The Equilibrium Law

Write equilibrium law expressions from balanced chemical equations for heterogeneous and homogeneous systems.   Use the value of the equilibrium constant to explain how far a system at equilibrium has gone towards completion. Additional KEY Terms

aA + bB cC + dD 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

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

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

Kc [NH3]2 [N2][H2]3 Writing the Equilibrium Law N2 (g) + 3 H2 (g) 2 NH3 (g) = Kc [N2][H2]3 [NH3]2 Homogeneous equilibria: Reactants and products are all in either gas (g) or aqueous (aq) phase.

Kc C(s) + H2O(g) CO(g) + H2(g) [CO][H2] [H2O] 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) + H2O(g) CO(g) + H2(g) = Kc [CO][H2] [H2O]

The Equilibrium Constant

As with most constants, the only factor affecting Kc is temperature. The Kc can indicate whether there are more products or reactants at equilibrium: = Kp PAa PBb PCc PDd = Kc [A]a[B]b [C]c[D]d

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

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

If Kc < 1 [product] < [reactant] Reactants are favoured. Case 3: If Kc < 1 [product] < [reactant] Reactants are favoured. Concentration Rate te te 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. Kc can be used to determine the equilibrium position.

CAN YOU / HAVE YOU? Write equilibrium law expressions from balanced chemical equations for heterogeneous and homogeneous systems.   Use the value of the equilibrium constant to explain how far a system at equilibrium has gone towards completion. Additional KEY Terms