# KINETICS -REACTION RATES

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KINETICS -REACTION RATES
AP Chem

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

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

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

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

Determining Reaction Order by Method of Initial Rates
Example:

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

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

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

Second Order 1/[A]t = kt + 1/[A]0 1/[A]t – 1/[A]0 = kt t1/2 = 1/k[A]0

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)

Zero Order rate is constant; doesn’t change with concentration
plot of [A] vs. time = linear Rate=k -k=slope

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

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