Dynamic Equilibrium

closed system: The total mass of products and reactants remains constant and energy cannot be lost to surroundings.

When reactions don't go to completion, they reach a state of dynamic equilibrium. A state of equilibrium is reached in a closed system when the rates of the forward and reverse reactions are equal.


  1. Forward and backward reaction occurs at same rate
  2. Concentration of reactants and products are the same
  3. The equilibrium occurs only in a closed system
  4. The are no changes in the macroscopic properties, like colour, pressure, pH

Physical equilibrium

Physical reaction: A reaction involving no changes of the chemical bonds. This is most often melting/boiling.

it is possible to have an equilibrium in a physical reaction. This is where you have two processes such as boiling and condensation occurring at the same time in a closed system. For example in a conical flask of water being heated up, the liquid may evaporate and then condense on the sides. If you close the system using a bung, then it will reach a physical equilibrium where condensation and evaporation occur at an equal rate.

The reaction quotient and equilibrium constant 

reaction quotient (Q)

In a reaction at any point in a reaction the relative quantities of products and reactants can be described using the reaction quotient. This is a number which tells whether the reaction favours the products or the reactants at that point. The reaction quotient is given by this formula

reaction: wa+xb  yc+zdQ= [C]y×[D]z[A]w×[B]x

Note: The symbol for the reaction quotient is Q

Equilibrium constant

At the dynamic equilibrium, the reaction quotient is called the equilibrium constant. The formula for the equilibrium constant is the same as the one for the reaction quotient except the concentrations of the products and reactants are specifically the concentrations which occur at the dynamic equilibrium. The formula is:

reaction: wa+xb  yc+zdKc= [Ceqm]y×[Deqm]z[Aeqm]w×[Beqm]x

magnitude of equilibrium constant

le chatelier's principle

Le Chatelier's principle says that any change to the products or reactants, which disturbs the equilibrium will be countered by a shift in equilibrium to to restore the equilibrium. In other words Kc must stay constant.

Factors affecting position of equilibrium

Effect of catalyst

A catalyst will increase the rate of reaction but will not change the equilibrium constant or position.





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