KINETICS & EQUILIBRIUM
Textbook chapter 17 & 18
KINETICS = the branch of chemistry concerned with the _____________ of
reactions
COLLISION THEORY = in order for a reaction to occur, reactant particles must
______________. That collision must be effective with the correct
____________________ and ___________________________________
Why isn’t this a good way to chop down a tree??
It is similar to the requirements for a chemical reaction! Reactions involve the
breaking of bonds and forming new bonds.
SIX FACTORS THAT AFFECT REACTION RATE BY CHANGING THE MECHANISM
Mechanism is the ______________ the reaction takes, or a series of steps that
leads from product to reactant.
The following factors change the number of effective collisions that occur
between particles.
1) Nature of Reactants
Depending on what the reactants are, it will take different amounts of energy to
elicit an effective collision.
In general, _____________ bonded substances are slower to react than
_________ substances due to the greater number of bonds that must be broken
for the reaction to occur.
2) Concentration
Most chemical reactions will proceed at a faster rate if the concentration of one
or more reactants is increased.
This affects the _______________ of the colliding particles. The more particles
there are in a given volume . . . the greater the chance for collision . . . the greater
the chance for proper orientation . . . the greater the chance for effective
collisions . . . and thus the higher the reaction rate.
WHICH HAS A HIGHER RATE OF COLLISIONS? A parking lot with 100 cars or 10
cars? ____________________________
3) Surface Area
The more surface that is exposed, the more chances that the reactant particles
can undergo effective collisions.
Crushing a salt cube to dissolve it exposes fresh surface to react. Smaller particles
of the same mass of a large cube of substance have a higher surface area. Affects
only solid reactants, since liquids and gases have maximum surface area (no
chunks to break up)
Which would react faster, crushed garlic or a whole garlic clove?
4) Pressure
An increase in pressure for gases is the same as an increase in _______________,
since the gas molecules are being pushed closer together, resulting in more
collisions and a faster rate.
Little to no effect for liquids or solids!!
Remember Boyle’s Law?
5) Catalyst
Catalysts are substances that increase the rate of reaction by providing a different
and easier pathway for a reaction.
A catalyst takes place in the reaction, but is _________________ when the
reaction is complete.
6) Temperature
Increasing the temperature directly affects the ___________ of the colliding
particles. The higher the temperature, the faster the particles are moving, and
therefore they collide with more energy. So there will be more collisions AND the
collisions will have more energy.
WHICH HAS A HIGHER RATE OF COLLISIONS? Cars travelling at 30 mph or 100
mph? __________________________________
FACTOR
Nature of reactants
Concentration
Surface Area
Pressure
Catalyst
Temperature
INCREASES RATE
SAMPLE REGENTS QUESTIONS
1. Explain how a catalyst may increase the rate of a chemical reaction.
2. As the number of effective collisions between reacting particles increases, the rate of the
reaction:
a) increases
b) decreases
c) remains the same
3. Which of the following pairs of reactants will react most quickly?
a) sodium chloride and silver nitrate
b) water and hydrogen chloride
4. In the reaction 2Mg (s) + O2 (g)  2MgO (s), as the surface area of the Mg (s) increases, the
rate of the reaction
a) increases
b) decreases
c) remains the same
5. Which statement best explains the role of a catalyst in a chemical reaction?
a) a catalyst is added as an additional reactant and is consumed but not regenerated
b) a catalyst limits the amount of reactants used
c) a catalyst changes the kinds of products produced
d) a catalyst provides an alternate reaction pathway that requires less activation energy
6. Consider the following reaction:
A (g) + B (g)  C (g)
As the concentration of A(g) increases, the frequency of the collisions between A(g) and
B(g)
a) increases
b) decreases
c) remains the same
7. Consider the following reaction:
A (g) + B (g)  C (g)
The rate of the reaction can be decreased by increasing the
a) pressure on the reactants
b) temperature of the reactants
c) concentration of A (g)
d) volume in the reaction chamber
POTENTIAL ENERGY DIAGRAMS = diagrams showing the changes in
____________________ contained in the chemical bonds as a reaction proceeds.
X-axis = _____________________________, or progress of the reaction
Y-axis = ____________________________
A potential energy diagram shows the potential energy changes that occur as
reactants become products. It has five distinct regions:
1. The potential energy of the reactants
2. The potential energy gain that must take place in order for old bonds to be
stretched to the breaking point
This is called the ___________________________ (Ea).
3. The potential energy of the transition state, also known as the activated complex.
As particles approach each other in a collision, kinetic energy is converted to
potential energy. If the particles collide in the proper orientation, and activated
complex is formed.
Activated complex = a temporary transitional product that may either break apart
and reform ____________ or rearrange the atoms and form ___________.
4. The potential energy released as new bonds form during a chemical change
5. The potential energy of the products.
When this graph is read from left to right, it is also said to be the energy
of the __________________ reaction. Note that this graph can also be
read for the ___________________ reaction.
Potential Energy Diagram for C2H4 + HCl  C2H5Cl + heat
1) PE of the Reactants
The flat region labeled "Reactants" shows the potential energy of the reactants.
This is the difference between the X-axis and the flat reactants line
2) Activation Energy
The rising part of the graph represents the increase in potential energy that occurs when
reactants collide.
This is the difference between the flat reactant line and the top of the bump
3) PE of activated complex
This is the difference between the X-axis and the top of the bump
4) Heat of Reaction
The falling part of the curve represents the energy released when the final products are
formed. The potential energy difference between the reactants and the products is called the
heat of reaction (ΔH). It represents the net energy change of the reaction.
TABLE I lists the Heats of Reactions for many common reactions
5) PE of the Products
The second flat region represents the potential energy of the products.
This is the difference between the X-axis and the flat products line
Endothermic Reactions
If the potential energy of the products is greater than that of the reactants, then the reaction is
classified as endothermic. Energy must be ___________ into the reaction.
in an ENdothermic reaction energy ENters the system
ΔH is _________________ (PE products – PE reactants)
Energy + A + B --> AB
Exothermic Reactions
If the potential energy of the products is less than that of the reactants, then the reaction is
classified as exothermic. The extra energy is __________________ to the surroundings.
in an EXothermic reaction energy EXits the system
ΔH is _______________________
A + B --> AB + Energy
CATALYSTS
When a catalyst is added, the energies of the reactants and products remain the same,
therefore ΔH also remains the same. The only difference is the ____________________is
lowered!
SAMPLE REGENTS QUESTIONS
Base your answers on the information and diagram below, which represent the changes in
potential energy that occur during the given reaction. Given the reaction: A + B --> C
1) Does the diagram illustrate an exothermic or an endothermic reaction? State one reason, in
terms of energy, to support your answer.
2) On the diagram provided in your answer booklet, draw a dashed line to indicate a potential
energy curve for the reaction if a catalyst is added.
3) Given the reaction: 2 H2(g) + O2(g) --> 2 H2O(l) + 571.6 kJ What is the approximate ΔH for the
formation of 1 mole of H2O(l)?
(1) -285.8 kJ
(2) +285.8 kJ
(3) -571.6 kJ
(4) +571.6 kJ
4) According to Table I, which potential energy diagram best represents the reaction that forms
H2O(l) from its elements?
5) According to Table I, which salt releases energy as it dissolves?
(1) KNO3
(2) LiBr
(3) NH4NO3
(4) NaCl
6) Which statement correctly describes an endothermic chemical reaction?
(1) The products have higher potential energy than the reactants, and the ΔH is negative.
(2) The products have higher potential energy than the reactants, and the ΔH is positive.
(3) The products have lower potential energy than the reactants, and the ΔH is negative.
(4) The products have lower potential energy than the reactants, and the ΔH is positive.
7) A catalyst is added to a system at equilibrium. If the temperature remains constant, the
activation energy of the forward reaction (1) decreases (2) increases (3) remains the same
8) The potential energy diagram below represents a reaction.
Which arrow represents the activation energy of the forward reaction?
What do the other lines represent?
Equilibrium
We read the potential energy diagrams from left to right….reactants  activated complex 
products. Can the reverse happen? YES! Not only can the forward and reverse reaction both
occur, they both occur at the same time. When both forward and reverse reactions occur at the
same rate, the condition is called __________________________.
An equation representing equilibrium uses a double arrow
to show that reactions are
proceeding in both directions. Equilibrium is a state of balance between the rates of two
opposite processes.
It can only occur in a _____________ system (neither reactant
nor product can leave), such as a sealed container.
** Quantities of reactants and products are not necessarily
equal at equilibrium – it is the rates of the forward and reverse
reaction that is equal.
2 types of Equilibrium
PHYSICAL EQUILIBRIUM
Phase equilibrium can exist between two phases of
a substance. For example, melting/freezing, or
evaporation/condensation.
H2O (l)
H2O (g) The double pointed arrow shows
that both reactions are taking place at the same rate.
CHEMICAL EQUILIBRIUM
When reactants are first mixed and no products are
present, only the forward reaction can occur. As
time progresses the concentrations of the reactants will decrease causing the forward reaction
to slow. At the same time concentrations of the products will increase, causing the reverse
reaction to increase. This will continue until the rate of the forward and reverse reaction are
equal and chemical equilibrium is reached.
Solution equilibrium exists between the dissolving and recrystallizing of a solute
NaCl (s)
NaCl (aq)
This equilibrium can be disturbed by temperature or pressure (for gases only!)
Le Chatelier’s Principle
Any change in temperature, concentration or pressure on an equilibrium system
is a stress. Le Chatelier’s Principle says that a system will respond to relieve any
stress on the system.
Systems will shift ______________ from the stress!
Concentration Changes
Addition of any substance will mean the reaction that reduces the amount of that
substance will occur.
If reactants are added = rate of _____________ reaction will increase and more
products will form
If products are added = rate of _________________ reaction will increase and
more reactants will form
Removal of any substance will mean the reaction that produces that substance
will occur.
If reactants are removed = rate of __________________ reaction will decrease
If products are removed = rate of _________________ reaction will decrease
4NH3 + 5O2
4NO + 6H2O + heat
Stress
Effect
System shift
Effect
Effect
Effect
+ NH3
- NH3
Temperature Changes
In the following reaction, heat can be considered to be a product
3 H2 + N 2
2 NH3 + heat
If the temperature is raised, the rates of both reactions will increase, but not
equally.
Stress
+ H2
- N2
Effect
System shift
Effect
Effect
Pressure Changes
Pressure changes do not have an effect on the rate of reaction when only solids or
liquids are involved. Pressure changes only affect _____________. An increase in
pressure increases the concentration of the gas.
CO2 (g)
CO2 (aq)
When the pressure is increased, the concentration of
the gas increases shifting the reaction to the _______________.
When gas is present on only one side of the equation and the pressure is
increased, the reaction will shift _______________ from the gas.
In the case where gas molecules are present on both sides of the equation, an
increase in pressure will have a greater affect on the side with __________ gas
molecules (so reactions will shift toward the side with fewer gas molecules).
When there are the same number of gas molecules on either side of the reaction,
pressure changes have _______________ on the system.
What about a catalyst? A catalyst will affect the rate of both the forward and
reverse reactions equally. A catalyst may cause equilibrium to be established
more quickly but does not change any of the equilibrium concentrations.
12.6 kcal + H2(g) + I2(g)
Stress
Add H2
Add I2
Add HI
Remove H2
Remove I2
Remove HI
Increase
Temperature
Decrease
temperature
Increase pressure
Decrease pressure
2HI (g)
Equilibrium Shift [H2]
xxxxx
[I2]
[HI]
xxxxx
xxxxx
xxxxx
xxxxx
xxxxx
Entropy & Enthalpy
There are two tendencies in nature that help determine whether or not these chemical and
physical changes will occur.
ENTHALPY = tendency in nature to change to a state of lower ____________ (or enthalpy).
______________ reactions move toward a lower energy state because some of the energy
contained in the reactants is released. The products have less Potential Energy than the
reactants do.
Looking at PE diagrams we can see that the activation energy of exothermic reactions is less
than the activation for the reverse endothermic reactions. This means at any given
temperature, the particles are more likely to collide with enough energy to react in the
exothermic direction.
ENTROPY = a measure of ______________________________________
There is a tendency in nature to change to a state of greater randomness or disorder. The
greater the disorder the higher the entropy.
_____ is used to represent entropy
+ S = ______________ in entropy
- S = ______________ in entropy
Examples of entropy change are:

Physical changes from solid to liquid to gas. ___________ has the highest entropy!

More ______________ than _____________. Generally the side of the equation with
the greater number of molecules has a greater amount of entropy.

Higher ______________________ = higher entropy