Transcript Kinetics II (download)

Factors that affect reaction rate
 Concentration of REACTANTS
 Temperature (kinetic energy of molecules)
 Presence of a catalyst
 Physical state of reactants (surface area)
Higher concentration = more
collisions
Ways over (or around) the barrier
 Temperature increases reaction rate by
increasing the fraction of molecules which
have sufficient energy to jump the barrier
 A catalyst is a way to remove, or at least
lower the barrier. A catalyst acts to increase
the chemical reaction, but is not consumed
itself during the reaction
Catalysts modify the pathway
 Addition of a chlorine
catalyst increases the rate
of decomposition of ozone
into O2 – the reason for the
destructive influence of
these compounds in the
atmosphere
 Although two barriers are
present, both are smaller
than the one without the
catalyst, and the reaction
proceeds more rapidly
Without catalysts, there would be no
life at all, from microbes to humans
 ENZYMES are
biological catalysts
 Most enzymes are
proteins – large
molecules
 Have correct shape to
bring reactant
molecules together in
correct orientation
Natural selection produces
prodigious catalysts
 Green lines are the
amino acid backbone
of the enzyme
 The white molecule is
the fatty acid
 Red are iron atoms
Catalysts can be solids, liquids or
gases
The catalyst shape can direct certain
products: zeolite molecular sieve
Reactant
molecules
Product
molecules
Mathematical treatment of reaction
rate
 In the reaction
2N2O5 = 4NO2 + O2
We describe the rate according to the form
Rate = k[N2O5]n
 k is the rate constant
 n is the order of reaction with respect to N2O5
 NB: n is not the same as the coefficient in the equation!
 Rate is normally measured by monitoring growth
in concentration of a product, or decline in
concentration of a reactant
More than one reactant
 A + B = products
Rate = k[A]m[B]n
m and n are individual orders for the reactants A
and B. The overall order of reaction = m + n
Strategies for determining orders of
reaction
 Method of initial rates
 At t = 0, product concentration is zero
 Vary concentration of one reactant, keeping the other(s) constant
NO2 
Rate 
 k[ NO]2 [O2 ]
t
 Reaction is second order in NO and first order in O2
Effect of temperature: activation
energy
 As temperature increases rate increases
k  Ae
 EA
RT
 Arrhenius expression: EA is activation energy
 Plot of ln k vs 1/T is constant (T in Kelvin)
Activation energy using two
temperatures
 k2   Ea  1 1 
ln        
 k1   R  T2 T1 
 Measure ratio of rates at two temperatures
(Rate is proportional to k)
 k2 
RT1T2 ln  
k1 

EA 
(T1  T2 )