the product of the equilibrium concentrations of the products to that of the reactants with each concentration term raised to the power equal to the stoichiometric coefficients of the substance in the balanced chemical equation is called concentration ratio or concentration quotient.
  • It is denoted by QC.
  • At equilibrium, the concentration ratio is equal to the equilibrium constant Kc.
  • Its significance is that it helps in the prediction of the direction in which the net reaction is proceeding at given concentrations or partial pressures of reactants and products.
  • The generalization is
    • If QC > KC: The reaction is taking place in a backward direction i.e. in the direction of reactants.
    • If QC < KC: The reaction is taking place in a forward direction i.e. in the direction of products.
    • If QC = KC: The reaction is in an equilibrium state and hence no net reaction is taking place.


Law of Chemical Equilibrium:

  • At a given temperature, the ratio of the product of the equilibrium concentrations of the products to that of the reactants with each concentration term raised to the power equal to the stoichiometric coefficients of the substance in the balanced chemical equation has a constant value.

The Expression for Equilibrium Constant in Terms of Partial Pressure:

Partial Pressure:

  • It is the pressure exerted by a gas in a mixture of gases if it alone occupies the entire volume of the mixture of gases.
  • The partial pressure of a gaseous component is proportional to mole fraction.
  • The partial pressure of a gas is calculated using following formula

Law of mass action 03



Where,

n = Number of moles of gaseous component

N = Total moles of a gaseous system

P = Total pressure of the gaseous system

The Expression for Equilibrium Constant in Terms of Partial Pressure:

  • In the gaseous system, the concentrations in concentration quotients are replaced by partial pressure, because for given temperature the partial pressure of the gas is directly proportional to its concentration.

Consider a hypothetical reversible reaction



aA(g) + bB(g) ⇌  cC(g) + dD(g)

Then the equilibrium constant in terms of partial pressures is given by

Law of mass action 04

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