Advanced Equilibrium Theory in Chemistry: Understanding Ionic Strength Effects
Explore the adjustments to equilibrium theory in chemistry, focusing on the qualitative effects of ionic strength and real equations for pH calculations. Discover how high concentrations of ions and multiple equilibria can impact results, along with practical demonstrations showcasing the influence of NaCl on MgCO3 solubility.
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Presentation Transcript
Announcements I Quiz 5 (last quiz) 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
Past Demonstration - sorry, not enough time to do this semester Summary of Observation: Two saturated solutions of MgCO3are 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 MgCO3and 6.0 mL needed in 0.1 M NaCl Saturated MgCO3 Saturated MgCO3 in NaCl(aq)
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
Demonstration Slide 3 What was the affect of the NaCl? 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
Ionic Strength Effects Spheres Surrounding Ions Low Ionic Strength High Ionic Strength Ion dipole interaction + CO32- H O CO32- H O H H Na+ Stronger ion ion interaction replaces ion - dipole Mg2+ Mg2+ Cl-
Ionic Strength Definition : = 0.5* CiZi2 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
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 = X[X], where X = activity coefficient The activity coefficient depends on the ionic strength
Effects of Ionic Strength on Equilibria What is the equilibrium equation (including activity coefficients) for the following reaction? Ca2+ + 2C2O42- Ca(C2O4)22-
Determination of Activity Coefficients Use of Debye-H ckel Equation: 2 x . 0 51 Z = log x + x 1 305 - where Zx = ion charge, x = hydrated ion radius (pm) - useful for 0.0001 M < < 0.1 M Can also use Table 7-1 for specific value Calculate (Mg2+) at = 0.050 M (Mg2+) = 800 pm Note: neutral compounds Zx = 0 so log x = 0 and x = 1
Factors Influencing Ionic Strength: as increase, decreases Charge of Ion: a larger decrease in occurs for more highly charged ions Size of Ion: Note: very small ions like Li+ actually have large hydrated spheres ion Gamma Plots 1.20 1.00 Activity coefficient Li+ Rb+ 0.80 Li+ Ba2+ PO43- 0.60 0.40 0.20 0.00 0.00 0.02 0.04 0.06 0.08 0.10 0.12 Ionic Strength (M) Hydrated sphere
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 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( H+[H+]) Example Problem: Determine the pH of a solution containing 0.0050 M Ba(OH)2
