1. Chemical Equilibrium

2. Complete and Reversible Reactions  Complete – Forms a precipitate or evolves gas, all reactants are used up  Reversible - When products formed in a chemical reaction decompose back to the original reactants

3. Reversible Reactions  The arrows go in both directions –  forward reaction –  reverse reaction  Must be in a closed system where nothing can escape

4. Equilibrium  Occurs when the forward and reverse reactions happen at an equal rate: there is no net change –Based on a specific temperature and pressure –The total amount of particles remains the same and therefore so does the concentration –The concentration of a substance is denoted by the use of brackets around the formula [H 2 ] – The reaction is dynamic - in constant motion

6. Equilibrium Constant  For the reaction: aA + bB cC + dD, K eq = [C] c [D] d [A] a [B] b –K eq = equilibrium constant –[ ] = concentration in M (mol/L)  Do not include any solids or liquids in the K eq expression –Both solids and liquids are pure substances, their concentration cannot change by definition

7. Write the formula for the equilibrium constant for each of the following reactions: 1. H 2 (g) + I 2 (g) 2HI (g) 2. As 4 O 6 (aq) + 6C (s) As 4 (g) + 6CO (g) 3. Hg (l) Hg (g) 4. NH 3 (aq) + H 2 O (l) NH 4 + (aq) + OH - (aq)

8. Equilibrium Constant Calculations At a temp of 25°C, the following concentrations of the reactants and products for the reaction involving carbonic acid and water are present: [H 2 CO 3 ] = 3.3 x 10 -2 M; [H 3 O + ] = 1.1 x 10 -6 M; and [HCO 3 - ] = 7.1 x 10 -1 M. What is the K eq value for the following reaction at equilibrium in a dilute aqueous solution? H 2 CO 3 (aq) + H 2 O (l) H 3 O + (aq) + HCO 3 - (aq)

9. Equilibrium Constant Calculations What is the equilibrium constant of formic acid, HCHO 2 ? In water, the equilibrium concentrations are [HCHO 2 ] = 2.00M; [H 3 O + ] = 6.00 x 10 -6 M; and [CHO 2 - ] = 6.00 x 10 -6 M. HCHO 2 (aq) + H 2 O(l) H 3 O + (aq) + CHO 2 - (aq)

10. What is favored: Products Reactants Reactants ⇋ Products X ⇋ Y Time Concentration

11. System Equilibria  Equilibria can favor the formation of reactants or products  K eq can determine which direction is favored in a rxn –K eq > 1 means forward rxn favored –K eq < 1 means reverse rxn favored –K eq = 1 means neither is favored  If conditions of the reaction are changed, the reaction will shift from its original equilibrium state to compensate for the change

12. Le Chatelier’s Principle  When a system at equilibrium is disturbed it attains a new equilibrium position to accommodate the change –Used in industry to increase efficiency A R dd way emove eplace

13. System Equilibria – 3 Factors  Factors that alter chemical equilibrium: –Concentration of reactants or products –Pressure –Temperature

14. Concentration  Adding a substance to a system at equilibrium drives the system to consume that substance  Removing a substance from a system at equilibrium drives the system to produce more of that substance

15. N 2 (g) + 3H 2 (g) ⇋ 2NH 3 (g) + 22.0 kcal StressEquilibrium Shift [N 2 ][H 2 ][NH 3 ]K eq Add N 2 Add H 2 Add NH 3 Remove N 2 Remove H 2 Remove NH 3 Increase Temp Decrease Temp Increase Pressure Decrease Pressure

16. Temperature  Only factor that affects the value of the equilibrium constant  Affects how completely a reaction proceeds to products  Remember – exothermic: releases heat – endothermic: absorbs heat

17. Temperature  Heat can be treated as a product or a reactant –EXOTHERMIC if ∆H is negative A + B ⇋ C + HEAT –ENDOTHERMIC if ∆H is positive A + HEAT ⇋ C + D  Adding heat to an exothermic reaction will shift the equilibrium towards the reactants  Adding heat to an endothermic reaction will shift the equilibrium towards the products

18. Pressure  Increase system pressure - the system will shift to reduce that pressure by proceeding in the direction that produces fewer molecules of gas  An equilibrium reaction that has the same # of moles of gas on both sides of the equation will not be affected by changes in pressure