Ochem Ch. 6 Reactivity & Mechanisms - 6.5 KINETICS...

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6.5 KINETICS Spontaneous: If a rxn is - G = spontaneous Rxn is thermodynamically favorable Rxn favors products Has NOTHING to do with speed Rate Equations Rate Equation: rate of any reaction The rate of a rxn is dependent on the rate constant (k) and the concentration of the reactants (k) is specific to each rxn and dependent on a number of factors A rxn = result of a collision btwn reactants Increasing con’c of reactants increases frequency of collisions that lead to a rxn → increases rate of rxn Rate = k[A] x [B] y Exponents must be determined how the rate is affected when the con’c of A & B are each separately doubled First order: [B] is absent -- doubling con’c of [A] had effect of doubling the rate of the rxn… but doubling con’c of [B] has no effect at all. Sum of exponents is 1 Second order: doubling both [A] & [B] = doubles rate Both A & B are present → exponents of each is 1 Sum of exponents is 2 Third order: doubling [A] = quadrupling rate, doubling [B] = doubling rate Exponent of [A] = 2, exponent of [B] = 1 Sum of exponents is 3
Factors Affecting Rate Constant Fast rxn = large rate constant Slow rxn = small rate constant Factors on value of rate constant 1. Energy of Activation Energy barrier btwn reactants & products = energy of activation (E a ) Represents minimum amount of energy required for the rxn to occur btwn 2 reactants that collide If a collision does not have this much energy, they will not react with each other to form products Number of collisions is dependent on the number of molecules that have a certain threshold kinetic energy At any specific temp, reactants will have a specific average kinetic energy (KE) but NOT ALL MOLECULES will possess this avg energy Most molecules have less or more than the average Only a certain number of molecules will have the minimum energy to produce a rxn Number of molecules with this energy will be dependent on the value of E a E a = small → large % will have energy to produce a rxn Low E a = faster rxn 2. Temperature

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