CHEM 162: Gilbert Chapter 17 Part II W2013 page 1 Chapter 17: Equilibrium in the Aqueous Phase (Part II) 17.7 THE COMMON-ION EFFECT Ex. 1. a. Calculate the percent dissociation for a 0.10M acetic acid solution. (Ka=1.8×10-5) b. Calculate the pH of a solution 0.10M acetic acid and 0.10M sodium acetate, then calculate the percent dissociation for the acid under these conditions. b. Compare the percent dissociations for acetic acid in parts a and b. d. Use Le Châtelier’s Principle to explain the difference in percent dissociation for parts a and b above.CHEM 162: Gilbert Chapter 17 Part II W2013 page 2 Thus, the extent of dissociation for an acid will be decreased by the presence of either its conjugate base or H+ in solution—the products in its dissociation equilibrium. – This is known as the common ion effect, in which the equilibrium shifts due to the addition or presence of an ion already involved in an equilibrium reaction. – The common ion effect is also responsible for the first dissociation step inhibiting the subsequent dissociation steps for polyprotic acids. Ex. 2 Compare the percent dissociation for a 0.10M ammonia solution versus the percent dissociation for ammonia when 0.050 mol of ammonium chloride is added to 500.0 mL of a 0.10M ammonia. (Kb=1.8×10-5 for NH3) Acid Rain and Fossil Fuel Combustion Since carbon dioxide (CO2) makes up about 0.039% of the atmosphere, rain is naturally acidic because it reacts with CO2 in the atmosphere: CO2(g) + H2O(l) → H2CO3(aq) However, carbonic acid, H2CO3(aq), is a relatively weak acid, so even rain saturated with CO2 is only slightly acidic with a pH of 5.6.CHEM 162: Gilbert Chapter 17 Part II W2013 page 3 Example: Calculate the initial concentration of carbonic acid that results from carbon dioxide in the atmosphere reacting with rain water to produce rain with a pH of 5.6. Ka=4.3×10-7 for carbonic acid. However, strong acids would have a bigger impact on the pH of rain. About 90% of the energy in the US comes from burning fossil fuels like natural gas and coal. – Since coal contains impurities like sulfur, the combustion of coal results in the production of oxides of these impurities that are then dispersed in the atmosphere. – Consider the combustion of sulfur impurities to produce SO2: S(s) + O2(g) → SO2(g) – In addition, since air is about 78% nitrogen (N2), during any combustion reactions the N2 in air often reacts with oxygen (O2) to produce NO, which reacts with more O2 to produce NO2. N2(g) + O2(g) → 2 NO(g) 2 NO(g) + O2(g) → 2 NO2(g) – Note that NO2 has a brown color, so it is the primary component that gives smog its characteristic color. In the Midwest where there are many coal-burning power plants, the concentrations of SO2, NO, and NO2 build up in the atmosphere, and they can react with water molecules in the atmosphere to produce acid rain: 2 SO2(g) + O2(g) + 2 H2O(l) → 2 H2SO4(aq) 4 NO2(g) + O2(g) + 2 H2O(l) → 4 HNO3(aq) Since the acids produced are strong acids, they can significantly decrease the pH of the resulting rain.CHEM 162: Gilbert Chapter 17 Part II W2013 page 4 Ex. 1: The pH of acid rain can be as low as 4.5. If acid rain were caused solely by sulfur dioxide, calculate the mass of sulfur dioxide that must have reacted with rain water to produce 1.00 L of acid rain that has a pH of 4.5 (1.1 pH points lower than “normal rain”). Ex. 2: If acid rain were caused solely by nitrogen dioxide, calculate the mass of nitrogen dioxide that must have reacted with rain water to produce 1.00 L of acid rain that has a pH of 4.5 (1.1 pH points lower than “normal rain”).CHEM 162: Gilbert Chapter 17 Part II W2013 page 5 Consider the following map of the coal-fired power plants in the US: Then consider the map of rain pH throughout the US:CHEM 162: Gilbert Chapter 17 Part II W2013 page 6 17.10 INDICATORS AND ACID-BASE TITRATIONS Acid-Base Neutralization Reactions Ex. 1: Indicate the products for the following sets of reactants then write the net ionic equation: a. HCl(aq) + NaOH(aq) total ionic: __________________________________________________________________ net ionic: ___________________________________________________________________ b. HNO3(aq) + Ca(OH)2(aq) total ionic: __________________________________________________________________ net ionic: ___________________________________________________________________ c. Classify each of the reactants as strong or weak acids or bases: HCl(aq): __________________ NaOH(aq): __________________ HNO3(aq): __________________ Ca(OH)2(aq): __________________ If Kw = [H+] [OH−] = 1.0×10-14 for the reaction: H2O(l) H+(aq) + OH−(aq), then ===+][OH ][H1K1K-w'w1.0×1014 for the reverse: H+(aq) + OH−(aq) → H2O(l). Ex. 2: What does this equilibrium constant indicate about the extent of the reaction between H+ and OH−—i.e., the reaction between a strong acid and a strong base?CHEM 162: Gilbert Chapter 17 Part II W2013 page 7 Ex. 3: Indicate the products for the following sets of reactants then write the net ionic equation: a. HF(aq) + NaOH(aq) total ionic: __________________________________________________________________ net ionic: ___________________________________________________________________ b. HC2H3O2(aq) + Ca(OH)2(aq) total ionic: __________________________________________________________________ net ionic: ___________________________________________________________________ c. Classify each of the reactants as strong or weak acids or bases: HF(aq): __________________ NaOH(aq): __________________ HC2H3O2(aq): __________________ Ca(OH)2(aq): __________________ d. Consider the following reactions and their corresponding equilibrium constants, and calculate the equilibrium constant for the overall reaction for each: HF(aq) H+(aq) + F−(aq) Ka=7.2×10-4 H+(aq) + OH−(aq) → H2O(l) 'wK =1.0×1014 HCN(aq) H+(aq) + CN−(aq) Ka=4.9×10-10 H+(aq) + OH−(aq) → H2O(l) 'wK =1.0×1014 e. What does these equilibrium constants indicate about the extent of reaction between a weak acid and a strong base?CHEM 162: