Dynamic Equilibrium Chapter 15 Example liquid + heat  vapour an endothermic physical change

Le Chatelier’s Principle When a dynamic equilibrium is upset by a disturbance, the system responds in a direction that tends to counteract the disturbance and, if possible, restore equilibrium liquid + heat  vapour Increase the temperature (add heat)

Le Chatelier’s Principle When a dynamic equilibrium is upset by a disturbance, the system responds in a direction that tends to counteract the disturbance and, if possible, restore equilibrium liquid + heat  vapour Liquid evapourates to absorb the added heat and the position of the equilibrium shifts to the right

Chemical Equilibrium CH 3 COOH + H 2 O  H 3 O + +CH 3 COO - For a given composition of a system, equilibrium is independent of which side you approach it from…..reactions are REVERSIBLE N 2 O 4 (g)  2NO 2 (g) C init mol0 or mol C equil mol mol

Equilibrium Law simple relationship between molar concentrations (or pressures) of reactants and products at equilibrium H 2 (g)+I 2 (g)  2HI(g) (at 440C) Mass action expression: Where Q is the reaction quotient LAW:at equilibrium the reaction quotient is equal to the equilibrium constant, K. In general for dD + eE  gG + hH

Heterogeneous Equilibria (involving more than one phase)

Equilibrium Law for Gaseous Reactions For gaseous reactions use partial pressures, P, to give an equilibrium constant (K p )

The Magnitude of Equilibrium Constants The equilibrium constant, K, is the ratio of products to reactants. Therefore, the larger K the more products are present at equilibrium. Conversely, the smaller K the more reactants are present at equilibrium. If K >> 1, then products dominate at equilibrium and equilibrium lies to the right. If K << 1, then reactants dominate at equilibrium and the equilibrium lies to the left.

Predicting direction of equilibrium changes (Le Chatelier redux) Adding or removing reactant or product Cu(H 2 O) 4 2+ (aq.) + 4Cl - (aq.)  CuCl 4 2- (aq.) + 4H 2 O Changing volume (or pressure)-gases only as liquids and solids are incompressible Decrease V=>increase P System opposes this by reducing the number of molecules

Predicting direction of equilibrium changes (Le Chatelier redux) Changing temperature (heat) System opposes this by trying to absorb the added heat ONLY factor that actually changes the value of K Effect of catalyst Speeds up approach to equilibrium, but does not alter K or the system (kinetics)