Chem. 31 – 11/15 Lecture

Announcements I Quiz 5 (last quiz) Next Labs Due Today’s Lecture next Monday Next Labs Due Formal Part A 11/22 IC Part 1 11/22 Today’s Lecture Chapter 8 – Advanced Equilibrium Overview Ionic Strength Replacement equation for equilibrium Calculation of activity coefficients

Announcements II Today’s Lecture Chapter 8 – Advanced Equilibrium – cont. Qualitative effects of ionic strength Equilibrium problems – modified to account for activity Real equation for pH and use in calculations

Chapter 8 “Adjustments” to Equilibrium Theory There are two areas where the general chemistry equilibrium theory can give wrong results: When the solution has high concentrations of ions When multiple, interacting equilibria occur I used to do a demonstration, but skipping this semester 4

Past Demonstration - sorry, not enough time to do this semester Summary of Observation: Two saturated solutions of MgCO3 are prepared. One is prepared in water and the other is prepared in ~0.1 M NaCl. 5.0 mL of each solution was transferred (and filtered) into a beaker. Each solution was titrated with 0.002 M HCl 3.5 mL needed for saturated MgCO3 and 6.0 mL needed in 0.1 M NaCl SaturatedMgCO3 SaturatedMgCO3 in NaCl(aq) 5

Demonstration – Slide 2 Did the moles of HCl used match expectations? and Why did the solution containing NaCl need more HCl? First Question: How many mL of HCl were expected? MgCO3(s)  Mg2+ + CO32- Ksp = 3.5 x 10-8 T = 25°C Ksp = 3.5 x 10-8 = [Mg2+][CO32-] since [Mg2+] = [CO32-] (assuming no other reactions), [CO32-] = (3.5 x 10-8)0.5 = 1.87 x 10-4 M n(HCl) = (2 mol HCl/mol CO32-)(1.87 x 10-4 mmol/mL)(5.0 mL) = 0.001875 mmol HCl Calculate V(HCl) = 0.001875 mmol HCl/[HCl] = 0.001875 mmol HCl/0.002 mmol/mL = 0.935 mL Actual V(HCl) > 1 mL Conclusions It takes more HCl than expected, so more CO32- dissolved than expected. Also, the NaCl increased the solubility of MgCO3 6

Demonstration – Slide 3 What was the affect of the NaCl? Why? More CO32- (and Mg2+) was found to dissolve in the 0.10 M NaCl Why? The Na+ and Cl- ions stabilize CO32- and Mg2+ ions 7

Ionic Strength Effects Spheres Surrounding Ions Low Ionic Strength High Ionic Strength H O Ion – dipole interaction H O d+ CO32- H O CO32- H O H O Na+ H O Stronger ion – ion interaction replaces ion - dipole H O Mg2+ H O Mg2+ H O d- H O Cl- 8

Ionic Strength Definition : m = 0.5*SCiZi2 where i is an ion of charge Z and molar concentration C. But What is Ionic Strength A measure which allows us to correct for ion – ion effects Examples: 0.10 M NaCl 0.010 M MgCl2 0.010 M Ce(SO4)2 9

Effects of Ionic Strength on Equilibria Equilibrium Equation Learned Previously: for reaction A ↔ B, K = [B]/[A] Replacement Equation: K = AB/AA So what is AX? AX is the activity of X AX = gX[X], where gX = activity coefficient The activity coefficient depends on the ionic strength 10

Effects of Ionic Strength on Equilibria What is the equilibrium equation (including activity coefficients) for the following reaction? Ca2+ + 2C2O42- ↔ Ca(C2O4)22- 11

Determination of Activity Coefficients Use of Debye-Hückel Equation: - where Zx = ion charge, ax = hydrated ion radius (pm) - useful for 0.0001 M < m < 0.1 M Can also use Table 7-1 for specific m value Calculate g(Mg2+) at m = 0.050 M a(Mg2+) = 800 pm Note: neutral compounds – Zx = 0 so loggx = 0 and gx = 1 12

Factors Influencing g Ionic Strength: as m increase, g decreases Charge of Ion: a larger decrease in g occurs for more highly charged ions Size of Ion: Note: very small ions like Li+ actually have large hydrated spheres ion Li+ Rb+ Hydrated sphere 13

Ionic Strength Effects on Equilibria Qualitative Effects An increase in ionic strength shifts equilibria to the side with more ions or more highly charged ions Example Problems: (predict the shift as m increases) NH3(aq) + H2O(l) ↔ NH4+ + OH- Cu2+ + 4OH- ↔ Cu(OH)42- 2HSO3- ↔ S2O32- + H2O(l) HSO4- ↔ SO42- + H+

Ionic Strength Effects Effects on Equilibrium - Quantitative Calculate expected [Mg2+] in equilibrium with solid MgCO3 for cases both with and without NaCl. Go to Board

Ionic Strength Effects Real Equation for pH pH = -logAH+ = -log(gH+[H+]) Example Problem: Determine the pH of a solution containing 0.0050 M Ba(OH)2