Transcript Chemical Kinetics “Rates of Reactions”

Chemical Kinetics
“Rates of Reactions”
Reaction Rates
Average rate:
Change of reactant or product concentrations
over a specific time interval
Initial rate:
Rate at the beginning of a reaction.
Question?
For the reaction:
The dependence of the
concentration of H2 on
time is shown below.
Is the reaction rate
faster at point A or
point B?
A, B, both rates are the
same
Rate Determining Step
• slowest step in a multi-step mechanism
• the step which determines the overall rate of
the reaction
Rate Law
• an expression which relates the rate to the
concentrations and a specific rate constant
Rate Law
Reaction rate = k [A]m [B]n
where
m => order with respect to A
n => order with respect to B
overall order = m + n
Question?
The empirical rate law for the reaction
is Rate = k[NO2][F2].
Which of the following mechanisms is consistent with this rate law?
A) NO2 (g) + F2 (g) <=> NO2F(g) + F(g) fast
NO2(g) + F(g) --->NO2F(g) slow
B) NO2 (g) + F2(g) <==> NO2F(g) + F(g) slow NO2(g) + F(g) ---> NO2F(g) fast
C) F2(g) <==> F(g) + F(g) slow
2NO2(g) + 2F(g) ---> 2NO2F(g) fast
A, B, or C
Order of Reaction
• The exponent of the concentration for a
reactant implies the number of molecules of
that species involved in the rate determining
step
• first order, exponent equals one
• second order, exponent equals two
Integrated Rate Laws
A ---> products
rate = - (D[A]/Dt) = k[A]m
average rate
rate = - (d[A]/dt) = k[A]m
instantaneous rate
This plot of ln[H2O2] vs. time
produces a straight line, suggesting
that the reaction is first order.
Elementary Reactions
• Molecularity is the number of particles coming
together in a reaction step.
• unimolecular - rearrangement of a molecule
• bimolecular - reaction involving the collision of
two particles
• termolecular - reaction involving the collision of
three particles (statistically unlikely)
Unimolecular Reaction
Bimolecular Reaction
Collision Rate Model
Three conditions must be met at the
molecular level if a reaction is to occur:
• the molecules must collide;
• they must be positioned so that the reacting
groups are together in a transition state
between reactants and products;
• and the collision must have enough energy
to break bonds, form the transition state
and convert it into products.
Transition State:
Activated Complex or
Reaction Intermediates
• an unstable arrangement of atoms that has
the highest energy reached during the
rearrangement of the reactant atoms to give
products of a reaction
Activation Energy
the minimum energy required to start a reaction
Temperature & Reaction Rate
Catalyst
• A substance which speeds up the rate of a
reaction while not being consumed
Homogeneous Catalysis - a catalyst which is
in the same phase as the reactants
Heterogeneous Catalysis - a catalyst which is
in the different phase as the reactants
catalytic converter
– solid catalyst working on gaseous materials
Question?
Which curve
illustrates the
effect of a catalyst
on the reaction
diagram, given
that it speeds up
the rate of a
reaction?
A, B, C, D
Reaction Mechanism
• A set of elementary reactions which
represent the overall reaction
Car Catalytic Converter
catalyst
H2O(g) + HCs -------> CO(g) + H2(g)
(unbalanced)
catalyst
2 H2(g) + 2 NO(g) ------> N2(g) + 2 H2O(g)
catalyst
HCs + O2(g) --------> CO2(g) + H2O(g) (unbalanced)
catalyst
CO(g) + O2(g) --------> CO2(g)
catalyst = Pt-NiO
HCs = unburned hydrocarbons
(unbalanced)
Chain Mechanisms
chain initiating step - the step of a mechanism
which starts the chain
chain propagating step(s) - the step or steps
which keeps the chain going
chain terminating step(s) - the step or steps
which break the chain
Chain Mechanisms
combustion of gasoline in an internal
combustion engine
chain initiating step - additives which generate
free radicals, particles with unpaired
electrons
chain propagating step(s) - steps which
generate new free radicals
chain terminating step(s) - steps which do not
generate new free radicals
Enzymes
Any one of many specialized organic
substances, composed of polymers of
amino acids, that act as catalysts to
regulate the speed of the many
chemical reactions involved in the
metabolism of living organisms.