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KINETICS - REACTION RATES AP Chem

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RATE quantity / time mol / L s Average rate of product produced = negative average rate of reactant consumed Rate of a Rxn is always Positive Reaction rates are determined experimentally by measuring concentrations at various times

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Rate Laws Equation that expresses the mathematical relationship between the rate of a rxn and the concentration of reactants Include the specific rate constant k k = numerical value that relates rxn rate & concentration of reactants at a given temp

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Specific Rate Constant (k) k is unique for every rxn k will NOT change with concentration k will change with temperature A large k value means that the reactants rapidly form products Units can vary depending on situation ie: L/mols, L 2 /mol 2 s, 1/s

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General Form of Rate Laws General Reaction: aA + bB products General Rate Law: Rate = k [A] m [B] n ( m & n = reactions orders for A and B) Reaction Order defines how the rxn rate is affected by concentration of reactants

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Determining Reaction Order by Method of Initial Rates Example:

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Integrated Rate Laws Shows how concentration depends on time Used for aA products Three types of Integrated Rate Laws: Zero Order First Order Second Order

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Zero Order [A] t = -kt + [A] 0 [A] 0 = initial concentration [A] t = concentration at time t Half life – time required to reach half of original concentration t 1/2 = [A] 0 / 2k

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First Order ln[A] t = -kt + ln[A] 0 ln[A] t – ln[A] 0 = -kt ln([A] t /[A] 0 ) = -kt t 1/2 = 0.693/k

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Second Order 1/[A] t = kt + 1/[A] 0 1/[A] t – 1/[A] 0 = kt t 1/2 = 1/k[A] 0

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Determining Integrated Rate Law Order Graphically You can determine the rxn order if given [A] and time by making a graph. Plot: [A] vs. time ln[A] vs. time 1/[A] vs. time (see which graph is linear)

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Zero Order rate is constant; doesnt change with concentration plot of [A] vs. time = linear Rate=k -k=slope

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First Order Concentration and rate are directly proportional Plot of ln[A] vs. time = linear Rate = k[A] -k=slope

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Second Order If [reactants] doubles, rate quadruples Plot of 1/[A] vs. time = linear Rate = k[A] 2 k=slope

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