Reaction Rate
and
Equilibrium
Reaction Rate
• Not all reactions occur at the same speed.
• Some reactions are very slow while others are
fast.
Reaction Rate
• The reaction rate of a chemical reaction is a
measurement of the increase in the
concentration of a product or the decrease in
the concentration of a reactant as the reaction
proceeds over time.
N2 + 3H2  2NH3
• The units generally used to express reaction
rate are mol/L . sec.
• What does it mean if the rate of the reaction
above was given as 4.5 x 10-2 mol N2/L . sec?
C(s) + O2(g) → CO2(g)
• Occurs slowly at room T°
and normal [O2].
• Can speed up the reaction
by increasing the T° or
increasing [O2].
• We can explain these
changes in reaction rate
using the “collision theory”.
Collision Theory
• All substances are comprised of millions of
tiny particles in constant motion. These
particles are colliding with each other
constantly in any substance.
• All collisions between particles do not result in
a reaction.
• There are two factors that determine whether
or not a reaction will occur between two
particles that are colliding.
Collision Theory
• Substances most come into contact, (collide) with
enough energy.
Activation Energy
has been supplied
by the force of the
collision.
Activation Energy
• The activation energy is the amount of energy
that must be available in order for a reaction
to occur.
• The sparks generated by
striking steel against a flint
provide the activation
energy to initiate
combustion in this Bunsen
burner.
• The blue flame will sustain
itself after the sparks are
extinguished because the
continued combustion of
the flame is now providing
the necessary energy
through an exothermic
reaction.
Activation Energy
Collision Theory
• Substances most come into contact, (collide) in
the correct orientation (facing the correct way).
Collision Theory
• The collision theory states that reacting
substances most come into contact, (collide)
with enough activation energy, and in the
correct orientation (facing the correct way), so
that their electron shells can rearrange to
form the products of the reaction.
• Therefore any factor which changes the
frequency (how often), or energy of the
collisions will change the rate of the reaction.
Five Factors Affecting
Reaction Rate
• nature of the reacting substances
• concentration
• surface area
• temperature
• catalysts
Nature of the reacting substances
• The type, strength, and number of chemical
bonds or attractions between atoms differ
from one substance to another.
• These differences determine the energy and
orientation of the reacting particles that is
necessary to create an effective collision
resulting in a reaction.
TNT vs Gunpowder
• Explosive materials which react very violently
are known as high explosives. In contrast,
there are some materials that react more
slowly. These are known as low explosives.
They release a large amount of energy, but
due to the relatively slow rate of reaction.
TNT vs Gunpowder
TNT C6H2(NO2)3CH3 is a
high explosive.
Gunpowder is a mixture of
potassium nitrate (KNO3),
sulphur (S8) and charcoal (C).
It is a high explosive.
As the concentration of the reactants
increases, the reaction rate increases.
Why?
Concentration
• Concentration of the reactant refers to the
number of reactant particles within a given
volume.
• If the concentration of the reactants increases
there will be a greater number of collisions.
• The greater the number of total collisions, the
greater the number of “effective” collisions
(collisions that will form product) and the
greater the rate.
Concentration
Concentration and Reaction Rate
A
Steel wool burning in air
(21% oxygen)
Steel wool burning in pure oxygen
(100% oxygen)
As the surface area of the reactants
increases, the reaction rate increases.
Why?
Surface Area
• Increasing the surface area of the reactants
results in a higher number of reaction sites.
• Reaction sites - specific sites on molecules at
which reactions occur.
• Increasing the number of reaction sites
increases the number of total collisions.
• The greater the number of total collisions, the
greater the number of “effective” collisions
(collisions that will form product) and the greater
the rate.
Surface Area
Reaction Rate and Surface Area – Lycopodium Powder
Exit slideshow and play using Windows Media Player. There is a
delay if we try to show this file within the slide show.
As the temperature of the reactants
increases, the reaction rate increases.
Why?
Temperature
• Increasing the temperature increases the
kinetic energy of the particles.
• This results in more frequent collisions and
more energetic collisions.
• Therefore not only are there more collisions
but also a greater percentage of the collisions
have the needed activation energy.
Both light sticks have been activated however
the one on the left was placed in ice water and
the one on the right in boiling water.
Temperature and Reaction Rate
Catalysts increase the rate of
reactions.
Why?
Catalysts
• Catalysts lower the activation energy by
providing an alternate pathway by which the
reaction can occur at a lower energy.
• This results in a greater percentage of the
collisions having the necessary energy to be
effective resulting in an increase in reaction
rate.
• Catalysts are remain unchanged at the end of a
reaction.
Catalysts
lower the
activation
energy
Catalysts lower the activation energy
Left: Partially caramelized cube sugar,
Right: burning cube sugar with ash as catalyst
Enzymes
• Enzymes act as catalysts that lower the activation
energy of a chemical reaction within a living
organism.
• Enzymes carry out their function of lowering
activation energy by temporarily combining with
the chemicals involved in the reaction. These
chemicals that the enzyme combines with are
called the substrate.
• When the enzyme and substrate combine, the
substrate is changed to a different chemical called
the product. The enzyme is not consumed or
altered by the reaction.
Enzymes
Enzymes
• Enzymes are specific for their substrate: A
particular substrate molecule will combine
temporarily with one enzyme type, and the
active site of a particular enzyme will fit only
one kind of substrate. For example, the
enzyme sucrase will attach only to the
substrate sucrose.
Enzymes
Catalysts – Beakman’s World ≈ 6:25
Homework
• Worksheet: Reaction Rate
Reversible Reactions
N2 + 3H2 → 2NH3
• Reactions can normally be reversed.
2NH3 → N2 + 3H2
• Reversible reactions are often indicated by a double
arrow (↔).
N2 + 3H2 ↔ 2NH3
• This shows the forward and reverse reaction.
For a reversible reaction
such as
A + B
↔ C + D
there are actually two reactions
Forward reaction:
A+B → C+D
Backward reaction: C
+
D → A+B
A + B
↔
C + D
Forward reaction:
A + B →
C + D
A and B are used up
C and D are formed
Backward reaction
C + D →
A + B
C and D are used up
A and B are formed
At beginning of the reaction
A + B
↔ C + D
• There is only A and B in the reaction
container (no C and D formed yet)
• Forward reaction is very fast
• No backward reaction occurs yet
A little later
A + B
↔ C + D
• Forward reaction is still fast (since the container
still has mainly A and B)
• Some C and D have been formed
•
•
The backward reaction starts
Backward reaction is very slow (since there is
only a small amount of C and D)
Still some time later
A + B
↔ C + D
• More A and B have been used up
• The forward reaction slows down
• More C and D have been formed
• The backward reaction speed up
Eventually…
A + B
↔ C + D
• the point is reached where the speed (rate) of
the two reactions become equal.
The system is then said to be in
EQUILIBRIUM
Chemical Equilibrium
At equilibrium
• The rate of the
forward reaction
becomes equal to
the rate of the
reverse reaction.
• How could this
graph be adjusted
and still show
equilibrium?
Chemical Equilibrium
At equilibrium
• The forward and
reverse reactions
continue at equal
rates in both
directions.
• For this reason we
often refer to a
“dynamic equilibrium”
Dynamic Equilibrium
Dynamic Equilibrium
• It often appears that a
reaction at equilibrium
has “stopped”. This
however is only
somewhat true.
• What would happen if
the person stopped
running?
Chemical Equilibrium
When equilibrium is
Reached:
• There is no further
change in the amounts
(concentrations) of
reactant and product.
• Concentrations at
equilibrium are constant
(not equal).
Chemical Equilibrium
N2 + 3H2 ↔ 2NH3
• A chemical reaction is at equilibrium when
the forward and reverse reactions are
occurring at the same rate.
• A reaction that has reached equilibrium is
assigned an equilibrium constant (Keq or just
K).
The equilibrium constant expression
N2 + 3H2 ↔ 2NH3
K
eq
=
[products]
[reactants ]
=
[NH 3 ]
2
[N 2 ] [H 2 ]
3
• All we need to write an equilibrium expression is a
balanced equation.
Write the equilibrium expression for:
4HCl+ O2↔2Cl2+ 2H2O
•If we know the concentrations (molarities) we can
calculate a numerical value for K.
Given [CO] = 0.200, [H2O] = 0.500,
[H2] = 0.32 and [CO2] = 0.42
Find K for:
CO + H2O ↔ H2 + CO2
Given [H2S] = 0.706, [H2] = 0.222 and [S2] =
0.111
Find K for:
2H2S ↔ 2H2 + S2
K = 0.0110
Given K = 0.0875 and [N2O4] = 0.0172M.
Find [NO2] for:
N2O4 ↔ 2NO2
Given K = 0.0140 and
[H2] and [I2] are each 2.00 x 10-4M.
Find [HI] for: 2HI ↔ H2 + I2
[HI] = 0.00169 M
LeChâtelier’s Principle
• states that when a stress is applied to
a system at equilibrium, the system
will respond (shift) in a manner that
attempts to undo the stress.
Stresses are…
• Change in concentration ([ ])
(adding or removing substances)
• Change in temperature
(heating or cooling the system)
• Change in pressure
(increasing or decreasing pressure)
CONCENTRATION CHANGE
Increase concentration of a reactant
(add more nitrogen)
N2 + 3H2 ↔ 2NH3 + heat
Equilibrium shifts to the right
(FORWARD reaction is favored because it will
use up the nitrogen)
CONCENTRATION CHANGE
Increase concentration of a product
(add more ammonia)
N2 + 3H2 ↔ 2NH3 + heat
Equilibrium shifts to the left
(REVERSE reaction is favored because it will
use up the ammonia)
CONCENTRATION CHANGE
Decrease concentration of a reactant
(remove some nitrogen)
N2 + 3H2 ↔ 2NH3 + heat
Equilibrium shifts to the left
(REVERSE reaction is favored because it will
replace the nitrogen)
CONCENTRATION CHANGE
Decrease concentration of a product
(remove the ammonia)
N2 + 3H2 ↔ 2NH3 + heat
Equilibrium shifts to the right
(FORWARD reaction is favored because it will
replace the ammonia)
TEMPERATURE CHANGE
Increase the temperature.
(heat is added)
N2 + 3H2 ↔ 2NH3 + heat
Equilibrium shifts to the left
(REVERSE reaction is favored because it will
use up the added heat)
TEMPERATURE CHANGE
Decrease the temperature.
(heat is removed)
N2 + 3H2 ↔ 2NH3 + heat
Equilibrium shifts to the right
(FORWARD reaction is favored because it will
replace the heat that was removed)
Pressure Change
• Pressure can change by adjusting the volume.
Pressure Changes
• The side of the reaction with the greater
number of moles of gas will create higher
pressure.
• The side of the reaction with the lesser
number of moles of gas will create lower
pressure.
PRESSURE CHANGE
Increase the pressure.
(volume of the container is decreased)
N2(g) + 3H2(g) ↔ 2NH3(g) + heat
Equilibrium shifts to the right
(FORWARD reaction is favored because it will change
4 moles of gas into 2 moles of gas therefore returning
to a lower pressure)
PRESSURE CHANGE
Decrease the pressure.
(volume of the container is increased)
N2(g) + 3H2(g) ↔ 2NH3(g) + heat
Equilibrium shifts to the left
(REVERSE reaction is favored because it will change 2
moles of gas into 4 moles of gas therefore returning
to a higher pressure)
Le Chatelier’s Principle
2 NO2(g)
N2O4(g)
Ho = -57.20 kJ
Disturbance
Equilibrium Shift
Add more NO2………………
Add more N2O4…………….
Remove NO2………………
Add a catalyst……………..
Decrease pressure…………
Decrease temperature….…
no shift
Homework
• Worksheet: Equilibrium