Chemical Equilibrium Chapter 13

Equilibrium is a state in which there are no observable changes as time goes by. Chemical equilibrium is achieved when: the rates of the forward and reverse reactions are equal and the concentrations of the reactants and products remain constant (requires a closed system) Physical equilibrium H 2 O (l) Chemical equilibrium N2O4 (g)N2O4 (g) H 2 O (g) 2NO 2 (g) ⇌ ⇌

Characteristics of a system at equilibrium 1.Reversible reaction must be possible 2.There is a dynamic state of balance between both the forward and backward reaction 3.There is no change in concentration of reactants or products once chemical equilibrium is reached 4.There is no bulk change in properties of the system (ex: no colour or pressure change)

5.It is a closed system (no heat or matter in or out) 6.The temperature of the system remains constant 7.It can be reached from either direction 8.Any change to the system at equilibrium can be reversed and restored back to original equilibrium conditions.

Examples at Equilibrium Water evaporating and condensing in a jar with lid A saturated solution with a few crystals added A bottle of unopened cola, carbon dioxide is in equilibrium in solution and air above pop.

Writing equilibrium expressions – k eq expressions See rules pg Hebden 1.All gases must be included. 2.Aqueous ions must be included. 3.Solids are NEVER included. 4.Pure liquids are NEVER included. 5.Mixtures of liquids must be included. Assign Hebden pg. 28 #1-10

Changing the temperature affects the rates of forward and reverse reactions differently because they have different activation energies. ie. Changing the temperature will change the equilibrium and establish a NEW and DIFFERENT equilibrium. All other changes maintain the same equilibrium.

N 2 O 4 (g) 2NO 2 (g) Start with NO 2 Start with N 2 O 4 Start with NO 2 & N 2 O 4 equilibrium ⇌

K >> 1 K << 1 Lie to the rightFavor products Lie to the leftFavor reactants Equilibrium Will K = [C] c [D] d [A] a [B] b aA + bB cC + dD ⇌

constant

Homogeneous equilibrium applies to reactions in which all reacting species are in the same phase. N 2 O 4 (g) 2NO 2 (g) K c = [NO 2 ] 2 [N 2 O 4 ] K p = NO 2 P2P2 N2O4N2O4 P In most cases K c  K p ⇌

Homogeneous Equilibrium CH 3 COOH (aq) + H 2 O (l) CH 3 COO - (aq) + H 3 O + (aq) K c = ‘ [CH 3 COO - ][H 3 O + ] [CH 3 COOH][H 2 O] [H 2 O] = constant K c = [CH 3 COO - ][H 3 O + ] [CH 3 COOH] General practice not to include units for the equilibrium constant. ⇌

Heterogenous equilibrium applies to reactions in which reactants and products are in different phases. CaCO 3 (s) CaO (s) + CO 2 (g) K c = ‘ [CaO][CO 2 ] [CaCO 3 ] [CaCO 3 ] = constant [CaO] = constant K c = [CO 2 ] K p = P CO 2 The concentration of solids and pure liquids are not included in the expression for the equilibrium constant. ⇌

N 2 O 4 (g) 2NO 2 (g) = 4.63 x K = [NO 2 ] 2 [N 2 O 4 ] 2NO 2 (g) N 2 O 4 (g) K = [N 2 O 4 ] [NO 2 ] 2 ‘ = 1 K = 216 When the equation for a reversible reaction is written in the opposite direction, the equilibrium constant becomes the reciprocal of the original equilibrium constant. ⇌⇌

If an external stress is applied to a system at equilibrium, the system adjusts in such a way that the stress is partially offset as the system reaches a new equilibrium position. Le Châtelier’s Principle Changes in Concentration N 2 (g) + 3H 2 (g) 2NH 3 (g) Add NH 3 Equilibrium shifts left to offset stress ⇌ K eq value remains unchanged

Le Châtelier’s Principle Changes in Concentration continued ChangeShifts the Equilibrium Increase concentration of product(s)left Decrease concentration of product(s)right Decrease concentration of reactant(s) Increase concentration of reactant(s)right left aA + bB cC + dD Add Remove ⇌

Le Châtelier’s Principle Changes in Volume and Pressure A (g) + B (g) C (g) ChangeShifts the Equilibrium Increase pressureSide with fewest moles of gas Decrease pressureSide with most moles of gas Decrease volume Increase volumeSide with most moles of gas Side with fewest moles of gas ⇌

Le Châtelier’s Principle Changes in Temperature ChangeExothermic Rx Increase temperatureK decreases Decrease temperatureK increases Endothermic Rx K increases K decreases colder hotter

Changing the temperature affects the rates of forward and reverse reactions differently because they have different activation energies

uncatalyzedcatalyzed Catalyst lowers E a for both forward and reverse reactions. Catalyst does not change equilibrium constant or shift equilibrium. Adding a Catalyst does not change K does not shift the position of an equilibrium system system will reach equilibrium sooner Le Châtelier’s Principle

ChangeShift Equilibrium Change Equilibrium Constant Concentrationyesno Pressureyesno Volumeyesno Temperatureyes Catalystno