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Chapter 14

CHEM 1220 Chapter Notes - Chapter 14: Reaction Rate, Rate Equation, Reaction Rate Constant


Department
Chemistry
Course Code
CHEM 1220
Professor
Allen Heather
Chapter
14

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Chapter 14: Chemical Kinetics
Chemical Kinetics: the area of chemistry concerned with speeds or rates
Reaction Rate: the speed at which a chemical reaction occurs
14-1| Factors that Affect Reaction Rates
1) Physical state of reactants. Homogeneous (involving one state of matter) or heterogeneous
(involving more than one state of matter). As surface area increases in solids, reaction rates
increase
2) Reactant Concentrations. As concentration increases, reaction rate increases
3) Reaction Temperature. As temperature increases, reaction rate increases
4) Presence of catalyst (agents that increase reaction rates without being used up
5) The greater the frequency of collisions, the greater the reaction rate
14-2| Reaction Rates
Speed is defined as change that occurs within given time interval, Molarity/second (M/s)
Average rate of appearance of B= (change in concentration of B/change in time)
Average rate of disappearance of A= -(change in concentration of A/change in time)
Rates are always expressed as positive quantities
Change of Rate with Time
Rates decrease as reaction proceeds because concentration of reactants decreases
Instantaneous Rate
Instantaneous Rate: the rate at a particular instant during the reaction. Determined from slope
of curve at a point in time. Find by calculating tangent line slope by “drawing” triangle
Reaction Rates and Stoichiometry
If disappearance of A and appearance of B is not 1:1 ratio, take into account reaction
stoichiometry
14-3| Concentration and Rate Laws
Rate Law: for reaction aA+bB  cC+dD; Rate=k[A]m[B]n
k is rate constant, changes with temperature; m and n are typically small whole numbers
Reaction Orders
Reaction orders: exponents m and n
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First order: exponent m or n =1. If both 1, overall reaction order is second order overall
Overall reaction order: sum of orders with respect to each reactant represented in rate law
Magnitudes and Units of Rate Constants
A high k (109 or higher) means fast reaction and low k (10 or lower) means slow reaction
Units of rate= (units of rate constant)(units of concentration)2=usually 1/(Ms)
14-4| The Change of Concentration with Time
First Order Reactions
First order reaction: one whose rate depends on the concentration of a single reactant raised to
the first power
For reactions like A  products, rate law is Rate= k[A]
Ln[A]t-ln[A]0=-kt
Second Order Reactions
Second Order Reaction: one for which rate depends on reactant comparison raised to second
power or on concentrations of two reactants raised to the first power
Rate=k[A]21/[A]t=kt+(1/[A]0)
Zero Order Reactions
Zero order reaction: one in which the rate of disappearance of A is independent of [A].Usually
occurs when a gas undergoes decomposition on the surface of a solid
[A]t=-kt+[A]0
Half-Life
Half life: the time required for the concentration of a reaction to reach half its original value
T1/2=(-ln(1/2)/k)=.693/k
In a first order reaction, the concentration of the reactant decreases by one-half in each of
series of regularly spaced time intervals, each interval equal to a half life
Second order reaction half life equation: t1/2=1/(k[A]0)
14-5| Temperature and Rate
The Collision Model
Collision Model: based on kinetic-molecular theory, accounts for effect of reactant
concentrations and temperature on reaction rates at the molecular level
As collisions increase, reaction rate increases
The Orientation Factor
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