Transcript Equilibrium - Paul M. Dorman High School

Equilibrium
A state in which opposing processes of a
system are occurring at the same rate.
1. Physical
(a) Saturated Solution – dissolution and crystallization
(b) Contained Liquid – vaporization and condensation
2. Chemical
(a) Reaction – Forward and reverse reactions occurring at the
same rate.
A true equilibrium can be reached from
either direction.
H2(g) + I2(g) ↔ 2HI(g)
Starting with reactants
Starting with products
Haber Process
Fritz Haber
• Perfected the synthesis of ammonia from hydrogen (H2)
and nitrogen (N2) – 1913
N2(g) + 3H2(g) ↔ 2NH3(g)
• (1) Why is this difficult? . & . (2) Why was this important?
• (1) Nitrogen is triple bonded and a very stable element. :NΞN:
• (2) Ammonia needed for making explosives on a large scale
• (2) Now needed for the production of fertilizers to feed the world.
• Haber used equilibrium concepts to determine what set
of conditions would cause the greatest amount of
ammonia to be produced.
Law of Mass Action
a.k.a.
Equilibrium Expression
• Expresses concentrations, [ ]’s, of
reactants and products @ equilibrium in
terms of a constant, K, the equilibrium
constant.
K = [products]
/ [reactants]
. . . . Equilibrium expression
For reaction:
jA + kB ↔ pR +
/
qS
K = [R]p x [S]q [A]j x [B]k
Characteristics of “K”
1.
2.
3.
4.
Determined experimentally
Value changes with ΔT
Must be determined at equilibrium
If K is large, >1, reaction favors products,
equilibrium lies to the “right”.
5. If K is small, <1, reaction favors the reactants,
equilibrium lies to the “left”.
6. K for the reverse reaction = 1/K
. . . . . K continued
7. In heterogeneous systems, involving
solids, liquids, and/or gases, solids and
liquids are omitted from the expression.
CaCO3(s) ↔ CaO(s) + CO2(g)
K = ??
K = [CO2]1
. . . . . K continued
8. K can be calculated by using molarity or
atm’s, if a gas phase reaction.
if molarity: K expressed as Kc
if atm: K expressed as Kp
Also
Kp = Kc(RT)Δn
R = .0821
atm x L / K x mol
Δn = # gas mol products - # gas mol reactants
??? When will Kp
= Kc?
ICE Calculations / Tables
You are often given incomplete information
about an equilibrium system and asked to
calculate K.
I = initial
C = change
E = equilibrium
Example
• A mixture of 1.50 mol of SO2 and 1.00 mol of O2 is
placed in a 2.00 L vessel. At equilibrium 0.828 mol SO3
has formed.
2SO2(g) +
O2(g)
 2SO3(g)
I
C
E
1.50 mol
1.00 mol
?
?
?
?
?
?
0.828 mol
Kc =
Factors affecting equilibrium
Changes in:
1. Temperature
2. Pressure
3. Concentrations
Le Chatelier’s Principle
“When a stress is applied to a system, the system
behaves in a way to reduce that stress.”
Stresses applied at equilibrium
1. Δ[ ] of a reactant or product *
(a) Addition of a reactant or product
System/reaction will shift to consume that substance
(b) Removal of a reactant or product
System/reaction will shift to form more of the
substance
N2(g) + 3H2(g) ↔ 2NH3(g)
*Changing [ ] of solids and liquids does not effect
equililbrium.
. . . More stresses
2. ΔPressure by changing volume*
(a) V↓ then P↑ - how can we reduce P?
Shift in direction of fewer gas moles
(b) V↑ then P↓ - What happens?
Shift in direction of more gas moles
N2(g) + 3H2(g) ↔ 2NH3(g)
*no shift when # of gas moles equal on both sides
. . . Still more stresses
3. ΔT
Almost always changes value of K
(a) Addition of heat causes a shift in the
endothermic direction.
(b) Removal of heat causes a shift in the
exothermic direction
4. Addition of a catalyst NOT A STRESS
only gets you to equilibrium faster.
Quiz
1.
2.
3.
4.
5.
6.
When a chemical reaction reaches equilibrium we say
that __________ are equal and ______________ are
constant.
The Law of Mass Action is also known as the _____.
Write the law of mass action for the following rxn:
H2(g) + Cl2(g) ↔ 2HCl(g)
What concentration units are used to calculate Kc?
What concentration units are used to calculate Kp?
What does the magnitude of K tell you about the
equilibrium that has been reached?
. . . Quiz continued
7. Write the equation relating Kp to Kc.
8. When does Kp = Kc?
9. If heat is added to a system at
equilibrium, what will the system do?
10. What is the name of the principle that
describes what reactions will do when
they come under a stress?
Solubility Equilibria (Ksp)
• Solubility
• The amount of substance that will dissolve
in a given amount of solvent at a given
temperature.
• Units: grams/100 g water or molarity
(molar solubility)
• Can be affected by adding other
substances to the solution.
Solubility Equilibria cont…..
• BaSO4  considered insoluble according
to solubility rules. . . . . . . . But . . . . . . . .
• Even salts considered insoluble in water
will, to a very small degree, dissociate and
establish an equilibrium.
• For BaSO4 in water:
BaSO4(s) ↔ Ba+2(aq) + SO4-2(aq)
BaSO4(s) ↔
+2
Ba
(aq)
+ SO4
• Write the Equilibrium Expression:
Ksp = [Ba+2] x [SO4-2]
Ksp
-2
(aq)
• Solubility Equilibria problem types:
1.Calculate Ksp from molar solubility
2.Calculate molar solubility from Ksp
3.Will a precipitate form?