ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 12

1. ANALYTICAL CHEMISTRY CHEM 3811CHAPTER 12 DR. AUGUSTINE OFORI AGYEMAN Assistant professor of chemistry Department of natural sciences Clayton state university

2. CHAPTER 12 CHEMICAL EQUILIBRIUM CALCULATIONS

3. SOLUBILITY - A measure of how much of a solute can be dissolved in a solvent - Units: grams/100 mL Three factors that affect solubility - Temperature - Pressure - Polarity

4. SOLUBILITY OF SALTS - Most nitrate (NO3-) salts are soluble - Most salts of alkali metals (Group 1A) and ammonium (NH4+) are soluble - Most salts containing Cl-, Br-, and I- soluble Exceptions: salts of Ag+, Hg22+, Pb2+

5. SOLUBILITY OF SALTS - Most sulfate salts are soluble Exceptions: BaSO4, PbSO4, Hg2SO4 - Most hydroxides are slightly soluble Hydroxides of Ba2+, Sr2+, and Ca2+ are marginally soluble - Most salts containing S2-, CO32-, PO43-, CrO42- are insoluble Exceptions: salts of alkali metals and NH4+

6. SOLUBILITY OF SALTS - Solubility increases when soluble salts are added to solutions of marginally soluble salts - Cations are surrounded by anions to create a net negative ionic atmosphere - Anions are surrounded by cations to create a net positive ionic atmosphere - The net charges are less than those of the cation or anion alone

7. SOLUBILITY OF SALTS - The attraction between ions in solution is decreased which increases solubility - Increasing the concentration of ions in solution decreases the attraction between ions and increases solubility - Increasing concentration of ions increases ion dissociation

8. IONIC STRENGTH - A measure of the total concentration of ions in solution µ = the ionic strength ci = the concentration of the ith species zi = the charge on the ith species

9. IONIC STRENGTH Find the ionic strength of 0.0250 M Na2SO4 Na2SO4↔ 2Na+ + SO42- [Na+] = 2 x 0.0250 M = 0.0500 M [SO42-] = 0.0250 M

10. IONIC STRENGTH For 1:1 electrolytes (NaCl, NaNO3, KBr) The ionic strength is equal to the molarity 1:1 µ = molarity For any other stoichiometry The ionic strength is greater than the molarity 2:1 µ = 3 x molarity 3:1 µ = 6 x molarity 2:2 µ = 4 x molarity

11. ACTIVITY COEFFICIENT Consider the equilibrium for the reaction aA + bB ↔ cC + dD The equilibrium constant (K) is given by K does not account for the effect of ionic strength

12. ACTIVITY COEFFICIENT - Activities (A) are used in place of concentrations to account for ionic strength A = [ ] x γ where γ is the activity coefficient - Activity coefficient depends on ionic strength - Activity coefficient is 1 when there is no effect of ionic strength - Activity coefficient decreases with increasing ionic strength

13. ACTIVITY COEFFICIENT - K is generally expressed as follows

14. ACTIVITY COEFFICIENT Debye-Hückel Equation - Relates activity coefficients to ionic strength (at 25 oC) γ = activity coefficient z = ion charge (±) α = ion size in picometers (1 pm = 10-12 m) µ = ionic strength

15. ACTIVITY COEFFICIENT Effects (limited to dilute aqueous solutions) - Activity coefficient increases with decreasing ionic strength (approaches unity as ionic strength approaches zero) - Activity coefficient depends on the magnitude of the charge but not on the sign (departs from unity as charge increases) - Effect of activity on ions increases with decreasing ion size

16. ACTIVITY COEFFICIENT Neutral Molecules - Activity coefficient is assumed unity (no charge and no ionic atmosphere) - Activity is assumed to be equal to its concentration

17. ACTIVITY COEFFICIENT Gases Activity (called fugacity) is written as Agas = Pgas x γgas P = pressure in bars γgas = fugacity coefficient of a gas For most gases at or below 1 bar γgas≈ 1

18. ACTIVITY COEFFICIENT pH = negative logarithm of the hydrogen ion activity pH electrodes measure activity of hydrogen ions - Ionic strength of pure water is very low - Activity coefficient of pure water is very close to unity

19. CHARGE BALANCE - In a given solution sum of positive charges = sum of negative charges - The coefficient of each term equals the magnitude of the charge on the respective ion - 1 mole of an ion An+/n- contributes n moles of positive/negative charge

20. CHARGE BALANCE n1[C1] + n2[C2] + ….. = m1[A1] + m2[A2] +….. [C] = concentration of a cation n = magnitude of the charge on the cation [A] = concentration of an anion m = magnitude of the charge on the anion - Activity coefficient do not appear in charge balance

21. CHARGE BALANCE Consider a solution containing the following species Na+, CO32-, HCO3-, H+, Ca2+, OH-, PO43-, HPO42- total positive charge = total negative charge [Na+] + [H+] + 2[Ca2+] = 2[CO32-] + [HCO3-] + [OH-] + 3[PO43-] + 2[HPO42-]

22. MASS BALANCE - Also called the material balance - Conservation of matter the quantity of a particular atom (or group of atoms) equals the amount of that atom (or group of atoms) delivered - Mass balance includes all products of compounds that dissociate in several ways

23. MASS BALANCE Consider 0.0200 mol of H3AsO4 in 1.00 L of solution 0.0200 M = [H3AsO4] + [H2AsO4-] + [HAsO42-] + [AsO43-] For KH2AsO4 in water [K+] = [H3AsO4] + [H2AsO4-] + [HAsO42-] + [AsO43-] For K2HAsO4 in water [K+] = 2 x {[H3AsO4] + [H2AsO4-] + [HAsO42-] + [AsO43-]} For K3AsO4 in water [K+] = 3 x {[H3AsO4] + [H2AsO4-] + [HAsO42-] + [AsO43-]}

24. FRACTIONAL COMPOSITION F = initial concentration of acid HA (formal concentration) Fraction of species in the form HA Fraction of species in the form A-