acid_base_equilibria - Chapter 16 Acids and Bases and...

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Unformatted text preview: Chapter 16 Acids and Bases and Equilibrium brium Stro Strong Acids Memorize • The seven strong acids: HCl, HBr, HI, HNO , 3 H2SO , HClO , and HClO 4 3 4 ØThese are, by definition, strong electrolytes and exist totally as ions in aqueous solution ØFor the monoprotic strong acids, [H3O ] = [acid]. + Stro Strong Bases • Strong bases are the soluble hydroxides, which are the alkali metal and heavier 2 2 alkaline earth metal hydroxides (Ca +, Sr + , 2 and Ba +). Ø Again, these substances dissociate completely in in aqueous solution. We Weak Acids/Bases Weak Acids/Bases • Remember weak acids/bases only partially dissociate. • Weak Acid/Base are equilibrium reactions: HA(aq) + H2 O(l) B(aq) + H2O(l) − H3O (aq) + A (aq) + BH+ (aq) + OH− (aq) Ac Acid and Base Strength • Strong acids are completely dissociated in water. Ø Their conjugate bases are quite weak. • Weak acids only dissociate partially in water. Ø Their conjugate bases are weak bases. Ac Acid and Base Strength • Substances with negligible acidity do not dissociate in water. Ø Their conjugate bases are exceedingly strong. Ac Acid and Base Strength In any acid­base reaction, the equilibrium will favor the reaction that moves the proton to the stronger base. e. − HCl(aq) + H2O(l) ¾¾® H3 O (aq) + Cl (aq) + − H2 O is a much stronger base than Cl , so the equilibrium lies so far to the right K is not measured (K>>1). Ac Acid and Base Strength C2H3O (aq) + H2O(l) 2 H3O (aq) + C2H3O − (aq) + 2 Acetate is a stronger base than H2O, so the equilibrium favors the left side ( 1). (K< Dis Dissociation Constants • For a generalized acid dissociation, HA(aq) + H2O(l) − A (aq) + H3O (aq) + the equilibrium expression would be [H3O ] [A ] + − K = c [HA] • This equilibrium constant is called the acid­dissociation constant, K . tant, a Dis Dissociation Constants The greater the value of K , the stronger a the acid. Calculating K from the pH K a • The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is 2.38. Calculate K for e a formic acid at this temperature. • We know that − [H3O ] [COO ] + K = a [HCOOH] Calculating K from the pH K a • The pH of a 0.10 M solution of formic acid, HCOOH, at 25°C is 2.38. Calculate K for e a formic acid at this temperature. • To calculate K , we need the equilibrium a concentrations of all three things. • We can find [H3O ], which is the same as + − [HCOO ], from the pH. Calculating K from the pH K a pH = −log [H3 O ] + 2.38 = −log [H3O ] + −2.38 = log [H3O ] + − l 10 2.38 = 10 og [H 3O+] = [H3O ] + − − 4.2 ´ 10 3 = [H3 O ] = [HCOO ] + Cal Calculating K from pH a Now we can set up a table… [HCOOH], M Initially Change At Equilibrium 0.10 −4.2 ´ 10­3 + − [H O ], M [HCOO ], M 3 0 0 +4.2 ´ 10­3 +4.2 ´ 10−3 4.2 ´ 10−3 0.10 − 4.2 ´ 10−3 4.2 ´ 10−3 = 0.0958 = 0.10 Cal Calculating K from pH a − [4.2 ´ 10−3 ] [4.2 ´ 10 3 ] 10 K = a [0.10] − = 1.8 ´ 10 4 Cal Calculating Percent Ionization [H O+ ] q 3 e • Percent Ionization = [HA] ´ 100 initial • In this example −3 [H O+ ] q = 4.2 ´ 10 M e 3 [HCOOH] al = 0.10 M initi Cal Calculating Percent Ionization −3 4.2 ´ 10 Percent Ionization = ´ 100 0.10 = 4.2% 4.2% Cal Calculating pH from K a Calculate the pH of a 0.30 M solution of acetic a 0.30 acid, HC H O2, at 25°C. 2 3 HC2H3 O (aq) + H2O(l) 2 H3O (aq) + C2H3O − (aq) + 2 −5 K for acetic acid at 25° s 1.8 ´ 10 . C i a Cal Calculating pH from K a The equilibrium constant expression is [H O+ ] [C H O2− ] 3 2 3 K = a [HC 3 [HC H O2] 2 Cal Calculating pH from K a We next set up a table… [C H O ], M 2 3 2 Initially Change 0.30 −x + [H O ], M 3 [C H O − ], M 2 3 2 0 +x x 0 +x x At Equilibrium 0.30 − x » 0.30 We are assuming that x will be very small compared to 0.30 and can, therefore, be ignored. Try also using the quadratic Equation Cal Calculating pH from K a Now, 2 (x) − 1.8 ´ 10 5 = (0.30) −5 (1.8 ´ 10 ) (0.30) = x2 −6 5.4 ´ 10 = x2 −3 2.3 ´ 10 = x Cal Calculating pH from K a pH = −log [H O+ ] 3 −3 pH = −log (2.3 ´ 10 ) og (2.3 pH = 2.64 2.64 Pol Polyprotic Acids • Have more than one acidic proton. • If the difference between the K for the first he a dissociation and subsequent K values is equent a 3 10 or more, the pH generally depends only on the first dissociation. We Weak Bases Bases react with water to produce hydroxide ion. We Weak Bases The equilibrium constant expression for this reaction is [HB] [OH− ] H K = b [B− ] where K is the base­dissociation constant. tant. b We Weak Bases K can be used to find [OH− ] and, through it, pH. b pH pH of Basic Solutions What is the pH of a 0.15 M solution of NH ? a 0.15 3 NH3(aq) + H2O(l) NH4+(aq) + OH− (aq) − [NH + ] [OH ] 4 −5 K = = 1.8 ´ 10 b [NH ] 3 pH of Basic Solutions Tabulate the data. [NH3 ], M Initially At Equilibrium 0.15 0.15 ­ x » 0.15 [NH4+ ], M [OH− ], M 0 x 0 x pH pH of Basic Solutions 2 (x) − 1.8 ´ 10 5 = (0.15) −5 (1.8 ´ 10 ) (0.15) = x2 −6 2.7 2.7 ´ 10 = x2 −3 1.6 1.6 ´ 10 = x2 pH pH of Basic Solutions Therefore, − 1.6 −3 [OH ] = 1.6 ´ 10 M −3 pOH = −log (1.6 ´ 10 ) og (1.6 pOH = 2.80 pH = 14.00 − 2.80 pH = 11.20 11.20 Ka and K b K and K are related in this way: a b K ´ K = K K a b w Therefore, if you know one of them, you can an calculate the other. Rea Reactions of Anions with Water • Anions are bases. • As such, they can react with water in a − hydrolysis reaction to form OH and the conjugate acid: − X (aq) + H2O(l) HX(aq) + OH− (aq) Rea Reactions of Cations with Water • Cations with acidic protons (like NH4+) will lower the pH of a solution. • Most metal cations that are hydrated in solution also lower the pH of the solution. Rea Reactions of Cations with Water • Attraction between nonbonding electrons on oxygen and the metal causes a shift of the electron density in water. • This makes the O bond more O­H polar and the water more acidic. • Greater charge and smaller size make a cation more acidic. Effe Effect of Cations and Anions 1. An anion that is the conjugate base of a strong acid will not affect the pH. 2. An anion that is the conjugate base of a weak acid will increase the pH. 3. A cation that is the conjugate acid of a weak base will decrease the pH. Effe Effect of Cations and Anions 4. Cations of the strong Arrhenius bases will not affect the pH. 5. Other metal ions will cause a decrease in pH. 6. When a solution contains both the conjugate base of a weak acid and the conjugate acid of a weak base, the affect on pH depends on the K and K a b values. Fa Factors Affecting Acid Strength • The more polar the H­X bond and/or the weaker the H­X bond, the more acidic the compound. • Acidity increases from left to right across a row and from top to bottom down a group. Fa Factors Affecting Acid Strength In oxyacids, in which an OH is bonded to another atom, Y, the more electronegative Y is, the more acidic the acid. Fa Factors Affecting Acid Strength For a series of oxyacids, acidity increases with the number of oxygens. Fa Factors Affecting Acid Strength Resonance in the conjugate bases of carboxylic acids stabilizes the base and makes the conjugate acid more acidic. Le Lewis Acids • Lewis acids are defined as electron ron­pair acceptors. • Atoms with an empty valence orbital can be Lewis acids. Le Lewis Bases • Lewis bases are defined as electron ron­pair donors. • Anything that could be a Brønsted ed–Lowry base is a Lewis base. • Lewis bases can interact with things other than han protons, however. ...
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This note was uploaded on 04/26/2010 for the course CHEMISTRY 1b taught by Professor Kim during the Spring '10 term at District of Columbia.

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