Chapter 2 - Structure and Reactivity

Chapter 2 - Structure and Reactivity - Chapter 2: Structure...

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Chapter 2: Structure and Reactivity Acids and Bases, Polar and Nonpolar Molecules 2.1 Kinetics and Thermodynamics of Simple Chemical Processes Chemical thermodynamics – energy changes in chemical reactions; controls extent to which a reaction goes to completion Chemical kinetics – velocity or rate at which concentrations of reactants and products change; speed at which a reaction goes to completion Thermodynamic control is a transformation that yields most stable products; outcome is determined by net favorable change in energy in going from reactants to products Kinetic control is a reaction in which product obtained is the one formed fastest Equilibria are governed by the thermodynamics of chemical change o State of equilibrium is when concentrations of reactants and products do not change o Reactions with large K have large driving force o ΔG° = -RTlnK The free energy change is related to changes in bond strengths and the degree of energy dispersal in system o ΔG° = ΔH° - TΔS° o ΔH° is the enthalpy change which is the heat of a reaction at constant pressure, relates to changes in bond strengths in reaction o ΔS° is the entropy change which is the dispersal of energy The rate of a chemical reaction depends on the activation energy Collisions supply the energy to get past the activation energy barrier o Boltzmann distribution curve depicts the distribution of kinetic energy; shape of curve depends on temperature o Reaction proceeds at faster rate if temperature is higher The concentration of reactants can affect reaction rates o Rate of reaction = k [A][B]; where k is the rate constant of the reaction o Reaction is second-order if the rate depends on the concentrations of two molecules o First-order reaction depends on concentration of only one reactant The Arrhenius equation describes how temperature affects reaction rates o Arrhenius equation is k = A e –Ea / RT where A is the maximum rate constant; each reaction has its own characteristic value for A 2.2 Acids and Bases; Electrophiles and Nucleophiles Acid and base strengths are measured by equilibrium constants o Bronsted-Lowry acids are proton donors and bases are proton acceptors o The pK a is the pH at which acid is 50% dissociated; acid with pK a lower than 1 is a strong acid, greater than 4 a weak acid o Strong bases attract protons We can estimate relative acid and base strengths o More electronegative = more polar bond = more acidic o Increasing size of A = strong acid because overlap of larger outer-shell orbital with 1s hydrogen orbital is poor which weakens H-A bond; larger outer-shell also reduces electron-electron repulsion o Resonance in conjugate base – delocalized negative charge = more stable anion = weaker base o Acidity of acids increase to right and down in periodic table
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This note was uploaded on 09/28/2009 for the course CHEM 3A taught by Professor Fretchet during the Spring '08 term at Berkeley.

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Chapter 2 - Structure and Reactivity - Chapter 2: Structure...

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