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**KINETICS -REACTION RATES**

AP Chem

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**RATE Dquantity / Dtime 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/mol•s, L2/mol2•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 t1/2 = [A]0 / 2k**

[A]0 = initial concentration [A]t = concentration at time t Half life – time required to reach half of original concentration t1/2 = [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 t1/2 = 0.693/k

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Second Order 1/[A]t = kt + 1/[A]0 1/[A]t – 1/[A]0 = kt t1/2 = 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; doesn’t 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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