# CHMB16H3 Lecture Notes - Activated Complex, Rate Equation, Reaction Intermediate

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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

reactants

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

used

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

## Document Summary

The rate of a reaction describes how fast the concentration of a reactant or product changes with time. Rate of formation = [c]/ t: rate of disappearance is a negative quantity because concentration decreases with time. Rate of disappearance = - [a]/ t: the concentration at the end of a time period is less than t was at the start of the period. 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 o o o.