Chemical Equilibrium

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 [H2] • The reaction is dynamic - in constant motion

6. Equilibrium Constant • For the reaction: aA + bB cC + dD, Keq = [C]c[D]d [A]a[B]b • Keq = equilibrium constant • [ ] = concentration in M (mol/L) • Do not include any solids or liquids in the Keq 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: • H2 (g) + I2 (g) 2HI (g) • As4O6 (aq) + 6C (s) As4 (g) + 6CO (g) 3. Hg (l) Hg (g) • NH3 (aq) + H2O (l) NH4+ (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: [H2CO3] = 3.3 x 10-2M; [H3O+] = 1.1 x 10-6M; and [HCO3-] = 7.1 x 10-1M. What is the Keq value for the following reaction at equilibrium in a dilute aqueous solution? H2CO3 (aq) + H2O (l) H3O+ (aq) + HCO3- (aq)

9. Equilibrium Constant Calculations What is the equilibrium constant of formic acid, HCHO2? In water, the equilibrium concentrations are [HCHO2] = 2.00M; [H3O+] = 6.00 x 10-6M; and [CHO2-] = 6.00 x 10-6M. HCHO2 (aq) + H2O(l) H3O+ (aq) + CHO2- (aq)

10. System Equilibria • Equilibria can favor the formation of reactants or products • Keq can determine which direction is favored in a rxn • Keq > 1 means forward rxn favored • Keq < 1 means reverse rxn favored • Keq = 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

11. 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

12. System Equilibria • Factors that alter chemical equilibrium: • Concentration of reactants or products • Pressure • Temperature

13. 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

14. 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

15. Temperature • Heat can be treated as a product or a reactant • If the reaction is exothermic, heat is written on the product side of the equation • If the reaction is endothermic, heat is written on the reactant side of the equation • 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

16. 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

17. The Solubility Product Constant

18. Dissolution and Precipitation • Remember: ionic substances separate into their ions in solution and become uniformly distributed in the sol’n • Dissolution- the process in which an ionic solid dissolves in a polar liquid • Can write an equation for dissociation • Only dissociated substances are written as ions in equations • Must balance numerically and electrically

19. Precipitation • Precipitation- the process in which ions leave a sol’n and regenerate an ionic solid • Precipitate- insoluble solid formed • Dissolution and precipitation are opposite process • Solubility equilibrium- rate of dissolution= rate of precipitation

20. What is a solubility product constant, and what is it used for? • An equilibrium constant for slightly soluble ionic substances • symbolized Ksp • Used to determine solubility of sparingly soluble compounds • Cannot be applied successfully to salts that are more soluble

21. How is a solubility constant written? • The equation for a slightly soluble ionic substance in a saturated sol’n can be written in the following general form: • AaBb (s)  aA+(aq) + bB-(aq) • The solubility product constant is • Ksp = [A+]a[B-]b

22. Association Equations and Solubility Product Constants • Write the dissociation equation and solubility product constant for each of the following substances. • Strontium arsenite • Calcium oxalate • Barium sulfide • Magnesium hydroxide

23. Solubility Product • At 25°C, the concentration of Pb+2 ions in a saturated sol’n of PbF2 is 1.9 x 10-3M. What is the value of Ksp for PbF2?

24. PbF2 (s)  Pb+2 (aq) + 2F- (aq) • For every molecule of PbF2, there will be one Pb+2 ion and two F- ions. If [PbF2] = x, then [Pb+2] = x and [F-] = 2x • Ksp = [Pb+2] [F-]2 • Ksp = x (2x)2 • Ksp = 4x3

25. Solubility Product • A sample of Cd(OH)2 (s) is added to distilled water and allowed to come to equilibrium at 25°C. The concentration of Cd+2 is 1.7 x 10-5M at equilibrium. What is the value of Ksp for Cd(OH)2?

26. Solubility • What will be the equilibrium concentrations of lithium and phosphate ions in a saturated solution of lithium phosphate? (Ksp = 3.2 x 10-9)

27. Solubility • What will be the equilibrium concentrations of strontium and phosphate ions in a saturated solution of strontium phosphate? (Ksp = 1.0 x 10-31)

28. Precipitates • Supersaturated solutions are unstable • Non equilibrium state achieved by manipulating conditions • Precipitates will form in a supersaturated solution • To determine supersaturated solution calculate Q, the ion product • Ksp < Q = Supersaturated • Ksp > Q = Unsaturated • Ksp = Q = Saturated

29. Precipitation Reactions • Reaction in which 2 solutions are mixed and a precipitate is formed • Described by a chemical equation • Remember ionic substances dissociate in solution • The precipitate that forma is a combination of ions present • The precipitate formed can be identified by using solubility rules but can only be truly confirmed experimentally

30. What is the common ion effect? • Common ion: an ion that comes from two or more substances making up a chemical reaction • example: BaSO4 and Na2SO4; common ion is SO4-2 • Common ion effect: a process in which an ionic compound becomes less soluble upon the addition of one of its ions by adding another compound

31. Why does the common ion effect work? • The common ion effect is an example of Le Chatelier’s principle • When a product is added to a system in equilibrium, it will cause the equilibrium to shift to the left, making more insoluble reactant

32. a) Saturated silver sulfate solution, Ag2SO4 (aq), is colorless. A schematic of the solution is shown above, omitting the water for simplicity.(b) Following the addition of Na2SO4 (aq), most of the Ag+ ions originally present (about 7 of 8 shown) have precipitated. The schematic shows the only remaining silver ion as a silver + ball.