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Chemical Kinetics
Chapter 13
Chemical Kinetics
Thermodynamics – does a reaction take place?
Kinetics – how fast does a reaction proceed?
Reaction rate is the change in the concentration of a
reactant or a product with time (mol/(Ls)).
A B
D[A] D[A] = change in concentration of A over
rate = -
Dt time period Dt
D[B] D[B] = change in concentration of B over
rate =
Dt time period Dt
Because [A] decreases with time, D[A] is negative.
13.1
A B
time
D[A]
rate = -
Dt
D[B]
rate =
Dt
13.1
Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g)
time
393 nm Detector
light
D[Br2] a DAbsorption
13.1
Br2 (aq) + HCOOH (aq) 2Br- (aq) + 2H+ (aq) + CO2 (g)
slope of
tangent
slope of
tangent slope of
tangent
D[Br2] [Br2]final – [Br2]initial
average rate = - =-
Dt tfinal - tinitial
instantaneous rate = rate for specific instance in time
13.1
Reaction Rates and Stoichiometry
2A B
Two moles of A disappear for each mole of B that is formed.
1 D[A] D[B]
rate = - rate =
2 Dt Dt
aA + bB cC + dD
1 D[A] 1 D[B] 1 D[C] 1 D[D]
rate = - =- = =
a Dt b Dt c Dt d Dt
13.1
Write the rate for the following reaction:
CH4 (g) + 2O2 (g) CO2 (g) + 2H2O (g)
D[CH4] 1 D[O2] D[CO2] 1 D[H2O]
rate = - =- = =
Dt 2 Dt Dt 2 Dt
13.1
+
A+ B AB+ C+D
Exothermic Reaction Endothermic Reaction
The activation energy (Ea ) is the minimum amount of
energy required to initiate a chemical reaction.
13.4
Kinetic Molecular Theory
• Model of what happens to gas particles
during experimentation
– Large numbers of molecules in continuous
motion
– Attractive and repulsive forces are negligible
– Energy is transferred between molecules
during collisions, but average kinetic energy is
unchanged (as long as temp is constant)
– Kinetic energy of molecules is proportional to
the absolute temperature. (in K)
COLLISION THEORY
Collision theory states that...
• particles must COLLIDE before a reaction can take place
• not all collisions lead to a reaction
• reactants must possess at least a minimum amount of energy - ACTIVATION ENERGY
plus
• particles must approach each other in a certain relative way - the STERIC EFFECT
COLLISION THEORY
Collision theory states that...
• particles must COLLIDE before a reaction can take place
• not all collisions lead to a reaction
• reactants must possess at least a minimum amount of energy - ACTIVATION ENERGY
plus
• particles must approach each other in a certain relative way - the STERIC EFFECT
According to collision theory, to increase the rate of reaction you therefore need...
more frequent collisions increase particle speed or
have more particles present
more successful collisions give particles more energy or
lower the activation energy
A Reaction Profile
CO(g) + NO2(g) CO2(g) + NO(g)
Particles must collide with the proper geometry or
orientation for atoms to come in direct contact and form
the chemical bonds of the products. (steric factor)
• If both of these conditions are not met,
particles will merely collide and bounce
off one another without forming
products.
http://www.mhhe.com/physsci/chem
istry/essentialchemistry/flash/collis1
1.swf
• Although, the percentage of successful
collisions is extremely small, chemical
reactions still take place at a reasonable
rate because there are so many
collisions per second between reactant
particles.
INCREASING THE RATE
The following methods may be used to
increase the rate of a chemical reaction
• INCREASE THE SURFACE AREA OF SOLIDS
• INCREASE TEMPERATURE
• ADD A CATALYST
• INCREASE THE PRESSURE OF ANY GASES
• INCREASE THE CONCENTRATION OF REACTANTS
INCREASING SURFACE AREA
• Increasing surface area increases chances of a collision - more particles are exposed
• Powdered solids react quicker than larger lumps
• Catalysts (e.g. in catalytic converters) are in a finely divided form for this reason
+
In many organic reactions there are two liquid layers, one aqueous, the other non-
aqueous. Shaking the mixture improves the reaction rate as an emulsion is often
formed and the area of the boundary layers is increased giving more collisions.
1 1
CUT THE SHAPE 1
INTO SMALLER 1
PIECES
3
3
SURFACE AREA SURFACE AREA
9+9+3+3+3+3 = 30 sq units 9 x (1+1+1+1+1+1) = 54 sq units
INCREASING TEMPERATURE
Effect increasing the temperature increases the rate of a reaction
particles get more energy so they can overcome the energy barrier
particle speeds also increase so collisions are more frequent
ENERGY CHANGES
DURING A REACTION
As a reaction takes place the enthalpy of
the system rises to a maximum, then falls
A minimum amount of energy is required to
overcome the ACTIVATION ENERGY (Ea).
Only those reactants with energy equal to,
or greater than, this value will react.
If more energy is given to the reactants
Typical energy profile diagram
then they are more likely to react.
for an exothermic reaction
INCREASING TEMPERATURE
MAXWELL-BOLTZMANN
DISTRIBUTION OF
NUMBER OF MOLECUES WITH MOLECULAR ENERGY
A PARTICULAR ENERGY
MOLECULAR ENERGY
Because of the many collisions taking place between molecules, there is a spread of
molecular energies and velocities. This has been demonstrated by experiment.
It indicated that ... no particles have zero energy/velocity
some have very low and some have very high energies/velocities
most have intermediate velocities.
INCREASING TEMPERATURE
MAXWELL-BOLTZMANN
DISTRIBUTION OF
NUMBER OF MOLECUES WITH MOLECULAR ENERGY
A PARTICULAR ENERGY
T1
T2
TEMPERATURE
T2 > T1
MOLECULAR ENERGY
Increasing the temperature alters the distribution
• get a shift to higher energies/velocities
• curve gets broader and flatter due to the greater spread of values
• area under the curve stays constant - it corresponds to the total number of particles
INCREASING TEMPERATURE
T3 MAXWELL-BOLTZMANN
DISTRIBUTION OF
NUMBER OF MOLECUES WITH MOLECULAR ENERGY
A PARTICULAR ENERGY
T1
TEMPERATURE
T1 > T3
MOLECULAR ENERGY
Decreasing the temperature alters the distribution
• get a shift to lower energies/velocities
• curve gets narrower and more pointed due to the smaller spread of values
• area under the curve stays constant - it corresponds to the total number of particles
INCREASING TEMPERATURE
T3 MAXWELL-BOLTZMANN
DISTRIBUTION OF
NUMBER OF MOLECUES WITH MOLECULAR ENERGY
A PARTICULAR ENERGY
T1
T2
TEMPERATURE
T2 > T1 > T3
MOLECULAR ENERGY
REVIEW
no particles have zero energy/velocity
some particles have very low and some have very high energies/velocities
most have intermediate velocities
as the temperature increases the curves flatten, broaden and shift to higher energies
INCREASING TEMPERATURE
MAXWELL-BOLTZMANN
DISTRIBUTION OF
NUMBER OF MOLECUES WITH MOLECULAR ENERGY
A PARTICULAR ENERGY
NUMBER OF
Ea MOLECULES WITH
SUFFICIENT
ENERGY TO
OVERCOME THE
ENERGY BARRIER
MOLECULAR ENERGY
ACTIVATION ENERGY - Ea
The Activation Energy is the minimum energy required for a reaction to take place
The area under the curve beyond Ea corresponds to the number of molecules with
sufficient energy to overcome the energy barrier and react.
INCREASING TEMPERATURE
TEMPERATURE
MAXWELL-BOLTZMANN
T2 > T1 DISTRIBUTION OF
NUMBER OF MOLECUES WITH MOLECULAR ENERGY
A PARTICULAR ENERGY
T1
T2
EXTRA
Ea MOLECULES WITH
SUFFICIENT
ENERGY TO
OVERCOME THE
ENERGY BARRIER
MOLECULAR ENERGY
Explanation
increasing the temperature gives more particles an energy greater than Ea
more reactants are able to overcome the energy barrier and form products
a small rise in temperature can lead to a large increase in rate
ADDING A CATALYST
• Catalysts provide an alternative reaction pathway with a lower Activation Energy (Ea)
• Decreasing the Activation Energy means that more particles will have sufficient
energy to overcome the energy barrier and react
• Catalysts remain chemically unchanged at the end of the reaction.
WITHOUT A CATALYST WITH A CATALYST
ADDING A CATALYST
MAXWELL-BOLTZMANN
DISTRIBUTION OF
NUMBER OF MOLECUES WITH MOLECULAR ENERGY
A PARTICULAR ENERGY
NUMBER OF
MOLECULES WITH
SUFFICIENT
ENERGY TO
OVERCOME THE
ENERGY BARRIER
MOLECULAR ENERGY Ea
The area under the curve beyond Ea corresponds to the number of molecules with
sufficient energy to overcome the energy barrier and react.
If a catalyst is added, the Activation Energy is lowered - Ea will move to the left.
ADDING A CATALYST
MAXWELL-BOLTZMANN
DISTRIBUTION OF
NUMBER OF MOLECUES WITH MOLECULAR ENERGY
A PARTICULAR ENERGY
EXTRA MOLECULES
WITH SUFFICIENT
ENERGY TO OVERCOME
THE ENERGY BARRIER
MOLECULAR ENERGY Ea
The area under the curve beyond Ea corresponds to the number of molecules with
sufficient energy to overcome the energy barrier and react.
Lowering the Activation Energy, Ea, results in a greater area under the curve after Ea
showing that more molecules have energies in excess of the Activation Energy
CATALYSTS - A REVIEW
• work by providing an alternative reaction pathway with a lower Activation Energy
• using catalysts avoids the need to supply extra heat - safer and cheaper
• catalysts remain chemically unchanged at the end of the reaction.
Types Homogeneous Catalysts Heterogeneous Catalysts
same phase as reactants different phase to reactants
e.g. CFC’s and ozone e.g. Fe in Haber process
Uses used in industry especially where an increase in temperature results in
a lower yield due to a shift in equilibrium (Haber and Contact Processes)
INCREASING THE PRESSURE
• increasing the pressure forces gas particles closer together
• this increases the frequency of collisions so the reaction rate increases
• many industrial processes occur at high pressure to increase the rate... but
it can adversely affect the position of equilibrium and yield
The more particles there are in a given volume, the greater the pressure
The greater the pressure, the more frequent the collisions
The more frequent the collisions, the greater the chance of a reaction
INCREASING CONCENTRATION
Increasing concentration = more frequent collisions = increased rate of reaction
Low concentration = fewer collisions Higher concentration = more collisions
However, increasing the concentration of some reactants
can have a greater effect than increasing others
Concentration
• A higher concentration of reactants leads
to more effective collisions per unit time,
which leads to an increasing reaction rate
• We are not increasing the amount being
made for a given balanced equation with
limiting reactants, we are only speeding up
how quickly those products are made.
CuCO3(s) + 2HCl(aq) CuCl2(aq) + CO2(g) + H2O(l)
blue
Describe seven different ways to monitor the rate of the above reaction.
State how each property would change as the reaction proceeds.
1. Mass of CuCO3(s) over time decreases
2. [HCl] over time decreases
3. [CuCl2] over time increases
4. Volume of CO2 over time increases
5. Mass of a open beaker over time decreases
6. Pressure of a closed beaker over time increases
7. Colour of the blue Cu2+ over time increases
CuCO3(s) + 2HCl(aq) CuCl2(aq) + CO2(g) + H2O(l)
blue
Describe five different ways to increase the rate of the above reaction.
1. Increase the temperature
2. Increase [HCl]
3. Add a catalyst
4. Increase the surface area of CuCO3(s)
5. Agitate
We can't change the nature of the reactant because then we wouldn't
have the same reaction.
Replacing HCl with H2SO4 would be faster but a different reaction.
