Equilibrium Concepts

Properties of Chemical Systems and Equilibrium

Equilibrium constants can be calculated only if the system is closed and at equilibrium.

Values of equilibrium constants Kc and Kp provide valuable information about properties of chemical systems. However, some major factors must be noted while determining equilibrium constants.

  • Equilibrium can be achieved only in a closed system, which means no substance can enter or leave the system. If some substances are entering or exiting the system, an equilibrium cannot be reached.
  • The equilibrium constant Kc is equal to the reaction quotient Q at equilibrium. When Q is equal to Kc, no further changes in concentrations will happen.
  • When the reaction quotient is less than the equilibrium constant (Q<Kc) (Q\lt K_{\rm{c}}), the system is not at equilibrium, and the forward reaction is happening at a greater rate, producing more products.
  • When the reaction quotient is greater than the equilibrium constant (Q>Kc)(Q>K_{\rm{c}}), the system is not at equilibrium, and the reverse reaction is happening at a greater rate, producing more reactants.
Comparing the equilibrium constant Kp with the value of partial pressure term provides information about reactions when all of the reactants and products are gases. For a reaction at equilibrium
Kp=(PCc×PDd)(PAa×PBb)K_{\rm{p}}=\frac{\left({P_{\rm{C}}}^c\times{P_{\rm{D}}}^d\right)}{\left({P_{\rm{A}}}^a\times{P_{\rm{B}}}^b\right)}
  • The equilibrium constant Kp equals the partial pressure term only at equilibrium. At equilibrium, there will be no change in partial pressures of reactants or products.
  • When the partial pressure term is less than Kp, the system is not at equilibrium, and the forward reaction is happening at a greater rate, producing more products.
  • When the partial pressure term is greater than Kp, the system is not at equilibrium, and the reverse reaction is happening at a greater rate, producing more reactants.
The ideal gas law can be used to derive a relation between Kc and Kp. The ideal gas law is
PV=nRTPV = nRT
If all the reactants and the products are gases, the relation between them is given by the equation
Kp=Kc(RT)Δng{K_{\rm{p}}} = {K_{\rm{c}}}{\left( {RT} \right)^{\Delta {n_g}}}
Here, the term Δng\Delta n_g is the difference between moles of products and moles of reactants.