Transcript Equilibrium Notes
Equilibrium Notes
The Concept of Equilibrium
• Ex) elevator, football game, moving walkway
1. Reversible Reactions: Chemical reaction in which the products can regenerate the original reactants . A double arrow is used to express a reversible reaction.
Ex)
2NO 2(g) ↔ N 2 O 4(g)
reactants
yield
products ↔ or or forward reaction reverse reaction Forward Rxn:
2NO 2 → N 2 O 4
Reverse Rxn:
N 2 O 4 → 2NO 2
2. Some reactions can reverse on their own , some under certain conditions , ( temp, pressure or a catalyst ) and some will not reverse. Ex) Single Replacement Rxns:
3CuCl 2(aq) + 2Al (s) → 3Cu (s) + 2AlCl 3(aq)
3. Chemical Equilibrium:
State of a product’s and reactant’s concentrations remaining constant , not equal . This is because the rate of the forward and reverse reactions are equal . The symbol “[ ]” denotes concentration .
A. Reaction rates are affected by concentration . If the concentration of the reactant increases, the rate of the forward reaction will increase .
B. As the reaction proceeds, the concentration of the reactants decrease and the products increases . This will cause the forward reaction to decrease .
4. Reaching equilibrium does not mean the reaction has stopped . It only means the rate of the forward and reverse reactions are equal . This is not a static equilibrium, but a dynamic equilibrium that stays constant over time.
A ↔ B
Concentration becomes Constant
NOT
Equal
Rates
become Equal
Reaction Rates & Equilibrium:
A. Collision Theory: (Theoretical Model) 1. In order for reactions to occur between substances, the particles must
collide
. 2. According to the
Collision Theory
, a successful collision occurs when A. the collision is
energetic
enough, and B. the particles collide with the correct
orientation
.
3.
Effective
collisions
lead
to the formation of products;
ineffective
collisions do
not
lead to the formation of products.
Ineffective Collision – Insufficient Energy - No Products
Effective and Ineffective Collisions
Effective Collision – Sufficient Energy – Forms Products
Effective collisions have enough energy, and the correct orientation to form products. Ineffective collisions revert to the original reactants.
4. A successful collision results in bond
breaking
(endothermic) and bond
forming
(exothermic).
5. The minimum energy needed to produce an
effective energy
collision is called the
activation
for the reaction. It’s abbreviation is
E a
.
Activation Energy
6. A transitional structure results from a successful collision. The structure is present while old bonds are
breaking
and new bonds are
forming
. It is called an
activated complex
and is unstable and short-lived. It is neither
reactant
nor
product
. 7. A substance that increases the rate of a chemical reaction by providing a mechanism with a lower energy of activation is called a
catalyst
.
8. A system that has just one phase is called a
homogeneous
system, (g→g→g) while a system that has more than one phase is called a
heterogeneous
(s→l→g) system.
Endothermic
9. Energy Diagram: shows changes in
energy
during a reaction.
Exothermic
Energy Diagram for a Chemical Rxn.
Reaction Pathway Activated Complex E a forward reaction
(80 kJ)
Energy of Reactants Added Catalyst ∆H = -60 kJ (for.) +60 kJ (rev.) E a reverse reaction
(140 kJ)
Forward Reaction (Exo) Energy of Products Reverse Reaction (Endo)
Factors Affecting Reaction Rates
1. Chemical Kinetics is concerned with the
rate
which a reaction occurs.
at 2.
Reaction rate
is a
measure
of the rate or speed of a chemical reaction. Reaction rate is determined by measuring the change in
concentration
of reactants and products over a certain amount of
time
. These reaction rates are determined experimentally.
3. Rate-influencing factors are those factors that affect
rate
of reactions by altering the
frequency
,
orientation
, or
energy
at which particles collide.
4. According to the Collision Theory, the following 5 factors affect the rate of a reaction: 1.
Nature of Reactants:
A. Structure:
complexity
of bonds broken and formed & the
orientation
.
B. State: Homogeneous systems have reactants and products in the
same
state. Heterogeneous systems have reactants and products in
different
states. Homogeneous systems usually react
faster
and heterogeneous systems react
slower
.
2 liquids would usually react quickly Iron rusts in air very slowly
2.
Temperature
:
(Average kinetic energy!)
of thumb is that every
10°C increase
in temperature
doubles the reaction rate
.
Rule
3.
Concentration
:
increased
concentration
,
increases
# collisions.
Aqueous solutions can
change [conc] or
molarity
,
gases can change
pressure
. Solids and pure liquids like
water
cannot change concentration.
4.
Surface Area
:
Increased
surface area
,
increases
frequency of collisions. This is especially true for
heterogeneous
systems
(s→l→g)
.
5.
Catalyst:
Increases the rate of reaction without being
consumed
in the reaction. Catalysts speed up the reaction rate by
lowering
the
activation energy
needed for the reaction to occur.
Uncatalyzed reaction (slow) Catalyzed reaction (fast)
5.
Inhibitors:
Decrease
the rate of the reaction by taking the place of a
reactant
and
stopping
the reaction (opposite of a catalyst.)
Energy diagram involving a catalyst:
11. What is in the reaction vessel at time = 0?
H 2 + N 2 12. Write the forward reaction: 3H 2 + N 2 → 2NH 3 13. What kind of reaction is the forward reaction?
synthesis it slows down 14. Write the reverse reaction: 2NH 3 → 3H 2 + N 2 15. What kind of reaction is the reverse reaction?
decomposition it speeds up 16. Over time the concentration of which substance(s)
decreases
?
H 2 + N 2 17. Over time the concentration of which substance(s)
increases
?
NH 3 18. Mark on the graph with a dashed line when equilibrium is reached.
19. At equilibrium which substance(s) is(are) present in the greater concentration?
H 2 + N 2 20. Is the
forward
or
reverse
reaction favored?
reverse
(min)
Equilibrium Systems and Stress – Le Chatlier’s Principle
1. Le Chatlier’s Principle states that if a stress is imposed on a system at equilibrium, the equilibrium position will
shift
toward the direction that tends to minimize the stress. (Pure
liquids
and
solids
are not affected by changes in equilibrium.) 2. In chemistry, Le Chatlier’s Principle is used to manipulate the outcome of reversible reactions to
maximize
the amount of
product
produced by altering the temperature, pressure, or removing a reactant or product from a reaction.
Crash Course in Chemistry – Equilibrium Video Khan Academy – LeChatlier’s Principle
The Haber Process
3. An application of Le Chatlier’s Principle is
The Haber Process
. This was used by
Germany
in WWI to produce
ammonia
(NH 3 ) for nitrogen containing explosives .
Today this process is used to create ammonia (
NH 3
) for household cleaners and especially
fertilizers
allowing for a four-fold increase in food production from 1900 to 2000.
4. There are
4
factors which determine whether a reaction favors making reactants
“lies to the left”
or favors making products
“lies to the right”
.
A. Changes in Concentration -Adding/Removing Reactants -Adding/Removing Products B. Changes in Pressure C. Changes in Temperature
Changes in Concentration - (Reactants and Products)
1. A dding
a substance to a system at equilibrium drives the system
AWAY
from that substance and makes more of the substance(s) on the opposite side.
2. R emoving
a substance from a system at equilibrium drives the system toward
REPLACING
that substance.
Ex)
N 2 O 4(g) ↔ 2NO 2(g)
↑ N 2 O 4(g) ↑ NO 2(g) ↓ N 2 O 4(g) ↓ NO 2(g) shift
right
shift shift shift
left left right
making more
NO 2
making more
N 2 O 4
making more
N 2 O 4
making more
NO 2
B. Changes in Pressure
1. Avogadro’s Law
– Equal volumes of gases at the same temperature and pressure have the
same
number of molecules. (
1 mole of any gas at STP =
22.4L
= 6.02 x10 23 particles
) 2. According to
Boyle’s Law
an increase in pressure means a
decrease
in volume. So if the pressure is
increased
on a system at equilibrium, the side which occupies the
lower
volume will be favored. (inverse relationship) 3. If pressure is increased, the reaction will shift in the direction that produces
fewer
moles.
Ex)
2NO 2(g) ↔ N 2 O 4(g)
↑ pressure shift to
right
making more
N 2 O 4 2H 2 O 2(g) ↔ 2H 2 O (g) + O 2(g)
↑ pressure shift to
left
making more
H 2 O 2 H 2(g) + Cl 2(g) ↔ 2HCl (g)
↑ pressure shift to
neither!
making more
same # of moles each side
4.Pressure changes will only affect
gases!
.
C. Changes in Temperature
1. Exothermic Reactions release
heat. If temperature is increased on this system then the reaction which absorbs or uses heat will
increase
. If temperature is decreased on an exothermic reaction, then the reaction which releases heat will
decrease
.
Ex)
H 2(g) + I 2(g) ↔ 2HI (g) + Heat
Increasing the temperature will produce a higher yield of
H 2 + I 2
.
Lowering the temperature will produce a higher yield of
HI
.
C. Changes in Temperature
2. Endothermic Reactions absorb
heat. If temperature is increased on this system then the reaction which absorbs or uses heat will
increase
. If temperature is decreased on an endothermic reaction, then the reaction which releases heat will
decrease
.
Ex)
HEAT + NH 4 Cl (s) ↔ NH 3(g) + HCl (g)
Increasing the temperature will produce a higher yield of
NH 3 + HCl
.
Lowering the temperature will produce a higher yield of
NH 4 Cl
.
Conclusion: These examples have been
illustrations of
Le Chatlier’s Principle
which states that a system at
equilibrium
, when subjected to a
stress
, will temporarily adjust itself to relieve the stress. This means that the
shift
to the right or left, or the
increased
forward or reverse reaction, will be temporary and a new equilibrium will be reestablished.
Practice: 1. 2SO 2(g) + O 2(g)
2SO 3(g) + heat
What conditions of temperature and pressure favor high equilibrium concentrations of SO 3 ?
(
high / low
) pressure; (
high / low
) temperature
1.
3H 2(g) + N 2(g)
2NH 3(g) + hea
t The commercial production of ammonia uses the Haber Process which is expressed by the above equation.
What condition of temperature and pressure will provide a maximum yield of NH 3 ?
(
high / low
) pressure: (
high / low
) temperature
1.
4HCl (g) + O 2(g)
2H 2 O (g) + 2Cl 2(g) + heat
Increasing the temperature
of the reaction will (
increase / decrease
) the forward reaction.
Decreasing the pressure
on the system will (
increase / decrease
) the forward reaction.