This lecture covers the concept of chemical equilibrium focusing on reaction mechanisms. It discusses how to determine equilibrium concentrations of chemical species based on initial conditions and rate constants. The lecture also explores consecutive reactions with reversible steps and the derivation of expressions for concentrations of reactants and products. Additionally, it includes the analysis of concentration sketches for different scenarios and the interpretation of system behavior. Emphasis is placed on understanding the time dependence of concentrations and identifying similar behaviors in complex reactions.