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KINETICS AND
EQUILIBRIUM
HOW SUBSTACNCES REACT!
UNIT 6
KINETICS AND EQUILIBRIUM
CHEMICAL KINETICS
A. Definition: Branch of chemistry
concerned with the rate of chemical
reactions and the mechanisms by which
chemical reactions occur
B. Rate- measured in terms of moles of
reactants consumed or moles of
product formed in a unit of time
C. Mechanism –describes the sequence of
events (reactions) by which an overall
reaction takes place
REACTION RATES AND COLLISION THEORY
1. Chemical reactions depend on
collisions between reacting species (
atoms, molecules, ions, or particles)
2. Reactions occur only when reactants collide
with enough energy and in the correct
orientation to form an activated complex –
a high energy intermediate
3. The rate of the reaction is affected by the
number of collisions occurring and the
fraction of these collisions that are
effective.
a. Effective collisions are collisions
in which the reacting species collide
at the correct angle and at the right
speed so that they form an
activated complex
b. Therefore, any factor that causes more
collisions to occur or results in more
collisions being effective (leading to an
activated complex) will increase the
rate of a reaction
FACTORS AFFECTING REACTION RATE
1. Nature of reactants
a. During a chemical reaction bonds are
broken and new bonds are made
b. Covalent substance react slower
than ionic Ionic = metal + nonmetal
Covalent = nonmetal + nonmetal
c. Ionic substances are generally dissolved
in water before being reacted together.
When an ionic substance is dissolved in
water the crystal lattice is broken and ions
are free to move about, collide and react.
Meaning no bonds need to be broken for
the reaction to occur.
2. Concentration of Reactants
a. An increase in the concentration
(amount) of one or more reactant results
in an increase in the reaction rate
because there are more molecules
reacting therefore there is an
increase in the frequency of collisions
c. Concentration is generally measured in
mole/ liters or M
d. For reactants dissolved in a liquid
solvent the concentration is increased
by removing some of the solvent
through evaporation or by adding
more of the solute. Conversely
decreases in concentration can be
achieved by adding more solvent
2. Temperature
a. An increase in temperature will increase
the rate of almost all chemical reactions
because it increases the KE(speed) of
the particles and if the particles move
faster the number of collisions will
increase but so will the number of
effective collisions
3. Pressure
a. An increase in concentration for a gas is
achieved by decreasing the volume the
gas occupies, this is accomplished by
increasing the pressure on the gas.
4. Surface Area
a. Increase the surface area of
reactants increases the rate of the
reaction
b. Important for heterogeneous
reactions( one where the reactants
are in different phases)
c. To increase the surface area of a solid
grind it up.
d. Example.
A given amount of Zinc will react faster
with dilute HCl if the zinc is ground up due
to increased surface area.
5. Reaction Mechanism
a. Most reactions do not occur in a single
step, but in a series of steps known as the
reaction mechanism
b. More steps required the slower
the reaction
c. Reaction Mechanism is not indicated by
a chemical equation
6. Presence of a Catalyst
a. Increase the rate of a chemical reaction by
providing an easier path for the reaction
b. They take part in the reaction by
lowering the activation energy but
they are not used up.
POTENTIAL ENERGY DIAGRAMS
1. A diagram that shows the change in
potential (stored energy) that occurs
during the course of a chemical reaction.
2. During the course of a chemical
reaction if reactants collide with both the
proper orientation and sufficient energy an
activated complex is formed.
a. activated complex-temporary
intermediate product that may either
break apart and reform the reactants
or rearrange the atoms and form a
new product.
3. Activation Energy
a. Definition –Minimum energy
necessary to initiate (start) a
chemical reaction.
b. All reactions require some “start up”
energy.
4. Heat of reaction (ENTHALPY), H,
a. Heat energy is released or absorbed in
the formation of products. That is reactions
are either endothermic or exothermic
b. Δ H represents the difference in
Potential Energy (PE) between the
products and reactants.
c. H = Hproducts – Hreactants
5. Exothermic Reactions
a. Overall Energy is released
b. Products have lower potential energy
than reactants
c. H is negative
6. Endothermic Reactions
a. Overall energy is absorbed
b. Products have a higher PE than
reactants
c. H is positive
7. Sign used when energy is included in a
chemical reaction should not be confused
with the sign for H.
a. The sign of H tells us whether a
reaction is endothermic or exothermic.
b. If H is positive the energy term is
found on the reactant side the reaction
is endothermic.
c. conversely if H is negative the energy
term is found on the product side the
reaction is exothermic.
The units of H and the energy term is
kJ.
8. Table I
a. Heats of reactions for certain chemical
reactions and processes
b. H is measured in kJ.
d.
c. the reaction is exactly equal to H if the
equation is identical to the one listed.
Examples: CH4 + 2O2  CO2 + 2H2O
H = -890.4 kJ
Rewrite the equation with the heat term
added
CH4 + 2O2  CO2 + 2H2O + 890.5kJ
N2 (g) + O2 (g)  2NO (g) H = +182.6kJ
Rewrite the equation with the heat term
added
N (g) + O (g) + 182.6kJ 2NO (g)
d. If it is the same except that the
coefficients are multiples of the ones listed
then H and the energy written in the
reaction will also be a multiple.
Examples
2H2 (g) + 2I2(g)  4HI (g) H = 2(+53kJ)
Rewrite the equation with the heat term
added
2H2 (g) + 2I2(g) + 106kJ  4HI (g)
e. If the reactants and products are switched
then change the sign of H
2NO (g)  N2 (g) + O2 (g)
H = -(+182.6kJ)
Rewrite with the heat term in the equation
2NO (g)  N2 (g) + O2 (g) + 182.6kJ
EQUILIBRIUM
A. Definition: Equilibrium is said to exist
when any reaction takes place under fixed
conditions so that the rate of the forward
reaction is equal to the rate of the reverse
reaction.
B. At equilibrium the rate at which
reactants are converted to products
is equal to the rate at which products
are reconverted to reactants
C. THE QUANTITIES OF PRODUCTS AND
REACTANTS AT EQUILIBRIUM ARE NOT
NECESSARILY EQUAL IT IS THE RATES OF
THE FORWARD AND REVERSE REACTION
THAT ARE EQUAL.
D. A closed system is necessary so neither
reactant nor product escapes.
E. = forward reaction
F.  = Reverse reaction
PHYSICAL EQUILIBRIUM
A. Phase equilibria
1. In general for a closed system there
will exist an equilibrium between phases
where the rate of escape (from the phase)
= the rate of return (to that phase)
2. Equilibrium existing between a solid
and liquid phase at a substances melting
point. At the melting point the rate at
which a solid melts to become a liquid is
equal to the rate at which the liquid
freezes to reform the solid. Rate of
melting = Rate of freezing
3. Exists between liquid and gas
Rate of evaporation = Rate of
condensation
B. Solution equilibrium
1. Solids and liquids
a. Process that goes on in a saturated
solution if any additional material is added.
We see the added chemical fall to the
bottom. What actually occurs is the added
particle dissolves and anther particles falls
out of solution.
b. Solution equilibrium exists when
the rate of dissolving equals the rate
of crystallization.
c. A saturated solution is defined as : A
solution in which an equilibrium exists
between dissolved and undissolved
solute. It cannot hold any more solute
at a given temperature.
2. Gases in liquids.
a. In a closed system, equilibrium may
exist between a gas dissolved in a liquid and
the undissolved gas above the liquid
b. Equilibrium between dissolved and
undissolved gas is affected by temperature
and pressure
c. Increase in temperature decreases
solubility, converse is also true
d. Increases in pressure increases
solubility, converse is also true
C. Chemical Equilibrium
1. State in which the concentration of the
reactants and products of a reaction
remain constant.
2. When equilibrium is reached all
observable properties of the system, such
as color, pressure and temperature remain
constant.
3. When equilibrium is reached the rate of
the forward reaction is equal to the rate
of the reverse reaction
a. During a certain unit of time, the rate
at which the product is made is equal
to the rate at which the Product is
broken down
b. This results in the mass of each
substance remaining constant at
equilibrium
c. This results in a system that is dynamic,
constantly changing, but appears to be
static, unchanging
d. The mass of each substance may only be
changed if the system is disturbed,
meaning something is done so that the
rate of the forward and reverse
reactions are no longer equal.
LE CHATLIER’S PRINCIPLE
A. STATEMENT:
If a stress (disturbance) such as a change
in concentration, pressure or temperature,
is applied to a system in equilibrium, the
equilibrium is shifted in such a way that
tends to relieve the effects of the stress.
B. How to predict which direction the
equilibrium will shift.
1. Choose the arrow (forward or reverse) that points
away from the substance in the reaction that was
increased.
2. Choose an arrow (forward or reverse) that points at
the substance that was decreased
C. How to predict the change in
concentration (amount) of the substances in
the reaction
1. All substances at the tip of the chosen arrow
increase
2. All substances at the tail of the chosen arrow
decrease
D. Terminology
1. If the FORWARD reaction is favored,
the products are favored, or the equilibrium
shifts to the right, it means that the forward
reaction goes faster in the reverse reaction
once the stress is applied
2. If the reverse reaction is favored, the
reactants are favored, or the equilibrium
shifts to the left, it means that the reverse
reaction goes faster than the forward once
the stress is applied
E. Stresses that can be applied to a chemical
system
1. Effect of changing the concentration
a. Increasing the concentration either a
reactant or product in a reaction at
equilibrium will cause the reaction to
go(shift) in such a direction as to
consume the increase. This direction
is said to be favored.
b. Which ever reaction, forward or reverse
that consumes the increase will be
favored, meaning the rate of that reaction,
(forward or reverse) will be increased until
the stress is relieved. Eventually a new
equilibrium is reached.
c. Decreasing the concentration of either a
reactant or product) in a reaction at
equilibrium will cause the reaction to
go(shift) in such a direction as to
produce more of the substance
decreased. Whichever reaction,
forward or reverse, that produces
more of the substance will be favored,
d. Example: [ ] = reads concentration
N2 (g) + 3H2 (g) ↔2NH3 (g) This process is
called the Haber Process it is used in the
commercial production of ammonia
Stress: [ H2] is increased
► Result: forward reaction is favored
because the forward reaction
consumes H2 ,
therefore [N2] will decrease and [ NH3] will
increase. The equilibrium will shift to
the right
Stress: [N2] is increased
► Result: forward reaction is favored
because the forward reaction consumes N2
,therefore [H2]will decrease and [ NH3]
increase. The equilibrium will shift to
the right
Stress: [ NH3] is increased
Result: reverse reaction is favored because
the reverse reaction consumes NH3
,therefore the [N2] and [H2] will increase.
The equilibrium will shift to the left.
Stress: [ H2] is decreased
Result: reverse reaction is favored because
the reverse reaction produces [H2]
therefore the [N2] will increase and the [NH3]
will decrease. The equilibrium will shift
to the left.
►
Stress: [N2] is decreased
► Result: reverse reaction is favored
because N2 is produced by the reverse
reaction, therefore the [H2] is increased
and the [ NH3] will decrease. The
equilibrium will shift to the left.
Stress [NH3] is decreased
► Result: forward reaction is favored
because the forward reaction produces NH3
therefore the [H2] and [N2] will both
decrease. The equilibrium will shift to the
right.
g. Removal of a product as it is formed tends
to cause the forward reaction to go more
toward completion. Continuous removal of
the product may destroy the equilibrium
system by removing all of the substance
necessary for the reverse reaction
h. Products are removed from a reaction by
the formation of a gas, an insoluble
precipitate (table F), or in an ionic
reaction by producing water.
2. Effect of pressure
a. A change in pressure effects chemical
equilibria in which gases are involved.
There is no effect on a system that does
not contain gases
b. An increase in pressure will favor a shift in
equilibrium toward the side of the reaction
that contains the lesser number of
moles ( the side of the reaction with a
smaller sum of the coefficients)
c. A decrease in pressure will favor a shift in
equilibrium toward the side of the reaction
with the greater number of moles ( the side
of the reaction with a larger sum of the
coefficients)
d. If there is not change in the number
of moles than a change in pressure will
not shift the equilibrium
e. Example
N2 (g) + 3H2 (g) ↔2NH3 (g)
4 moles
2moles
Stress: Increase in pressure
► Result: The forward reaction is favored
because the product side has fewer total
moles therefore [N2] and [H2] will
decrease and the [NH3] will increase
Stress: Decrease in pressure
► Result: The reverse reaction is favored
because it has a greater number
of total moles therefore [N2] and [H2] will
increase and the [NH3] will decrease
H2 (g) + Cl2 (g)↔ 2HCl (g)
2 moles
2 moles
Stress: increasing the pressure
Result: No change the equilibrium will
not shift
3. Effect of temperature
a. When the temperature of a system in
equilibrium is raised, the equilibrium is
displaced (shifted) in such a way that heat
is absorbed. Meaning the endothermic
reaction is favored.
b. When the temperature of a system in
equilibrium is lowered, the equilibrium is
shifted in such a way as to release heat.
Meaning exothermic reaction is
favored.
c. Note: The rates of all chemical reactions,
both endothermic and exothermic are
increased when temperature is increased
(by increasing the number of effective
collisions). However, the opposing reactions
are increased unequally, resulting in a shift
in the equilibrium.
d. Example
N2 (g) + 3H2 (g) ↔2NH3 (g) + 22 kcal
Stress: Increase in temperature
► Result: The reverse reaction is favored
because it is the endothermic
reaction therefore the [N2] and [H2] will
increase and the [NH3] will decrease
Stress: Decrease in temperature
► Result: The forward reaction is favored
because it is the exothermic reaction
therefore the [N2] and [H2] will decrease
and the [NH3] will increase.
4. Effect of a Catalyst
a. Increases the rate of both the
forward and reverse reaction equally
and causes no shift in the equilibrium
b. May cause equilibrium to be reached
more quickly but does not affect the point
of equilibrium
SPONTANEOUS REACTIONS?
A. Definition: Spontaneous changes are
changes that are observed to occur
under given conditions
B. Example: If the temperature is above 0C
ice melts but if the reverse, freezing is not
observed at these temperatures.
1. Chemical reactions are also observed
to go in one direction (forward or reverse)
under one set of conditions and in the
opposite direction under other conditions,
or to remain in equilibrium under still
another set of conditions.
C. Factors determining the direction of
spontaneous change
1. There are two fundamental tendencies
in nature which together determine the
direction (forward or reverse) of a
spontaneous change.
a. Minimum Energy
b. Maximum Disorder
2. Energy Changes (Enthalpy Changes)
a. At constant temperature and
pressure a system tends to under go a
reaction that in its final state it has lower
energy than in its initial state
b. Therefore the tendency in nature
favors the exothermic reaction, -ΔH
3. Tendency toward randomness (Entropy)
a. Randomness is the disorder or lack of
order in a system
b. Entropy is a measure of the randomness
or disorder in a system
c. Symbol S
d. The more random the higher the
entropy
e. Nature favors more disorder or higher
entropy; +ΔS
f. Examples of entropy changes
Phase changes illustrate entropy changes
Changes from SL G show an
increase in entropy because the particles
are moving toward a state of greater
disorder. The reverse, cooling, shows a
decrease in entropy
► Systems were the number of particles
increases show increases in entropy, more
particles more disorder
2NO2(g)  2NO(g) + O2(g)
The dissolving of a solid in a solvent
(solutions) show increases in entropy
because dissolving breaks up the crystal
lattice of the solid giving the particles
increased freedom
g. Increase in entropy means that the final
state of a system is more disordered
than the initial state.