Transcript Kinetics and Equilibrium

Kinetics and Equilibrium
Kinetics
• The branch of chemistry known as
chemical kinetics is concerned with the
rates of chemical reactions and the
mechanisms by which they occur.
• The rate of a chemical reaction is
measured in terms of the # of moles of a
reactant consumed (or produced) in a
unit of time.
• The mechanism of a chem rxn attempts
to describe the order of events in which
the reaction takes place.
Energy and Reaction Rates
• Energy is required to initiate a rxn.
Activation energy is defined as the min
energy required to make a rxn occur.
• The heat of reaction illustrates the
difference in heat content before and
after the rxn.
• It is also the difference in potential
energy between the prod and reactants.
• If the products have less energy than the
reactants, the reaction is considered
exothermic.
• Endothermic reactions occur when the
products have more energy than the
reactants. They are given by a positive
heat of rxn.
Potential Energy Diagram
• We can show the relationship between
activation energy and heat of reaction
for a given rxn on a graph.
• We place PE on the vertical axis, and the
“reaction coordinate” on the horizontal
axis.
• In order for any rxn to occur, an
activated complex must be formed.
• This is an intermediate product, and may
be considered to be a temporary
association of atoms or molecules which
when finished, will give us our products.
• The highest point on the curve of a PE
diagram shows the PE of the
intermediate product (activated
complex).
• Once the formation of an activated
complex has occurred, the rxn can
continue to give the products.
• The PE of the products will be lower
than the peak energy of the activated
complex.
• If the PE is higher for the products, the
rxn was endothermic, and if the PE is
higher for the reactants, the rxn was
exothermic.
• The heat of reaction is strictly the
difference between the PE’s of the
products and reactants, and is the same
regardless of the amounts of each
involved.
Factors that Affect Reaction Rates
• Chem rxns depend on collisions
between reacting species - atoms,
molecules, ions, etc.
• A rxn only occurs when adequate energy
has been given to form an activated
complex.
• Generally speaking, anything that causes
more collisions to occur will increase
the rate of reaction.
Nature of the Reactants
• The nature of the reactants, with respect
to the types of bonds involved,
contributes to determining the rate of
rxn.
• If few bond rearrangements are
necessary, the rxn will occur quickly,
such as with ionic substances in water.
• If bonds need to be broken or newly
created, more time is necessary, such as
oxygen and hydrogen bonding to form
water.
Concentration
• If the concentration of one or more
reactants is increased, the rate of rxn
increases. Compressing a gas has the
same effect.
Temperature
• An increase in temperature will increase
the rate of reaction for two reasons increased frequency of collisions and
increased energy values in each
collision.
Reaction Mechanism
• Most reactions do not occur in a single
step, but in a series of steps called the
reaction mechanism.
• Each individual step is generally a twoparticle collision, and the number of
steps and their individual complexity
affects the rate of the overall rxn. The
more steps, the slower the rxn.
Catalysts
• Fast reactions have low activation
energies, while slow reactions have high
activation energies.
• The addition of a catalyst to a system
changes its activation energy, thereby
changing its rate of rxn.
• A catalyzed rxn requires less activation
energy, without changing the overall
rxn.
• Even though the activated complexes in
a catalyzed rxn have less PE than a noncatalyzed rxn, the heats of rxn are the
same.