1. Equilibrium in Solutions Chapter 16, 17, 18, 19

2. Review Equilibrium How is the equilibrium constant for a reaction defined, generally? How is the equilibrium constant for a reaction defined, generally? Answer: [Products]/[Reactants] Answer: [Products]/[Reactants] How is it defined specifically for the following reaction: N 2 + 2O 2 ⇌ 2NO 2 How is it defined specifically for the following reaction: N 2 + 2O 2 ⇌ 2NO 2 Answer: [NO 2 ] 2 /[N 2 ][O 2 ] 2 Answer: [NO 2 ] 2 /[N 2 ][O 2 ] 2 What do the brackets [products] mean, in the context above What do the brackets [products] mean, in the context above Answer: These represent product, or reactant “Activity”, measured at equilibrium. Answer: These represent product, or reactant “Activity”, measured at equilibrium.

3. Review Equilibrium How is chemical “activity” measured? How is chemical “activity” measured? Answer: For an aqueous solution “activity” means the molar concentration. For a gas, it can be measured in molar units or in partial pressure units. For a solid or a liquid, the effective concentration is “1”. In more advanced chemistry texts, “activity” is always treated as a unitless value. Therefore, Keq is also treated as a unitless value. Answer: For an aqueous solution “activity” means the molar concentration. For a gas, it can be measured in molar units or in partial pressure units. For a solid or a liquid, the effective concentration is “1”. In more advanced chemistry texts, “activity” is always treated as a unitless value. Therefore, Keq is also treated as a unitless value. What can be determined about a reaction just by looking at the magnitude of Keq? What can be determined about a reaction just by looking at the magnitude of Keq? Answer: Whether a reaction mixture will contain greater concentrations of products, reactants, or some of both at equilibrium. If Keq>>1, products dominate (equilibrium lies to the right). If Keq >1, products dominate (equilibrium lies to the right). If Keq<<1, reactants dominate (equilibrium lies to the left). When Keq is close to 1, significant amounts of both reactant and product are left over at equilibrium.

4. Review Equilibrium What can you tell about a reaction mixture if both Q and Keq are known? What can you tell about a reaction mixture if both Q and Keq are known? Answer: You can tell whether the reaction is already at equilibrium, or which direction it needs to proceed to get there. Answer: You can tell whether the reaction is already at equilibrium, or which direction it needs to proceed to get there. If Q>Keq Products are in excess  If Q>Keq Products are in excess  If Q=Keq reaction is at equilibrium If Q=Keq reaction is at equilibrium If Q<Keq Products are in short supply  If Q<Keq Products are in short supply 

5. Equilibrium in solution Many important chemical reactions take place in solutions, with water as the solvent (aqueous phase) Many important chemical reactions take place in solutions, with water as the solvent (aqueous phase) Solvation of a salt: Solvation of a salt: Auto ionization of water Auto ionization of water Hydrolysis of an acid Hydrolysis of an acid Hydrolysis of a base Hydrolysis of a base

6. Salt solvation A “salt” is a substance formed by an anion and a cation. Ionic compounds are salts. A “salt” is a substance formed by an anion and a cation. Ionic compounds are salts. “Solvation” means to go into solution. Ions separate when a salt goes into solution. “Solvation” means to go into solution. Ions separate when a salt goes into solution. Example: NaCl (s) ⇌ Na + (aq) + Cl - (aq) Example: NaCl (s) ⇌ Na + (aq) + Cl - (aq) The equilibrium constant for this type of reaction is given its own symbol: Ksp, for solubility product The equilibrium constant for this type of reaction is given its own symbol: Ksp, for solubility product In the preceding example, Ksp= [Na + ][Cl - ], which has a value of “Ksp=37” at 25 o C In the preceding example, Ksp= [Na + ][Cl - ], which has a value of “Ksp=37” at 25 o C Can you calculate the equilibrium concentration of Na and Cl ions, in a saturated solution which contains equal concentrations of each? Calculate both molar concetration, and grams/Liter. Can you calculate the equilibrium concentration of Na and Cl ions, in a saturated solution which contains equal concentrations of each? Calculate both molar concetration, and grams/Liter. Ksp for NaCl is fairly large, compared to other salts, indicating a high degree of solubility. Ksp for NaCl is fairly large, compared to other salts, indicating a high degree of solubility.

7. Self ionization of water Water, being a polar molecule, forms ions in solution also, though at a rate which is orders of magnitude lower than for the solvation of ionic compounds. Water, being a polar molecule, forms ions in solution also, though at a rate which is orders of magnitude lower than for the solvation of ionic compounds. 2H2O (l) ⇌ H 3 O + (aq) + OH - (aq) 2H2O (l) ⇌ H 3 O + (aq) + OH - (aq) The reaction above is called “self ionization of water.” The equilibrium constant for this reaction is given a special name: Kw. The reaction above is called “self ionization of water.” The equilibrium constant for this reaction is given a special name: Kw. Kw= [H 3 O + ][OH - ]= 10 -14 at 25 o C Kw= [H 3 O + ][OH - ]= 10 -14 at 25 o C Can you calculate the equilibrium concentration of hydroxide and hydronium in a solution which contains equal concentrations of the two ions? Can you calculate the equilibrium concentration of hydroxide and hydronium in a solution which contains equal concentrations of the two ions? Despite the very low equilibrium constant for this reaction, the reaction itself is important, and explains the behavior of acids and bases in solution. Despite the very low equilibrium constant for this reaction, the reaction itself is important, and explains the behavior of acids and bases in solution.

8. Hydronium and hydroxide 2H2O (l) ⇌ H 3 O + (aq) + OH - (aq) 2H2O (l) ⇌ H 3 O + (aq) + OH - (aq) H 3 O + is called the “hydronium ion” H 3 O + is called the “hydronium ion” Compounds which increase the concentration of hydronium ions in solution (such as HCl) are called “acids” Compounds which increase the concentration of hydronium ions in solution (such as HCl) are called “acids” OH - is called a hydroxide ion. Compounds which increase the concentration of hydroxide ions in solution (such as NaOH) are called bases. OH - is called a hydroxide ion. Compounds which increase the concentration of hydroxide ions in solution (such as NaOH) are called bases. Regardless of the addition of acid or base to a solution, the product of [H 3 O + ] and [OH - ] is always the same for a solution at equilibrium: Kw= 10 -14. Regardless of the addition of acid or base to a solution, the product of [H 3 O + ] and [OH - ] is always the same for a solution at equilibrium: Kw= 10 -14. Can you calculte the hydroxide ion concentration for an acidic solution containing 10 -4 M hydronium ions? Can you calculte the hydroxide ion concentration for an acidic solution containing 10 -4 M hydronium ions?

9. Acids Acids have the following properties (among others) Acids have the following properties (among others) They taste tart They taste tart They are corrosive They are corrosive Strong acids will burn your skin Strong acids will burn your skin They react with most metals to form hydrogen gas They react with most metals to form hydrogen gas Chemically, an acid is a compound which increases the hydronium ion concentration Chemically, an acid is a compound which increases the hydronium ion concentration

10. Acid hydrolysis An acid hydrolysis reaction is one in which the acid reacts with water to form hydronium ions. Water is not merely a solvent, but actually takes part in the reaction An acid hydrolysis reaction is one in which the acid reacts with water to form hydronium ions. Water is not merely a solvent, but actually takes part in the reaction HCl(aq) + H2O ⇌ H3O + (aq)+ Cl - (aq) HCl(aq) + H2O ⇌ H3O + (aq)+ Cl - (aq) HA(aq) + H2O ⇌ H3O + (aq)+ A - (aq) HA(aq) + H2O ⇌ H3O + (aq)+ A - (aq) The equilibrium constant has a special name: Ka is the acid dissociation constant. The equilibrium constant has a special name: Ka is the acid dissociation constant. Ka= [H3O + ][A - ]/[HA] Ka= [H3O + ][A - ]/[HA] For hydrochloric acid (HCl) where “A - ” represents the chloride ion, Ka=107, a huge number as acids go. For hydrochloric acid (HCl) where “A - ” represents the chloride ion, Ka=107, a huge number as acids go. For acetic acid (where “A - ” is the acetate ion) Ka=1.8 × 10 –5, a much weaker acid than HCl. There are many other acids which are weaker still. For acetic acid (where “A - ” is the acetate ion) Ka=1.8 × 10 –5, a much weaker acid than HCl. There are many other acids which are weaker still.

11. Bases Bases are compounds which increase the concentration of hydroxide ion in water. Bases are compounds which increase the concentration of hydroxide ion in water. Some properties of bases include (among others) Some properties of bases include (among others) Taste bitter Taste bitter Reacts with fats to make soap Reacts with fats to make soap Bases are caustic Bases are caustic Bases feel soapy (no surprise, since your skin contains fat molecules) Bases feel soapy (no surprise, since your skin contains fat molecules)

12. Base hydrolysis During base hydrolysis, the base reacts with water to form hydroxide ions. Water may take part in the reaction During base hydrolysis, the base reacts with water to form hydroxide ions. Water may take part in the reaction NaOH(aq) ⇌ Na + (aq)+ OH - (aq) NaOH(aq) ⇌ Na + (aq)+ OH - (aq) NH3 + H 2 O ⇌ NH 4 + (aq)+ OH - (aq) NH3 + H 2 O ⇌ NH 4 + (aq)+ OH - (aq) B(aq) + H 2 O ⇌ HB + (aq)+ OH - (aq) B(aq) + H 2 O ⇌ HB + (aq)+ OH - (aq) The equilibrium constant has a special name: Kb is the base dissociation constant. The equilibrium constant has a special name: Kb is the base dissociation constant. Kb= [HB + ][OH - ]/[B] Kb= [HB + ][OH - ]/[B] For NaOH, Kb is also the solubility product (Ksp), about 700. Very strong base. For NaOH, Kb is also the solubility product (Ksp), about 700. Very strong base. For ammonia, a much weaker base, Ksp=1.8 x 10 -5 For ammonia, a much weaker base, Ksp=1.8 x 10 -5