Topic #23: Chemical Kinetics EQ: How do the factors that affect
the rate of reaction slow down or speed up a chemical reaction?
1
Slide 3
Chemical Kinetics Studies the rate at which a chemical process
occurs. Besides information about the speed at which reactions
occur, kinetics also sheds light on the reaction mechanism (how the
reaction occurs) 2 2
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3 Rate of Reaction Some reactions occur rapidly Inflation of an
airbag Explosion of nitrogen triiodide Some reactions occur slowly
Reaction of pigments in paintings with light & pollutants
Tarnishing of silver/Iron rusting 3
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4 Rate of Reaction An increase in concentration of one of the
products per unit time or as the decrease in concentration of one
of the reactants per unit time. OR How fast reactants are being
converted to products during a chemical reaction either the rate of
formation of a product of the rate of consumption of a reactant may
be used (they are related to each other by dividing the
corresponding coeffficient in the stoichiometric equation) Units of
rate of reaction:moles per dm 3 per second We will use mol dm -3 s
-1 4
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Rate of Reaction Rates of reactions can be determined by
monitoring the change in concentration of either reactants or
products as a function of time. [A] vs t or [B] vs t 5 5
Slide 7
Kinetic Theory Tiny particles in all forms of matter are in
constant motion. The energy an object has because of motion is
called kinetic energy. The particles in any collection of atoms or
molecules at a given temperature have a wide range of kinetic
energies, from very low to very high. Overall, there is an average
kinetic energy. 6 6
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The Maxwell-Boltzmann Distribution In a sample gas or liquid,
the molecules are flying about and bumping into each other and the
walls all the time. When they do this, no energy is lost from the
system - theyre elastic collisions. So in a sample of gas or liquid
at any given time, the energy that molecules have is going to be
spread out, with some particles having more or less energy than
others. If you were to draw a graph of how the energy was spread
over different molecules, youd get the Maxwell-Boltzmann
distribution!
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This graph is important if you want to get those easy marks.
You have to remember that: the area under the curve = the total
molecules in the sample, and doesnt change there are no molecules
with no energy very few molecules have high energies, but there is
no maximum energy for a molecule, and the most probable energy
value is where the curve is at its highest, as shown.
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If you increase the temperature of the sample, the distribution
changes as shown below. It becomes stretched out and has a lower
peak, but the area under the curve remains the same. The reverse
happens when you cool down a sample.
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Activation Energy (E a ) Activation Energy: The minimum amount
of energy required for reaction: to happen is called the activation
energy, E a. Just as a ball cannot get over a hill if it does not
roll up the hill with enough energy, a reaction cannot occur unless
the molecules possess sufficient energy to get over the activation
energy barrier.
Slide 13
Activation energy is the minimum amount of kinetic energy that
must be given to the reactants before they will react. (a)
Activation energy (Ea) of forward reaction (b) Activation energy
(Ea) of reverse reaction (c) Heat of reaction (H) 1) What is the
heat of reaction for each of the graphs? Which is exothermic and
which is endothermic?
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In a chemical reaction, bonds are broken and new bonds are
formed. Molecules can only react if they collide with each other
with enough energy. (E a ) 13
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Collision theory: Furthermore, molecules must collide with the
correct orientation and with enough energy to cause bond to break
and reform again. 14
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COLLISION THEORY: Know this!!! 1. In order for a reaction to
occur, reactants (atoms, ions, radicals or molecules) must collide
2. Molecules must collide in the right orientation so that the
reactive parts of each of the two particles come into contact with
each other 3. Reactants must collide with sufficient kinetic energy
to bring about the reaction (at least the activation energy). Not
all collisions result in a reaction. The likelihood of these
happening determines the rate. 15
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16 Changing the rate of reactions increased temperature
increased concentration of dissolved reactants, and increased
pressure of gaseous reactants increased surface area of solid
reactants.(decrease particle size) use of a suitable catalyst.
Anything that increases the number of successful collisions between
reactant particles will speed up a reaction. What factors affect
the rate of reactions?
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17 Effect of temperature on rate Increasing the temperature
will make the particles move faster, so there will be more
collisions. At a higher temperature, many more particles will have
the necessary activation energy. The ratio of successful collision
to unsuccessful collisions will increase. Which one will contribute
more towards increasing the rate of reaction?
Slide 19
Temperature: greater T = more collisions with the required Ea
Increasing the temperature increases the frequency of collisions
but, more importantly, the proportion of molecules with E Ea
increases 18
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19 Effect of concentration on rate of reaction The higher the
concentration of a dissolved reactant, the faster the rate of a
reaction. Why does increased concentration increase the rate of
reaction? At a higher concentration, there are more particles in
the same amount of space. This means that the particles are more
likely to collide and therefore more likely to react. higher
concentration lower concentration The ratio of successful
collisions to unsuccessful collisions will stay the same, but there
will be more successful collisions.
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20 Effect of pressure on rate of reaction The gas particles
become closer together, increasing the frequency of collisions.
This means that the particles are more likely to react. Why does
increasing the pressure of gaseous reactants increase the rate of
reaction? As the pressure increases, the space in which the gas
particles are moving becomes smaller. lower pressure higher
pressure
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Concentration: higher conc = more particles in given volume =
more collisions per unit of time 21
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22 Effect of surface area on rate of reaction Any reaction
involving a solid can only take place at the surface of the solid.
If the solid is split into several pieces, the surface area
increases. What effect will this have on rate of reaction? The
smaller the pieces, the larger the surface area. This means more
collisions and a greater chance of reaction. This means that there
is an increased area for the reactant particles to collide with.
low surface areahigh surface area
Slide 24
Particle size: greater surface area = greater # of collisions
per unit of time 23
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24 What are catalysts? reaction (time) energy (kJ) Catalysts
are substances that change the rate of a reaction without being
used up in the reaction. Catalysts never produce more product they
just produce the same amount more quickly. Different catalysts work
in different ways, but most provide an alternative path with lower
activation energy (E a ). E a with catalyst E a without
catalyst
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Catalysts: provide alternative mechanism w/ lower Ea = more
collisions with sufficient Ea **You must not say the number of
collisions only... You must say number of collisions per unit time
or frequency 25
Slide 27
In order to increase reaction rates Factor Optimal Condition to
Increase Reaction Rate Reason Particle Size Concentration
Temperature Catalyst 26
Slide 28
Decreasing the particle size of solid reactants increases the
reaction rate. Increasing the temperature increases the reaction
rate. Only the surface particles of a solid react. Breaking the
solid into smaller pieces increases the surface area so more
particles available to react. At higher temperatures the particles
move faster and have more energy. Faster movement means greater
frequency of collisions, more energy means more collisions have the
activation energy required for reacting. 27
Slide 29
Increasing the concentration of the reactants increase the
reaction rate Increasing the pressure of a gaseous reaction
increases the reaction rate. More particles present means greater
frequency of collisions. Increasing pressure translates to
increased concentration by either reducing the volume or increasing
the number of particles. Higher concentration means greater
frequency of collisions. 28
