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Chemical Equilibrium and Equilibrium constant expression Question: What is law of mass action? Derive the equilibrium constant expression for a general reaction given as follows. Answer. Law of mass action: This law states that the rate at which a substance reacts is directly proportional to its active mass and the rate at which reaction takes place is directly proportional to product of active masses of all reactants. Consider a reaction in which a moles of a reactant  react with b moles of reactant B to produce a c moles of product c and d moles of product d. According to law of mass action: \[{\color{DarkGreen} Rate \: of \, forward\: reaction \propto \left [ A \right ]^{a}\left [ B \right ]^{b}}\] \[{\color{DarkGreen} Rate \: of \, forward\: reaction = {K}_{f}\left [ A \right ]^{a}\left [ B \right ]^{b}}\cdot \cdot \cdot (i)\] \[ {\color{DarkGreen} Rate \: of \, reverse\: reaction = {K}_{r}\left [ C \right ]^{c}\left [ D \right ]^{d}}\cdot \cdot \cdot (ii)\] \[Where \: K_{f}\:...