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Understanding Dissolution Reactions and Colligative Properties in Chemical Kinetics

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21 diapositives · 373 Views · Oct 15, 2014

This comprehensive overview discusses the principles of dissolution reactions, particularly focusing on Arrhenius-type acid/base reactions and various colligative properties such as vapor pressure lowering, boiling point elevation, freezing point depression, and osmotic pressure. It delves into the rates of chemical reactions as they transition from reactants to products, exploring rate laws, reaction orders, and the effects of solute concentration on freezing point depression and osmotic pressure. Additional emphasis is placed on the interaction of polar and non-polar liquids in dissolution processes.

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Understanding Dissolution Reactions and Colligative Properties in Chemical Kinetics
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  1. • Dissolution reactions and Arrhenius type Acid/Base rxns • Colligative properties, Vapor Pressure Lowering, Boiling Point Elevation, Freezing Point Depression and Osmotic Pressure, • The Rates of Chemical Reactions A-->B Rate=d[A]/dt= -d[B]/dt. units molL-1s-1 A is the Reactant and B is the product of the reaction • Rate Laws at early times =k[A]n k is the rate constant and n is the reaction order • Elementary reactions: single step reactions Unimolecular(1st), Bimolecular(2nd), Termolecular 20 B Week V Chapters 11 and 18 Colligative Properties and Chemical Kinetics

  2. e In Solutions, for example when NaCl(s) is dissolved in H2O(l). + H2O Dissolution of a polar solid by a polar solid by a polar liquid A non-polar liquid e.g., benzene, would not dissolve NaCl? NaCl(s) + H2O(l) Na+(aq) +Cl-(aq) (aq) means an aqueous solution, where water is the solvent, major component. The solute is NaCl, which is dissolved, minor component Water molecules solvates the ions the Cation (Na+) and the Anion (Cl-). The forces at play here are Ion dipole forces Fig. 10-6, p. 450

  3. Freezing Point Depression Fig. 10-21, p. 463

  4. Freezing Point Depression 0.01°C(triple pt.)

  5. Freezing Point Depression only consider cases where the pure solvent crystalizes from solution, e.g., ice Crystalizes from salt water and NaCl(s) does not DTf = - Kfm ( m=molality of soln) DTf = T’f - Tf Melting point(Tf) lowered to keep the vapor pressure over the pure solid and liquid solution the same at Equilibrium! Fig. 11-12, p. 496

  6. Example : again for 0.058 g of NaCl in 10 g H2O(l) over H2O(s) The molality is m = 0.02 gkg-1 (grams of solute per kg of solvent) DTf = - Kfm ( m=molality of soln) Table 11-2, p. 494

  7. Example : again for 0.058 gmol-1 of NaCl in 10 g H2O(l) over H2O(s) The molality is m = 0.02 gkg-1 (grams of solute per kg of solvent) DTf = - Kfm ( m=molality of soln) DTf = - (1.86)x(0.02) K= - 0.0372 °C T’f = 0 +(-0.037)°C=-0.037°C Table 11-2, p. 494

  8. Freezing Point Depression only consider cases where the pure solvent crystalizes from solution, e.g., ice Crystalizes from salt water and NaCl(s) does not DTf = - Kfm ( m=molality of soln) DTf = T’f - Tf Melting point(Tf) lowered to keep the vapor pressure over the pure solid and liquid solution the same at Equilibrium! DP1= - X2P°1 S= -DP1/DTf (pure solvent) Solved for DTf DP1 DTf = 0.037 °C = 0.037 K Fig. 11-12, p. 496

  9. Freezing point depression (DTb) v.s. Molality of the solution M(s)M(aq) } Closer to ideal soln MX(s)M(aq) +X(aq) MX2(s)M(aq) +2X(aq)x MX3(s)M(aq) +3X(aq)x Fig. 11-13, p. 497

  10. Salt solns Osmotic Pressure π Osmotic pressure forces water out of a carrot placed I n a salt soln Water doesn’t leave a carrot placed in pure water p. 499

  11. Measuring Pressure Hg Barometer mm

  12. Pressure= Force/unit area F=mg=Vg= hAg =mass/V Mass Density kg/m3 or kg/cm3 mm FHg Fair FHg = Fair Pair =Fair/A=FHg/A= hAg/A= hg: Pair= hg

  13. Osmotic Pressure π=[solute]RT Solute molar concentration Recall that pressure in the tube P=rghso π=rghVan’t Hoff proposed π=[solute]RT Which is similar to PV=nRT for an ideal gas. Note that is the solute Concentration but the Solvent mass density r(kg meter-3)! At Equilibrium the rate of The Solvent molecules Crossing the membrane from solution Is equal to the rate from the solvent Solute molecules Lowers the rate of Solvent molecules Crossing the Membrane From the solution Fig. 11-14, p. 498

  14. Kinetics of Approaching Equilibrium Equilibrium: Evaporation rate equal the condensation Rate  Equilibrium C2H4(l)  C2H4(g) Rate Law Rate=k1=const, , Order zero • C2H4(l)  C2H4(g) Evap • C2H4(g)  C2H4(l) Cond Rate Law Rate=k2[C2H4(g)] The order n =1 Fig. 10-16, p. 459

  15. B + surface(wall) Collision Rate Ch 9.7) Zwall= {(1/4)<u>A} [B] Average over incident velocities <v>=(1/4)<u> A = surface area B + surface condensation Rate Rate= --d[B]/dt ~ Zwall x (probability of sticking to the surface) --d[B]/dt ~{(1/4)<u>A} [B] exp{-Ea/kT} - d[B]/dt = k[B] k~{(1/4)<u>A} exp{-Ea/kT} Rate constant (molecule/surface property) Ea the activation energy

  16. A + B Collision Rate ZAB=[A][B]{(√2)<u>(πd2)}(N0)2 A + B Reaction Rate Reaction Rate= -d[A]/dt=-d[B]/dt ~ ZAB p exp{-Ea/kT} -d[A]/dt=-d[B]/dt =[A][B]{(√2)<u>(πd2)}(N0)p exp{-Ea/kT} -d[A]/dt=-d[B]/dt = k[A][B]

  17. The Rate Constant k k={(√2)<u>(πd2)}(N0)p exp{-Ea/kT} k = (collision rate {cross-section for a πd2N0} ) (p= fraction with correct orientation) (fraction with energy ≥Ea ) k=A exp{-Ea/kT} The Arrhenius Form and the A-factor A= {(√2)<u>(πd2)}(N0)p For Elementary Bimolecular Reactions only

  18. Ea:: the Activation Energy for a Reaction A-----B-----C AB + C A + BC Fig. 18-11, p. 858

  19. Boltzmann Factor

  20. Collision rate for one molecule moving throw a gas Z1 = {(√2)<u>(πd2) Fig. 9-20, p. 426