This chapter explores reversible and irreversible chemical reactions, detailing how chemical equilibrium is established in a closed system. It explains the dynamic nature of equilibrium and introduces the Law of Mass Action, which leads to the derivation of the equilibrium constant expression ($Kc$). Students learn how to interpret the numerical value of $Kc$ to predict the extent of a reaction and how Le Chatelier’s Principle can be applied to shift equilibrium positions by altering concentration, temperature, or pressure.