Chemistry · Ch 8 — Physical and Chemical Equilibrium

Summary

In daily life we observe both irreversible changes (a banana ripening, silver tarnishing) and reversible changes; in a reversible reaction, the state at which the rate of the reverse reaction equals the rate of the forward reaction is called equilibrium.
Equilibrium is classified into physical equilibrium (phase changes and dissolution, with no chemical change) and chemical equilibrium.
Chemical equilibrium is dynamic -- the forward and backward reactions keep proceeding at the same rate, so no macroscopic change is observed, even though reactant and product molecules keep interconverting.
The law of mass action states that the rate of a reaction is directly proportional to the product of the active masses of the reactants, each raised to its stoichiometric coefficient.
The equilibrium constant $K_C$ (or $K_P$ for an all-gas reaction) tells us the extent of a reaction, i.e. how far it has proceeded toward product formation at a given temperature; $K_P = K_C(RT)^{\Delta n_g}$ .
Under non-equilibrium conditions, the reaction quotient Q is defined the same way as $K_C$ , but from whatever concentrations are present at that instant; comparing Q to $K_C$ predicts the direction a reaction will move. …