General Chemistry Review: Chemical Equilibrium, Ionization | BIOC 460, Study notes of Biology

Download General Chemistry Review: Chemical Equilibrium, Ionization | BIOC 460 and more Study notes Biology in PDF only on Docsity! Bioc460 General Chemistry Review (2008) p. 1 BIOC 460 General Chemistry Review: Chemical Equilibrium, Ionization of H2O, pH, pKa General Equilibrium: What are the UNITS of Keq? Example reactions: A --> B units of Keq ? _________ A --> B + C units of Keq ? _________ A + B --> C units of Keq ? _________ A + B --> C + D units of Keq ? _________ 2A --> B units of Keq ? _________ 2A + B --> C + D units of Keq ? _________ Units of Keq depend on the reaction! Dissociation of H2O: Bioc460 General Chemistry Review (2008) p. 2 Table 2-6, from Nelson & Cox: Lehninger Principles of Biochemistry, 4th edition (2004) NOTE: •If pH > 7, [H+] < 10–7 M (solution is "basic") •If pH < 7, [H+] > 10–7 M (solution is "acidic") •If pH = 7, [H+] = 10–7 M (solution is "neutral") •pH scale is logarithmic: •If change in pH (ΔpH) = 1, Δ[H+] = 101 (10-fold). •If change in pH (ΔpH) = 2, Δ[H+] = 102 (100-fold). Bioc460 General Chemistry Review (2008) p. 5 Quantitative relationship between pKa and pH: Look again at the equilibrium dissociation reaction and remember LeChatelier's Principle: HA H+ + A– The higher the [H+] (i.e., the lower the pH), the more the equilibrium shifts to the left, so the more conjugate acid will be present. Conversely, the lower the [H+] (i.e., the higher the pH), the more the equilibrium shifts to the right, so the more conjugate base will be present. The exact ratio of base to acid depends on the pH and the pKa (Henderson-Hasselbalch Equation). Note that when pH = pKa, [A –] = [HA], so [base] /[ acid] ratio = 1/1 ([base] = [acid]). When pH = pKa, the “buffering capacity” of the mixture will be the greatest. That is, a given change in concentration of base (OH–) or acid (H+) will result in the smallest change in pH. If the pH is ABOVE the pKa, there will be more base than acid in the solution, i.e., the [base] /[ acid] ratio will be greater than 1/1. If the pH is BELOW the pKa, there will be more acid than base, i.e., the [base] /[ acid] ratio will be less than 1/1. When you do calculations, always stop at the end to think about whether your answer makes sense. Work the posted practice problems at the end of this review, on pH, pKa, and [base]/[acid] ratio, and learn how to convert a [base]/[acid] ratio to the FRACTION of the total ([base] + [acid]) that's in the form of the conjugate base (or the conjugate acid). Bioc460 General Chemistry Review (2008) p. 6 Titration Curves: •add measured volume of solution of strong base (NaOH) of known concentration •measure the pH, e.g. with a pH meter •can be used to determine °amount of acid in the solution (from mols of OH – required to completely deprotonate each acidic group), which permits conversion of volume of standard OH – added (which was measured) to equivalents OH – added (mols OH –/mol acidic group) °pKa values of acidic groups (from pH at which an acidic group is exactly half titrated) •example: titration of acetic acid, CH3-COOH °involves 2 reversible equilibria: Fig. 2-17: Titration curve of acetic acid (from Nelson & Cox: Lehninger Principles of Biochemistry, 4th edition (2004). When no OH– has been added, predominant form is HAc; at the endpoint, when 1 equivalent OH– has been added, all the HAc has been converted to Ac –. When exactly 0.5 equivalent of OH– has been added, exactly half the original HAc has dissociated, so [Ac –] = [HAc] , i.e., [base] = [acid]), and pH = pKa. Shaded zone shows the pH range of useful buffering power, from about 1 pH unit below to about 1 pH unit above the pKa, i.e. between about 10% and 90% titration of the weak acid. Fig. 2-18: Titration curves of acetic acid, H2PO4–, and ammonium ion (from Nelson & Cox: Lehninger Principles of Biochemistry, 4th edition (2004). Bioc460 General Chemistry Review (2008) p. 7 Fig. 2.13 from Garrett & Grisham, Biochemistry, 3rd ed., 2004. For a polyprotic acid (more than one acidic group, so more than one pKa on titration curve), it takes 1 equivalent of OH– to titrate each acidic group, so a triprotic acid like phosphoric acid requires 3 equivalents. The pKa values are pH values at each half equivalent added. Summary: 1) H2O and acids in aqueous solution dissociate to yield protons (H+), which in actuality would be hydrated to form H3O+. 2) The tendency of an acid to donate its proton to H2O (dissociate the proton) can be quantitatively described by its dissociation equilibrium constant Ka (or its pKa, which = –logKa). 3) pKa values can be very accurately measured by titration curves, as the pH at half equivalence points. 4) Relationship between pH, pKa, and ratio of [base]/[acid] can be described by the Henderson- Hasselbalch Equation: Buffers: •homeostasis: the maintenance of constant conditions in internal environment •Fluids in living systems have a characteristic, almost constant, pH. •How is pH controlled in living systems? By buffer systems. Buffer: aqueous system that tends to resist changes in pH when small amounts of acid or base are added. Buffer system: aqueous solution of a weak acid and its conjugate base Buffer range of a weak acid: near its pKa, about ±1 pH unit from the pKa. Maximum buffering capacity is at the pKa. Look again at the equilibrium dissociation reaction and remember LeChatelier's Principle: HA H+ + A–