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CHM101H1 Lecture Notes - Reaction Mechanism, Endothermic Process, Arrhenius Equation

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Chapter 14- Chemical Kinetics
Nerosanth Selvarajah
14.1- The Rate of a Chemical Reaction
The rate of a reaction describes how fast the concentration of a reactant or product changes with time
A + B C + D
Rate of Formation = ∆[C]/ ∆t Rate of Disappearance = -∆[A]/t
Rate of disappearance is a negative quantity because concentration decreases with time
oThe concentration at the end of a time period is less than t was at the start of the period
14.2 Measuring Reaction Rates
To determine rate of reaction, we need to measure changes in concentration over time
Reaction rate is not constant; the lower the remaining concentration of the reactant, the more slowly the
reaction proceeds
Instantaneous Rate of Reaction- is the exact rate of a reaction at some precise point in the reaction. It is
obtained from the slope of a tangent line to a concentration-time graph
Initial Rate of Reaction- is the rate of a reaction immediately after the reactants are brought together
14.3 Effect of Concentration on Reaction Rates: The Rate Law
Rate Law/ Rate Equation-for a reaction relates the reaction rate to the concentrations of the
Rate= k[A]m[B]n [Pg. 578]
Term order is related to the exponents in the rate law
The overall order of reaction is the sum of all the exponents: m + n + …
Rate constant (k)- is the proportionality constant in a rate law that permits the rate of a reaction to be
related to the concentrations of the reactants
oits values depend on the specific reaction, presence of a catalyst and temperature
othe larger the value of k, the faster a reaction goes
order of the reaction establishes the general form of the rate law & the appropriate units of k
if reaction is first order in one of the reactants, doubling the initial concentration of that reactant causes the
initial rate of reaction to double
ozero order in reactant- no effect on initial rate of reaction
ofirst order in reactant- initial rate of reaction doubles
osecond order in reactant- initial rate of reaction quadruples
othird order in reactant- initial rate of reaction increases eightfold
order of reaction (indicated through rate law) establishes units of rate constant, k [k = M-2 min-1
14.4 Zero-Order Reactions
zero-order reaction has a rate law in which the sum of the exponents, m + n+… is equal to 0
oreaction proceeds at a rate that is independent of reactant concentrations
Rate of Reaction = k [A]0 = k = constant
concentration-time graph is a straight line with a negative slope
rate of reaction which is equal to k and remains constant throughout the reaction is the
negative of the slope of the line
units of k are the same as units of rate of a reaction: mol/L*t = M/s
Integrated Rate Law- expresses the concentration of a reactant as a function of time
[A]t = -kt + [A]o
14.5 First-Order Reactions
First-order reaction has a rate law in which sum of the exponents, m + n + …is equal to 1
ln ([A]t / [A]o) = -kt or ln [A]t = -kt + ln[A]o
an easy test for a first-order reaction is to plot the natural logarithm of a reactant concentration versus time
& see if graph is linear k = -slope
Half-Life of a reaction is the time required for one-half of a reactant to be consumed; time during which the
amount of reactant or its concentration decreases to one-half of its initial value
t1/2 = ln 2 / k
Half-life is constant for a first-order reaction; it is also independent of the initial concentration
In Reactions involving gases, Rates are often measured in terms of gas pressure
Radioactive decay is a first-order process
14.6 Second-Order Reactions
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