Kinetics and Equilibrium
Rates of Chemical Reactions
Expressing Reaction Rates
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The change in the amount of
reactants or products over time is
called the reaction rate or rate of
reaction.
Expressing Reaction Rates
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When a reaction occurs between gaseous
species or in solution, the reaction rate is
expressed as a change in the
concentration of the reactant or product
per unit time.
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The concentration of a compound (in mol/L) is
symbolized by placing square brackets, [ ],
around the chemical formula.
Factors that affect Reaction Rate
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5.
The rate of a reaction can be increased by
increasing the temperature.
Increasing the concentrations of the reactants
usually increases the rate of the reaction.
Increasing the available surface area of a
reactant increases the rate of a reaction.
A catalyst is a substance that increases the
rate of a reaction. The catalyst is regenerated at
the end of the reaction and can be re-used.
The rate of a chemical reaction depends on what
the reactants are. In other words, the
reactivity of the reactants has a major impact
on reactant rate.
Section Review:
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P 467, #1,2,3,4,5
Theories of Reaction Rates
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kinetic molecular theory
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All matter is made up of microscopic-sized
particles (atoms, molecules, ions).
These particles are in constant motion,
because they possess kinetic (movement)
energy.
There are spaces between the particles of
matter. The speed and spacing determine the
physical state of matter.
Adding heat increases the speed of the moving
particles, thus increasing their kinetic energy,
as well as the space they occupy.
Collision Theory
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In order for a reaction to occur,
reacting particles (atoms,
molecules, or ions) must collide
successfully with one another.
If a collision is necessary for a
reaction to occur, then the rate of
the reaction will increase if there
are more collisions per unit time.
Collision Theory and Concentration
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greater concentration: increases the
number of collisions between the particles
per second.
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decreasing the concentration of reactants
decreases the reaction rate.
If the particles are gaseous, increasing
the pressure has the same effect as
increasing the concentration, because the
same mass of particles is confined to a
smaller volume.
Collision Theory and Surface Area
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When a solid undergoes a chemical
reaction, collisions can occur only at the
solid’s surface.
For example, if you want to make a
campfire, you will likely start with paper
and small twigs, rather than with logs.
The paper and twigs provide a greater
surface area, which enables more
collisions to occur.
Collision Theory and the Nature of
Reactants
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Reactions that involve ionic compounds
and simple ions are generally faster than
reactions involving molecular compounds.
Reactions that involve breaking weaker
bonds are generally faster than reactions
that involve breaking stronger bonds.
Reactions that involve breaking fewer
bonds are generally faster than reactions
that involve breaking a greater number of
bonds.
Collision Theory and Temperature
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Increasing the temperature of a
reaction results in an increase in the
reaction rate.
At higher temperatures, particles
move more quickly, so they have
greater kinetic energy and collide
more frequently.
Activation Energy
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In order for a collision to result in a
chemical reaction, the particles
must collide with enough energy to
break bonds in the reactants and
start to form bonds in the products.
The activation energy, Ea, of a
reaction is the minimum collision
energy that is required for a
successful reaction.
Reactions and Orientation of Reactants
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Reacting particles must collide with
the proper orientation in relation
to one another. This is also known
as having the correct collision
geometry.
Video
Summary
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For a collision between reactants to result
in a reaction, the collision must satisfy
both of these two criteria:
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correct orientation of reactant particles
sufficient collision energy
When both of these criteria are met, a
collision is said to be effective. Only
effective collisions result in the formation
of products.
Transition State Theory
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Transition state theory is used to
explain the transition, or change,
from reactants to products.
The kinetic energy of the reactants
is transferred to potential energy as
the reactants collide, due to the law
of conservation of energy.
Transition State Theory
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You can represent the increase in potential
energy during a chemical reaction using a
potential energy diagram: a diagram that
charts the potential energy of a reaction
against the progress of the reaction.
The y-axis represents potential energy. The
x-axis, labelled “Reaction progress,”
represents the progress of the reaction
through time.
The “hill” in each diagram illustrates the
activation energy barrier of the reaction. A
slow reaction has a high activation energy
barrier. A fast reaction, by contrast, has a
low activation energy barrier.
Potential Energy Diagrams
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The top of the activation energy barrier
on a potential energy diagram represents
the transition state of the reaction.
The chemical species that exists at the
transition state is referred to as the
activated complex.
The activated complex is a transitional
species that exists for a fraction of a
moment and is neither product nor
reactant.
Practice
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P 467 1-4
Elementary Reactions
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A reaction mechanism is a series of
steps that make up an overall reaction.
Each step, called an elementary
reaction, involves a single molecular
event, such as a simple collision between
atoms, molecules, or ions.
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It cannot be broken down into further, simpler
steps.
Elementary Reactions
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Reaction Mechanism:
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Two steps or elementary reactions :
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2NO(g) + O2(g) → 2NO2(g)
Step 1 NO(g) + O2(g) → NO3(g)
Step 2 NO3(g) + NO(g) → 2NO2(g)
Adding the two steps gives the
overall reaction.
Intermediates?
Problem
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Step 1 NO2(g) + NO2(g) → NO3(g) + NO(g)
Step 2 NO3(g) + CO(g) → NO2(g) + CO2(g)
Write the overall balanced equation
for this reaction and identify the
reaction intermediate.
Practice
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P 478 #5-8
The Rate-Determining Step
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Elementary reactions in
mechanisms all have different rates.
The rate-determining step, is the
slowest elementary reaction.
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So, it determines the overall rate.
The Rate-Determining Step
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Step 1 Bread → Toast (slow, rate-determining)
Step 2 Toast + Butter → Buttered toast (fast)
Step 1 Bread → Toast (fast)
Step 2 Toast + Frozen butter → Buttered toast
(slow, rate-determining)
Rate determining step
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2NO(g) + 2H2(g) → N2(g) + 2H2O(g)
Step 1
2NO(g) + H2(g) → N2O(g) + H2O(g) (slow, rate-determining)
Step 2
N2O(g) + H2(g) → N2(g) + H2O(g) (fast)
Catalysts
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A catalyst is a substance that increases
the rate of a chemical reaction without
being consumed by the reaction.
A catalyst works by lowering the
activation energy of a reaction so that
more reactants have sufficient energy to
react.
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It lowers the activation energy by providing an
alternative mechanism for the reaction.
Catalysts
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Step 1
Step 2
A + catalyst → Acatalyst
Acatalyst + B → AB + catalyst
Overall reaction A + B → AB
Section Review
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P 484 #1-12
Chapter Review:
P 486 #1-11 and 13
Quiz Wednesday