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• 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 -1 s -1
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• 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
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
Freezing Point Depression Fig. 10-21, p. 463
Freezing Point Depression 0.01°C(triple pt.)
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
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
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
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
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
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
Measuring Pressure Hg Barometer mm
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
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
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
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
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]
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
Ea:: the Activation Energy for a Reaction A-----B-----C AB + C A + BC Fig. 18-11, p. 858
Collision rate for one molecule moving throw a gas Z1 = {(√2)<u>(πd2) Fig. 9-20, p. 426