Review Initially pure water at 25oC ISOLATED from the atmosphere comes into contact with solid CaCO3 For the following pairs of species in solution determine at equilibrium which has the higher molar concentration mol L Explain your reasoning Choose A or B A Ca2 or B CO322 CaCO3 Ca CO3 Example Ca 10 3 3 A Once Ca2 dissolves from CaCO3 it will remain in solution as Ca2 On the other hand CO32 will distributed itself amount other carbonate species Ca2 will be higher CaCO3 Ca 2 CO32 CO2 aq H 2O H 2CO3 H HCO3 2 HCO3 H CO32 2 Solution M CO3 10 M 2 HCO3 H CO32 5 63 Review Initially pure water at 25oC ISOLATED from the atmosphere comes into contact with solid CaCO3 For the following pairs of species in solution determine at equilibrium which has the higher molar concentration mol L Explain your reasoning Choose A or B A H or B OH Example Ca 2 10 3 3 M CO32 10 5 M 64 Solution B Because CO32 will take up H OH will be higher i e pH 7 HCO3 H CO32 HCO3 H CO32 65 Now we know that pH 7 For the following pairs of species in solution determine at equilibrium which has the higher molar concentration mol L Choose A or B A HCO3 or B CO2 aq 2 CO2aq A Because OH is higher pH is above 6 3 pKa so formation of HCO3 is more favorable than CO2aq 2 H2CO3 3 CO32 HCO3 CO32 4 log C 3 HCO3 66 4 log C 5 5 6 6 7 7 pKa1 6 35 8 8 9 3 4 5 6 pKa1 6 35 pKa2 10 33 7 9 10 11 12 13 14 8 9 3 4 5 6 7 pKa1 10 33 8 9 10 11 12 13 14 pH pH 67 68 1 Review Select the WRONG electroneutrality equation A B C D E Review Select the WRONG statement H Na OH Cl H OH HCO3 2 CO32 H 2 Ca2 OH Cl H OH H OH HCO3 CO32 A When CO2 is released to the air above an open water body CO2 distributes itself among air and water phase B Concentrations of CO2 in water and in air is related by the following expression CO2aq KHPCO2 C When CO2 is released to the air above an open water body CO2 stays in the air Answer E 69 Review Select the CORRECT equilibrium equation for CO2 aq in water Answer C 70 Reaction Kinetics Are Expressed by Rate Equations A CO2 aq H 2O H HCO3 Generic form A B H HCO3 K 1 CO aq 2 B reaction rate rA CO2 aq H 2O H HCO3 H CO2 aq K 1 HCO3 dC A dt C HCO3 H CO32 H HCO3 K2 2 CO3 Answer A 71 Consider a zero order reaction A B reaction rate rA Unit of the reaction rate constant k for zero order reaction is dCA kC A0 k dt A Concentration unit B Concentration unit divided by time C Concentration unit multiplied by time D None of the above dCA kdt CA CA 0 72 t dC A kdt 0 CA C A C A0 kt C A C Ao kt time Intercept CA 0 Slope k reaction rate constant 73 74 2 Consider a first order reaction A B reaction rate rA Unit of the reaction rate constant k for first order reaction is dCA kC A1 kC A dt A Concentration unit B One over time C Time D None of the above dCA kCAdt CA CA 0 lnCA t dC A kdt C A0 0 ln C A C A0 kt C A C Ao e kt Intercept lnCA 0 Slope k reaction rate constant time 75 Examples of first order reactions of environmental interest Dissociation of hydrogen sulfide in solution H2S H HS Dissociation of bicarbonate into CO2 and OHHCO3 CO2 OH Hydrolysis of alum Al H2O 63 Al H2O 5OH H Inactivation of pathogens 76 In a bioreactor it is observed that the zero order rate constant for a pollutant removal k was 0 1 g L min 1 What water retention time would be required to reduce the concentration from 1 mg L in the influent to half of its initial value in the effluent of a reactor Assume the reactor is completely mixed A t 500 min B t 5000 min C t 5 sec D t 50 min 77 C A C A 0 kt kt C A 0 C A 0 5C A 0 half life t1 2 0 5C A 0 k 0 5 1mg L 1000 g mg 5000 min 0 1 g L min 78 In a bioreactor it is observed that the zero order rate constant for a pollutant removal k was 0 1 g L min 2 What water retention time would be required to reduce the concentration from 1 mg L in the influent to 1 g L in the effluent of a reactor Assume the reactor is completely mixed A t 9990min B t 9 min C t 9 sec D None of the above 79 80 3 Example 2 Pure water is carbonated by means of bubbling CO2 gas through it Then it is bottled If the air space in the bottle above the water contains only CO2 at a pressure of 2 atm what is the equilibrium pH of the carbonated water C A C A 0 kt t C A 0 C A k 1000 g L 1 g L 0 1 g L min a 5 6 b 3 8 c 6 5 d 7 5 e 10 t 9990 min 7 days 81 List species are present in the aqueous phase H HCO3 CO3 2 OH CO2aq We have the following equations 1 Henry s law CO2aq KHP CO2 0 0334 Matm1 2 atm 0 0668 M 2 Electroneutrality H OH HCO3 2 CO32 Each CO32 has 2 charges its charge contribution is twice as much per mole Assume that water should not be too basic so from the diagram carbonate should be much less than bicarbonate CO32 HCO3 The electroneutrality can be shorten to H OH HCO3 82 3 Water dissociation H OH Kw 10 14 You can express OH Kw H and electroneutrality becomes 10 14 H HCO3 H 83 84 4 Carbonic acid dissociation CO2 aq H 2O H 2CO3 H HCO3 The soda bottled described in the previous exercise is opened the water is poured into a glass Assume that the surrounding atmosphere is unpolluted and T 25 C What is the new equilibrium pH What changes takes place in the water during the approach to equilibrium H HCO3 K1 4 47 10 7 mol L CO2 aq HCO3 HCO3 K1 CO2 aq H K1 CO2 aq H H H K1 CO2 aq 4 47 10 M 0 0668M 3 0 10 2 7 8 H 1 7 10 4 or pH 3 8 You will need to check the assumption that pH 10 3 85 Yes pH 3 8 A 5 6 B 3 …