chemical equilibrium – condition where the concentration of products and reactants do not change with time 15.1: The Concept of Equilibrium at equilibrium \[k_f[A] = k_r[B] \nonumber \] there for the ratio \[\displaystyle\frac{[B]}{[A]} = \frac{k_f}{k_r} = \textit{constant} \nonumber \] 15.2: The Equilibrium Constant equilibrium condition can be reached from either forward or reverse direction Cato Maximillian Galdberg (1836-1902), and Peter Wauge (1833-1900) Law of mass action – relationship between concentrations of reactants and products at equilibrium If \(aA + bB\rightleftharpoons pP+qQ\) then an equilibrium expression can be constructed \[\displaystyle K_c=\frac{[P]^p[Q]^q}{[A]^a[B]^b} \nonumber \] equilibrium expression depends only on stoichiometry of reaction and not mechanisms equilibrium constant: does not depend on initial concentrations does not matter if other substances present as long as they do not react with reactants or products varies with temperatures no units 15.2.1 Expressing Equilibrium Constants in Terms of Pressure, \(K_p\) \[\displaystyle K_p=\frac{(P_P)^p(P_Q)^q}{(P_A)^a(P_B)^b} \nonumber \] 15.2.2 The Magnitude of Equilibrium Constants \(K\gg 1\); equilibrium lies to the right; products favored \(K \ll 1\); equilibrium lies to the left; reactants favored 15.2.3 The Direction of the Chemical Equation and \(K\) equilibrium expression written in one direction is the reciprocal of the one in the other direction 15.3: Interpreting & Working with Equilibrium Constants 15.4: Heterogeneous Equilibria homogeneous equilibria – substances in the same phase heterogeneous equilibria – substances in different phases a pure solid, a pure liquid, and a solvent in dilute solutions all appear in equilibrium laws, but they are all assigned activities that are equal to 1 by convention the actvities of the pure solid, pure liquid, or solute are not explicitly written as part of the equilibrium law 15.5: Calculating Equilibrium Constants determining unknown equilibrium concentrations tabulate known initial and equilibrium concentrations calculate change in concentration that occurs as system reaches equilibrium use stoichiometry to determine change in concentration of unknown species from initial concentrations and changes in concentrations, calculate equilibrium concentrations 15.5.1 Relating K c and K p \[PV = nRT \nonumber \] \[P = (n/V)RT = MRT \nonumber \] \[PA = [A](RT) \nonumber \] \[K_p=K_c(RT)D^n \nonumber \] D n = change in moles from reactants to products 15.6: Applications of Equilibrium Constants equilibrium constant: product direction reaction mixture will proceed calculate concentrations of reactants and products once equilibrium is reached 15.6.1 Predicting the Direction of Reaction reaction quotient at equilibrium Q=K Q>K; reaction moves right to left Q<K; reaction moves left to right 15.6.2 Calculating of Equilibrium Concentrations 15.7: Le Chatelier's Principle If system at equilibrium is disturbed by change in temperature, pressure or concentration then system will shift equilibrium position 15.7.1 Change in Reactant or Product Concentration addition of substance will result in consummation of part of added substance if substance removed, reaction will move to produce more of the substance 15.7.2 Effects of Volume and Pressure Changes reducing volume, reaction shifts to reduce number of gas molecules increase volume, reaction shifts to produce more gas molecules increase pressure, decrease volume reduces total number of moles pressure volume changes do not affect K as long as temperature is constant changes concentrations of gaseous substances 15.7.3 Effect on Temperature Change endothermic: reactants + heat « products exothermic: reactants « products + heat increase temperature, equilibrium shifts in direction that absorbs heat endothermic: increase T, increase K exothermic: increase T, decrease K cooling shifts equilibrium to produce heat 15.7.4 The Effect of Catalysts catalysts increase rate at which equilibrium is obtained does not change composition of equilibrium mixture