Transcript Collision theory, PE diagrams, Activation energy

Chemical Kinetics
Chemical Energy— results from the particular arrangement of atoms in
a chemical compound; heat and light produced in this reaction due to
energy released during the breaking and reforming of chemical bonds
Decomposition of Nitrogen Triiodide
Decomposition of
Nitrogen Triiodide
N2
NI3
2 NI3(s)
I2
N2(g) + 3 I2(g)
Collision Theory
• Describes how a reaction proceeds.
• The molecules have to “collide” with the
correct energy and orientation in order
to create a new compound.
Ineffective Collision
(no reaction occurs)
Effective Collision
(reaction occurs)
Potential Energy Diagrams
activated
complex
Shows
the PE
of the
Ea
components
chemical
reactants
energy
of a
activation
energy
reaction.
products
course of reaction
Animation by Raymond Chang
All rights reserved
H, Enthalpy
• heat content of a substance:
– natural systems tend to go from a state of
higher energy to a state of lower energy. For
instance, a ball rolls down a hill
spontaneously, but not up. The ball looses
potential energy as it rolls downhill. At the
bottom of the hill it has zero potential energy.
This same idea can be applied to chemical
potential.
Potential energy
Energy in
kinetic energy
Energy out
kinetic energy
The energy something possesses due to its motion, depending on mass and velocity.
Potential Energy Diagram Concepts
Enthalpy of Reaction – a measure of the heat energy
absorbed or released by a reaction system in the form of its
products:
∆Hrxn = Hproducts – Hreactants
Activated Complex – a short lived, unstable arrangment of
atoms
Reaction rate - the rate at which products form or reactants
are used up.
Activation energy - the energy needed to start a chemical
reaction.
Exothermic Reaction
Reactants  Products + Energy
10 energy
=
8 energy
+ 2 energy
activated
complex
Energy of reactants
Energy of products
Energy
Activation
Energy
Reactants
-H
Products
Reaction Progress
Exothermic Reaction
• A reaction that releases energy.
• The reactants have more energy than the
products.
• Energy must be released as the products
form.
• The container in which an exothermic
reaction is taking place will feel hot.
Endothermic Reaction
Energy + Reactants  Products
activated
complex
Energy
Activation
Energy
Reactants
Products
+H Endothermic
Reaction progress
Endothermic Reaction
• A reaction that absorbs energy.
• The products have more energy than the
reactants.
• Energy must be absorbed as the products
form.
• The container in which an endothermic
reaction is taking place will feel cold.
Effect of Catalyst on Reaction Rate
WhatCatalyst
is a catalyst?
does it do
duringfor
a chemical
reaction?
lowers What
the activation
energy
the reaction.
No catalyst
Energy
activation energy
for catalyzed reaction
reactants
products
Reaction Progress
Chemical Reaction Rates
• Reaction rate refers to one or more of the
following:
* The rate of disappearance of one of the
reactants.
* The rate of appearance of one of the
products.
Chemical Reaction Rates
• The nature of the reactants involved in a
chemical reaction will determine the kind
of bonding that occurs.
• Reactions with bond rearrangement or
electron transfer (covalent) generally take
longer than ionic reactions.
– Ionic reactions are almost instantaneous
because of the strong attraction between the
charged particles.
Other factors affecting reaction rate:
• Concentration - For a
chemical reaction to
occur, the particles must
collide.
• An increase in the number of particles per
unit volume (concentration) increases the
chance of their colliding. Increasing the
concentration of the reacting particles
generally increases the reaction rate.
Other factors affecting reaction rate:
lower temperature
Fractions of particles
higher temperature
minimum energy
for reaction
Kinetic energy
Other factors affecting reaction rate:
• Temperature - An increase in the speed
of molecules increases the number of
molecules that have the required
activation energy. Increasing the number
of molecules with activation energy
generally increases the reaction rate.
• Surface Area – more surface area, faster
reaction
Other factors affecting reaction rate:
Energy
• Catalysts - A catalyst is aNosubstance
that
catalyst
increases a reaction rateactivation
without
being
energy
catalyzed reaction
permanently changed. for
The
catalyst
appears to be chemically unaffected by the
reaction. A catalyst changes the reaction
mechanism in such a way that the
activation energy required is less
than that
products
in the uncatalyzed reaction.
Reaction Progress
Other factors affecting reaction rate:
• Inhibitors - An inhibitor ties up a
reactant in a complex so that it
will not react.