analytical chemistry chem 3811 chapter 12 n.
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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 12

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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 12

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ANALYTICAL CHEMISTRY CHEM 3811 CHAPTER 12

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  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-