1. Chem. 31 – 11/15 Lecture

2. 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

3. 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

4. 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

5. 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 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

6. 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 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) = mmol HCl
Calculate V(HCl) = mmol HCl/[HCl]
= mmol HCl/0.002 mmol/mL = 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

7. 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

8. 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

9. 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

10. 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

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

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

13. 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

14. 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+

15. 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

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