aA + bB cC + dD Equilibrium RegionKinetic Region

aA + bB cC + dD Equilibrium RegionKinetic Region Chemical equilibrium: State where the concentrations of reactants and products remain constant over time because the rates of the forward reaction equal the reverse reaction.

The Equilibrium Law

CAN YOU? / HAVE YOU? Write the equilibrium law given the chemical equation for the equilibrium system.

Law of mass action (equilibrium law): A ratio to show the relationship between [reactants] and [products] at equilibrium. The value of this ratio is called the equilibrium constant. aA + bB cC + dD

k f [A] a [B] b = k r [C] c [D] d At equilibrium rates are equal: rate forward = k f [A] a [B] b rate reverse = k r [C] c [D] d If the forward and reverse reactions were elementary reactions, the rate laws would be: aA + bB cC + dD

The ratio of rate constants was condensed to one constant, K c, called the equilibrium constant. The law of mass action or equilibrium law then becomes = kfkf krkr [A] a [B] b [C] c [D] d = KcKc [A] a [B] b [C] c [D] d = KcKc Reactants Products

Homogeneous equilibria: Reactants and products are all in either gas (g) or aqueous (aq) phase. N 2 (g) + 3 H 2 (g) 2 NH 3 (g) Writing the Equilibrium Law = KcKc [N 2 ][H 2 ] 3 [NH 3 ] 2

Heterogeneous equilibria - reactants and products in more than one state. When writing the mass action expression, omit solids (s) or liquids (l). Concentration of solids and liquids don't change at constant temperatures. C (s) + H 2 O (g) CO (g) + H 2(g) = KcKc [CO][H 2 ] [H 2 O]

The Equilibrium Constant

As with most constants, the only factor affecting K c is temperature. The K c can indicate whether there are more products or reactants at equilibrium: = KcKc [A] a [B] b [C] c [D] d = KpKp P A a P B b P C c P D d

Rate Concentration Case 1: If K c = 1 [products] = [reactants] aA + bB cC + dD

Concentration Rate Case 2: If K c > 1 [product] > [reactant] Products are favoured. aA + bB cC + dD

Concentration Rate Case 3: If K c < 1 [product] < [reactant] Reactants are favoured. aA + bB cC + dD

 Most chemical reactions are reversible.  Chemical equilibrium can only occur in a closed system.  Equilibrium is when rates are equal AND the [ ]s remains constant.  K c can be used to determine the equilibrium position.

CAN YOU? / HAVE YOU? Write the equilibrium law given the chemical equation for the equilibrium system.