Chapter 15: Equilibrium

1. Chapter 15: Equilibrium Chemistry 1062: Principles of Chemistry II Andy Aspaas, Instructor

2. Chemical equilibrium • Most reactions are reversible: they can react in the forward or reverse direction • kf = forward rate constant, kr = reverse rate constant • In the forward reaction, the rate decreases over time due to the consumption of A • But, in the reverse direction, the rate increases due to buildup of B • Eventually the system reaches equilibrium, where the forward rate equals the reverse rate

3. Equilibrium quantities • If initial amounts of reactants are known, and the quantity of any substance at equilibrium is known, the quantities of the other substances can be calculated • Use nx as the change in moles from starting to equilibrium states, where n is the coefficient for that particular substance (-nx for reactants since they decrease, +nx for products since they increase)

4. Equilibrium quantities

5. The equilibrium constant • Since the forward rate = the reverse rate at equilibrium, kf[A] = kr[B] • Rearranging gives (kf / kr) = ([B] / [A]) • This quotient is the equilibrium constant, Kc • In general, for the following reaction, at equilibrium,

6. Determining equilibrium constant • Convert any chemical quantities into molar concentration, since that’s what is used in Kcequation • Use the method with initial, change, and equilibrium values as before • Substitute the values into the Kcequation • If two reactions are summed (ex. elementary reactions) their respective Kcvalues are multiplied

7. Reaction quotient • Reaction quotient, Qc = same as equilibrium constant, except represents an instant in time where the reaction is not necessarily at equilibrium • If Qc > Kc, there is an overabundance of products, and the reaction will go to the left to reach equilibrium • If Qc < Kc, there is an overabundance of reactants, and the reaction will go to the right • If Qc = Kc, the reaction is at equilibrium

8. Le Chatelier’s Principle • How will the concentrations of products and reactants be affected by addition of extra product or reactant, or changing pressure or temperature? • Le Chatelier’s Principle: if a system at equilibrium is disturbed by a change in temperature, pressure, or a concentration, the system shifts in equilibrium composition to counteract the change

9. Concentration • When extra reactant is added or when product is removed, Qc < Kc, and the reaction will shift more to the right to counteract the change • When extra product is added, or when reactant is removed, Qc > Kc, and the reaction will shift more to the left to counteract the change • After the shift, the system is restored back to Kc, but the composition may be different

10. Pressure change in equilibrium • A pressure change in a gaseous reaction can change the composition of the equilibrium mixture • A pressure increase (which increases the partial pressure of gaseous components) will cause the reaction to shift to reduce the total number of gas molecules in the system • Pressure increases caused by introduction of an inert gas does not affect the equilibrium, since partial pressures remain the same

11. Effect of temperature change • Temperature effects equilibrium in two ways: • An increase in temperature will cause forward and reverse reaction rates to increase, and equilibrium will be reached faster • The equilibrium constant itself will change • In an exothermic reaction (ΔH = -), view heat as a product • When heat is added (temperature increased) the equilibrium will shift towards reactants • The opposite is true for endothermic reactions

12. Catalysis • Catalysts increase the rates of reactions, but have no effect on the equilibrium concentration of the mixture • The equilibrium is reached more quickly • Helpful for reactions with a large equilibrium constant, but are too slow to be run at room temperature.