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Law of Chemical Equilibrium

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Law of Chemical Equilibrium - Lesson Summary

In 1864, on the basis of experimental studies of many reversible reactions, two Norwegian chemists, Guldberg and Waage, established a relationship between the rate of chemical reaction and the concentration of the reactants, and formulated the Law of Mass Action.

According to this law, "At any instant, the rate of a chemical reaction, at a given temperature, is directly proportional to the active masses of the reactants at that instant".

The Law of Chemical Equilibrium states that the product of the concentrations of the reaction products raised to their respective stoichiometric coefficients in a balanced chemical equation divided by the product of the concentrations of the reactants raised to their individual stoichiometric coefficients has a constant value at a given temperature.

                                  aA + bB ⇌  cC + dD
                                      [C] c D d [A] a [B] b = K c

The equilibrium constant for the backward reaction is the inverse of the equilibrium constant for the forward reaction.

The equilibrium in a system with all the reactants and products in the same phase is known as homogeneous equilibrium.

                  H2(g) + I2(g) ⇌ 2HI(g)
                  N2(g) + 3H2(g) ⇌ 2NH3(g)

  CH3COOC2H5(aq) + H2O(l)  ⇌ CH3COOH(aq) + C2H5OH(aq)

The equilibrium in a system which has more than one phase is called heterogeneous equilibrium.

CaCO3(s) ⇌ CaO(s) + CO2(g)
Zn(s) + CuSO4(aq) ⇌ Cu(s) + ZnSO4(aq)

For a reversible reaction involving gases, the equilibrium constant is conveniently expressed in terms of the partial pressures of the reactants and products instead of their molar concentrations.

Pressure of Gas ∝ Concentration

Kc and Kp are related to each other by the equation

Kp = Kc * RT ∆n

When Δn = 0, Kp = Kc.
When Δn is negative, Kp > Kc.
When Δn is positive, Kp < Kc.


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