Some physical and chemical reactions are capable of reaching equilibrium.
Equilibrium occurs WHEN THE
EQUALS THE
OF THE FORWARD REACTION
OF THE REVERSE REACTION in a closed system.
When equilibrium is reached, IT DOES NOT MEAN that the reactants and
products are of equal QUANTITIES. So...
o Equilibrium is represented by DOUBLE ARROWS ^5 instead
of a single arrow. This allows us to illustrate that the
reactions are proceeding in both directions (forward and
reverse).
o Equilibrium is
which means that it is constantly
or
o Equilibrium means that reactant and product
are
.
* Equilibrium does NOT mean that reactant and product
concentrations are equal
Define equilibrium in terms of reactant and product concentrations:
Define equilibrium in terms of forward and reverse reaction rates:
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TYPES O F EQUILIBRIUM
(all occur in,
*irs
1.
AIL ABOUT
THE
PhysiCQl Ecyuilibrium* Equilibrium that involves physical changes
Q) Phase Equilibrium - occurs during a
Examples:
_ of MELTING =
of FREEZING
(sealed container @ OX)
of EVAPORATION =
CONDENSATION
(sealed container @ 100**C)
of
b) Solution Equilibrium - occurs at a solution's
of DISSOLVING = RATE of CRYSTALLIZATION
example:
2.
Chemical Equilibrium:
of the FORWARD RXN =
of the REVERSE RXN
OR
of BREAKING BONDS =
of FORMING BONDS
^«9ulllxlum.tlw
13
LE CHATELIER's PRINCIPLE
U Chatelier's principle explains H O W A SYSTEM
RESPOND T O
STRESS
WILL
.
= Any change in
,
, or
put upon an system at equilibrium
When a
is added to a system at equilibrium, the system will
order to relieve that
SHIFT
stress and reach a new equilibrium.
= an increase in the
of
the forward
the reverse rxn
SHIFT TO RIGHT (TOWARD PRODUCTS):
Rate of
reaction
(
Reactants
)
Products
*Favors
SHIFT TO LEFT (TOWARD REACTANTS):
Rate of
in
reaction
(
Reactants
Products
*Favors
14
)
bIFFERENT TYPES OF STRESSES:
1)
Concentration
as initial stress: Equilibrium changes (or shifts) when a
reactant or product is added (introduced) or decreased (taken away) in a
reaction that is at equilibrium
When the concentration of a reactant or product is INCREASED:
the reaction will SHIFT AWAY from the increase (use up the excess)
Example 1:
4NH3(g) + 502(g) ^ 4 N 0 ( g ) + 6H20(g) + H E A T
1. If we add H20(g), the system would shift to the
and the [NH3] would
.
2. If we add 02(g), the system would shift to the
and the [NO] would
.
3. If we add H20(g), the system would shift to the
and the [NO] would
.
4. If we added NO(g). which concentration(s) would
decrease?
When the concentration of a reactant or product is DECREASED:
the reaction will SHIFT TOWARD the side that has experienced the
decrease in concentration (replaces what was taken)
Example
2:
4NH3(g) + 502(g) ^ 4N0(g) + 6H20(g) + H E A T
1. If we remove oxygen, the system will shift to the
and the [NH3] will
.
2. If we remove water, the system will shift to the
and the [NO] will
.
3. If we remove ammonia, which concentration(s) will decrease?
4. If we remove NO(g), which concentration(s) would increase?
miOi: ^ AA ' what you Abb, the SYSTEM shifts AWAY from
TT- what YOU TAKE, the SYSTEM shifts TOWARbS
15
2) Temperature
as initial stress: (involves increasing or decreasing
the "HEAT' component of a reaction)
NOTE: HEAT/ENERGY/J/KJ will either be a reactant or a product
A + B
A
+
^
B
+
C
energy
+
D
^
+
HEAT
C
+
D
When temperature (or HEAT) is increased: the reaction will SHIFT
FROM the rxn side containing "HEAT
(in the
direction)
When temperature (or HEAT) is decreased: the reaction will SHIFT
the rxn side containing "HEAT"
(in the
direction)
Example #1:
4NH3(g) + 502(g) ^ 4 N 0 ( g ) + 6H20(g) + H E A T
1. If we added heat, which concentration(s) will decrease?
2. If we added heat, which concentration(s) will increase?
Example #2:
C02(g) + H20(£) + 890.4 k J
U
CH4(g) + 202(g)
3. If we remove heat; which concentration(s) will decrease?
4. If we remove heat, which concentration(s) will increase?
16
3) Pressure
as initial stress: Recall, pressure affects
ONLY!
So every other state (s, I, aq) in the reaction is UNAFFECTED for this
type of stress
INCREASE PRESSURE: rxn shifts to side with
MOLECULES (or least # n\oles of gas)
# GAS
DECREASE PRESSURE: rxn shifts to side with
MOLECULES (or greater # moles of gas)
# GAS
NOTE: If the rxn contains NO GAS MOLECULES or if the rxn has
the SAME # GAS MOLECULES on each side, there is NO EFFECT and
NO SHIFT results from an increase or decrease in pressure
Example 1:
COiCg) ^ COjiaq)
1. If we increase the pressure, the concentrations of which
species will increase?
2. If we increase the pressure, the concentrations of which
species will decrease?
3. If we decrease the pressure, the concentrations of which
species will increase?
4. If we decrease the pressure, the concentrations of which
species will decrease?
17
Example 2:
NaCg) + 3H2(g) ^ 2NH3(g)
1. If we increase the pressure, in which direction will the
equilibrium shift? (Count moles of gases on each side T )
2. If we Increase the pressure, the concentration of which
species will increase initially?
3. If we decrease the pressure, the concentration of which
species will decrease initially?
4. If we decrease the pressure, the concentration of which
species will increase initially?
AND LASTLY...
WHY DO CHEMICAL AND PHYSICAL CHANGES OCCUk?
Turn the page please...
18
ENTROPY (AS): degree of
or
or
or "MESSINESS" in a system; nature tends to proceed
to a state of
entropy, or disorder.
The MORE ORDER you have, the
The LESS ORDER you have, the
ENTROPY in your system.
ENTROPY in your system.
is the most significant factor in determining AS:
Changing from (s) ^ (£) ^ (aq) -> (g) =
ENTROPY
Draw particle diagrams to illustrate each of the following phases:
e
S
aq
INCREASES when a compound
*Entropy DECREASES when a compound
*Entropy
g-
is broken down.
is created and bonds ore formed.
NOTE: I f there is no phase change, count up the # molecules on each side
(RULE: # moles ^ = ENTROPY sP = - A S , # moles t = ENTROPY -t = -AS)
For the following determine if there is an
increase, decrease,
or
no change
in entropy:
1. 2KCI03(s) -> 2KCI(s) + 302(g)
9. U^fyJ * Ch(g) ^
2. HaOd) ^ H20(s)
10. Ag*(ag) + Cl'caq) ^ AgCl(s)
3. Nacg) + 3H2(g) ^
2NH3(9)
11- 2N205(g) ^
2HCI(g)
2N02(g) + 02(g)
4. NaCI(s) ^ Na%g) + C\\^^)
12. 2AI(s) + 2l2(s)
5.
13. H%q) + OH"(oq)^ H20(i)
KCI(s)
6. COzis)
^
^
KCI(i)
COz(s)
^l^^(9) ^
7. H*(aq) * C2H302-(aq)^ HC2H303(I)
15. H20(g) ^
8. C(s) + 02(g) ^ C02(g)
19
2All3(s)
hJ2(g) * 02(g)
H20(s)
ENTHALPY (AH) = the
in a system; nature tends to
enthalpy, or energy
proceed to a state of
energy <
& move to
Exothermic reactions ^
energy state
Most common types of rxns because less energy has to be put in
to get the rxn started (
activation energy)
EXOTHERMIC
activated
complex
Potential
energy
(KJ)
reaction pathway
energy <
& move to a
Endothermic reactions ->
energy state (HIGHER ACTIVATION ENERGY NEEDED)
** Not as common because more energy must be put in to get
the rxn started (
activation energy)
ENDOTHERMIC
activated
complex
Potential
energy
(KJ)
reaction pathway
20