Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 1 of 52 Philip Dutton University of Windsor, Canada Prentice-Hall 2007 CHEMISTRY Ninth Edition GENERAL Principles and Modern Applications Petrucci Harwood Herring Madura Chapter 16: Acids and Bases Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 2 of 52 Objectives 1.Define Arrhenius, Brnsted-Lowry, and Lewis theories of acids & bases 2.Identify Brsted-Lowry conjugate acids and bases & write equations of acid-base reactions. 3.Identify Lewis acid-base reactions and write equations for acid-base reactions that involve them. 4.Explain what self-ionization (or autoionization) is and describe the nature of the proton in aqueous solution. Focus On Acid Rain Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 3 of 52 Definitions Acids: substances that ionize in water to produce H + ions* Acids have sour taste (i.e., acetic acid in vinegar (CH 3 COOH and citric acid in lemons) React with certain metals (i.e., Zn, Mg, an Fe to produce H 2 (g)) React with carbonates and bicarbonates (i.e., Na 2 CO 3, CaCO 3, and NaHCO 3 ) to produce CO 2 (g) Bases: are substances that ionize in water to produce OH - Have a bitter taste Feel slippery (i.e. soaps) Aqueous base solutions conduct electricity *defined by Svante Arrhenius Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 4 of The Arrhenius Theory: A Brief Review HCl(g) H + (aq) + Cl - (aq) NaOH(s) Na + (aq) + OH - (aq) H2OH2O H2OH2O Na + (aq) + OH - (aq) + H + (aq) + Cl - (aq) H 2 O(l) + Na + (aq) + Cl - (aq) H + (aq) + OH - (aq) H 2 O(l) Arrhenius theory did not handle non OH - bases such as ammonia very well. Limited b/c they only apply to the aqueous solutions Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 5 of Brnsted-Lowry Theory of Acids and Bases NH 3 + H 2 O NH OH - NH OH - NH 3 + H 2 O baseacid baseacid conjugate acid conjugate base ?? Conjugate base : formed by the removal of a proton from the acid A base is a proton acceptor. An acid is a proton donor. Conjugate acid : formed by the addition of a proton to the base. conjugate acid conjugate base Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 6 of 52 Classify each of the following as a Brnsted acid or base: a) HBr b) NO 2 - c) HCO 3 - a)HBr dissolves in water to yield H+ and Br- ions: HBr (aq ) H + (aq) + Br - (aq) Therefore HBr Brnsted acid b) In solution the nitrite ion can accept a proton to form nitrous acid: NO 2 - (aq) + H + (aq) HNO 2 (aq) This makes NO 2 - a Brnsted base. c) The bicarbonate ion is a Brnsted acid b/c it ionizes in a solution: HCO 3 - (aq) H + (aq) + CO 2- 3 (aq) It is also a Brnsted base b/c it can accept a proton. Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 7 of 52 Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 8 of 52 A Weak Base Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 9 of 52 A Weak Acid Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 10 of 52 A Strong Acid Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 11 of The Self-Ionization of Water and the pH Scale Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 12 of 52 Acid-Base Properties of Water H 2 O (l) H + (aq) + OH - (aq) Express this autoionization of water in the Brnsted framework, we write: H 2 O + H 2 OH 3 O + + OH - Acid 1 Base 1 Conjugate Acid Conjugate Base K c = [H + ][OH - ] [H 2 O] Since a very small fraction of water molecules are ionized, the concn of water, doesnt really change. K c = [H 2 O] = K w = [H + ][OH - ] K w is the ion-product constant: which is the molar concentrations of H + & OH - = 1.0 x [H + ] = [OH - ] (neutral solution) Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 13 of 52 Ion Product of Water Kc=Kc= [H 2 O][H 2 O] [H 3 O + ][OH - ] H 2 O + H 2 O H 3 O + + OH - baseacid conjugate acid conjugate base K W = K c [H 2 O][H 2 O] = = 1.0 [H 3 O + ][OH - ] Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 14 of 52 Autoionization of water examples Calculate the concentration of H + (aq) in (a) a solution in which [OH - ] is M ; (b) a solution in which [OH - ] is 2.0 x M. Answer K w = [H + ][OH - ] 1.0 x = [H + ][OH - ] Solve for [H + ] [H + ] = 1.0 x [OH-] = 1.0 x = 1.0 x M (a) Solution is basic b/c [OH - ] > [H + ] (b) [H+] = 5.0 x M Solution is acidic b/c [H + ] > [OH - ] Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 15 of 52 pH and pOH The potential of the hydrogen ion was defined in 1909 as the negative of the logarithm of [H + ]. pH = -log[H 3 O + ] pOH = -log[OH - ] -logK W = -log[H 3 O + ]-log[OH - ]= -log(1.0 ) K W = [H 3 O + ][OH - ]= 1.0 pK W = pH + pOH= -(-14) pK W = pH + pOH = 14 pH = -log[H + ] b/c the H+ concentration in an aqueous is usually quite small Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 16 of 52 pH and pOH Scales Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 17 of Strong Acids and Bases HCl CH 3 CO 2 H Thymol Blue Indicator pH < 1.2 < pH < 2.8 < pH Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 18 of Weak Acids and Bases Lactic AcidGlycine General Chemistry: Chapter 16Prentice-Hall 2007 Acetic Acid Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 19 of 52 Acetic Acid Weak Acids Ka=Ka= = 1.8 [CH 3 CO 2 H] [CH 3 CO 2 - ][H 3 O + ] pK a = -log(1.8 ) = 4.74 General Chemistry: Chapter 16Prentice-Hall 2007 Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 20 of 52 Weak Bases Kb=Kb= = 4.3 [CH 3 NH 2 ] [CH 3 NH 3 + ][HO - ] pK b = -log(4.2 ) = 3.37 Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 21 of 52 Table 16.3 Ionization Constants of Weak Acids and Bases Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 22 of 52 Relationship Among [H+], [OH-], and pH at 25 C Solution type[H + ][OH - ]pH value Acidic> 1.0 x < 1.0 x < 7.00 Neutral= 1.0 x = 7.00 Basic< 1.0 x > 1.0 x > 7.00 Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 23 of 52 Base Ionization Constant, K b NH 3 + H 2 O NH OH - Kc=Kc= [NH 3 ][H 2 O] [NH 4 + ][OH - ] K b = K c [H 2 O] = [NH 3 ] [NH 4 + ][OH - ] = 1.8 baseacid conjugate acid conjugate base K b refers to the equilibrium in which a base reacts with H 2 O to form the conjugate acid and OH -. Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 24 of 52 Acid Ionization Constant, K a CH 3 CO 2 H + H 2 O CH 3 CO H 3 O + Kc=Kc= [CH 3 CO 2 H][H 2 O] [CH 3 CO 2 - ][H 3 O + ] K a = K c [H 2 O] = = 1.8 [CH 3 CO 2 H] [CH 3 CO 2 - ][H 3 O + ] baseacid conjugate acid conjugate base The larger the value of K a, the stronger the acid Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 25 of 52 Determining a Value of K A from the pH of a Solution of a Weak Acid. Butyric acid, HC 4 H 7 O 2 (or CH 3 CH 2 CH 2 CO 2 H) is used to make compounds employed in artificial flavorings and syrups. A M aqueous solution of HC 4 H 7 O 2 is found to have a pH of Determine K A for butyric acid. HC 4 H 7 O 2 + H 2 O C 4 H 7 O 2 + H 3 O + K a = ? EXAMPLE 16-5 Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 26 of 52 HC 4 H 7 O 2 + H 2 O C 4 H 7 O 2 + H 3 O + Initial conc M00 Changes-x M+x M+x M Equilibrium (0.250-x) M x Mx M Concentration EXAMPLE 16-5 Solution: For HC 4 H 7 O 2 K A is likely to be much larger than K W. Therefore assume self-ionization of water is unimportant. Prentice-Hall 2007 General Chemistry: Chapter 16 Slide 27 of 52 Log[H 3 O + ] = -pH = HC 4 H 7 O 2 + H 2 O C 4 H 7 O 2 + H 3 O + [H 3 O + ] = = 1.9 = x [H 3 O + ] [C 4 H 7 O 2 - ] [HC 4 H 7 O 2 ] Ka=Ka= 1.9 1.9 (0.250 1.9 ) = K a = 1.5 Check assumption: K a >> K W. EXAMPLE 16-5