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Rtlnk Kinetics and Thermodynamics of Chemical Reactions Kinetics and Thermodynamics of Chemical Reactions Thermodynamic control outcome is determined by the most thermodynamically stable products Kinetic control: outcome is determined by the fastest route (product may not be the most thermodynamically stable) ΔG°= ΔH°-TΔS° ΔG°= -RTlnK Spontaneous at S.S. if negative Equilibrium constant changes Change in amount of order exponentially with ΔG° Bond breakage and formation (more disorder is positive) Magnitude depends on temp Notice: +ΔG°= favors reactants -ΔG°= favors products Kinetics and Thermodynamics of Chemical Reactions Rate=k[A][B] Rate constant depends on activation energy and temperature k= Ae(-Ea/RT) Figure 2-2 Maxwell-Boltzmann curves at two temperatures. At the higher temperature (green curve), there are more molecules of kinetic energy E than at the lower Figure 2-1 temperature (blue curve). Molecules with higher kinetic A (highly oversimplified) potential-energy diagram for the combustion reaction of energy can more easily overcome the activation-energy methane. Despite its thermodynamic favorability, as shown by the large negative barrier. ΔH°, the process is very slow because it has a high-energy transition state and a large activation energy. (In actuality, the process has many individual bond- breaking and bond-forming steps, and the applicable potential-energy diagram therefore has multiple maxima and minima.) Acids and Bases Arrhenius acid and base: Bronsted-Lowry acid and base: + − + − HCl → H + Cl HCl + H2O → H3O + Cl + − + − NaOH → Na + OH H2O + NH3 → NH4 + OH Lewis acid: electron pair acceptor Lewis base: electron pair donor What are the conjugate bases of the following? What are the conjugate acids of the following? - + 2- H2S, HSO4 , CH3CH3OH2 S , CH3(CO)CH3, CF3CH3O- For an acid in water: [H O ][A ] K = 3 [HA][H2O] and: Ka = K[H2O] For water: Ka = K[H2O] [H3O ][OH ] Ka = 2 •[H2O] [H2O] [H ][OH ] Ka = [H2O] 1x1014 K = = 1.8x10-16 a [55.55] What is the significance of the pKa? Strength of an Acid (HA) 1) Electronegativity 2) Size of A (bond length, bond strength, stablized A−) Increasing electronegativity of A Increasing size of A H4C < H3N < H2O < HF HF < HCl < HBr < HI Increasing acidity of HA Increasing acidity of HA 3) Stabilization of A− through resonance acetic acid is a stronger acid than methanol because acetate is stabilized by resonance Even more pronounced in sulfuric acid Amphoterism nitric acid acting as an acid nitric acid acting as a base acetic acid acting as a base Why does protonation occur on carbonyl carbon? Movement of Electrons 1) Dissociation of polar covalent bond into ions (B more electronegative than A) 2) Formation of a polar covalent bond from ions 3) Simultaneous formation and dissociation of two covalent bonds (bond substitution) (A-B bond is polarized) 3) Addition to a multiple bond addition of e− pair donor addition of e− pair acceptor Electrophiles and Nucleophiles Lewis acids Lewis bases base acid nucleophile electrophile (Dispersion) Figure 2-6 (A) Coulombic attraction in an ionic compound: crystalline sodium acetate, the sodium salt of acetic acid. (B) Dipole–dipole interactions in solid chloromethane. The polar molecules arrange to allow for favorable coulombic attraction. (C) London (dispersion) forces in crystalline pentane. In this simplified picture, the electron clouds as a whole mutually interact to produce very small partial charges of opposite sign. The charge distributions in the two molecules change continually as the electrons continue to correlate their movements. .
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