Equilibrium in Solutions Chapter 16, 17, 18, 19
Review Equilibrium • How is the equilibrium constant for a reaction defined, generally? • Answer: [Products]/[Reactants] • How is it defined specifically for the following reaction: N2 + 2O2⇌ 2NO2 • Answer: [NO2]2/[N2][O2]2 • What do the brackets [products] mean, in the context above • Answer: These represent product, or reactant “Activity”, measured at equilibrium.
Review Equilibrium • 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. • 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, 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.
Review Equilibrium • 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. • If Q>Keq Products are in excess • If Q=Keq reaction is at equilibrium • If Q<Keq Products are in short supply
Equilibrium in solution • Many important chemical reactions take place in solutions, with water as the solvent (aqueous phase) • Solvation of a salt: • Auto ionization of water • Hydrolysis of an acid • Hydrolysis of a base
Salt solvation • 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. • Example: NaCl (s) ⇌ Na+ (aq) + Cl- (aq) • 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 oC • 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.
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. • 2H2O (l) ⇌ H3O+ (aq) + OH- (aq) • The reaction above is called “self ionization of water.” The equilibrium constant for this reaction is given a special name: Kw. • Kw= [H3O+ ][OH-]= 10 -14 at 25 oC • 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.
Hydronium and hydroxide • 2H2O (l) ⇌ H3O+ (aq) + OH- (aq) • H3O+ is called the “hydronium ion” • 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. • Regardless of the addition of acid or base to a solution, the product of [H3O+] 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?
Acids • Acids have the following properties (among others) • They taste tart • They are corrosive • Strong acids will burn your skin • They react with most metals to form hydrogen gas • Chemically, an acid is a compound which increases the hydronium ion concentration
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 • HCl(aq) + H2O ⇌ H3O+ (aq)+ Cl- (aq) • HA(aq) + H2O ⇌ H3O+ (aq)+ A- (aq) • The equilibrium constant has a special name: Ka is the acid dissociation constant. • Ka= [H3O+][A- ]/[HA] • 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.
Bases • Bases are compounds which increase the concentration of hydroxide ion in water. • Some properties of bases include (among others) • Taste bitter • Reacts with fats to make soap • Bases are caustic • Bases feel soapy (no surprise, since your skin contains fat molecules)
Base hydrolysis • 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) • NH3 + H2O ⇌ NH4+ (aq)+ OH- (aq) • B(aq) + H2O ⇌ HB+ (aq)+ OH- (aq) • The equilibrium constant has a special name: Kb is the base dissociation constant. • Kb= [HB+][OH- ]/[B] • 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