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University of Science and Technology of Southern Philippines
1st
Cagayan de Oro City
Semester SY 2021 - 2022
TOPIC 1
CHEMICAL EQUILIBRIUM
2
Outline
V
...
State these ideas in their own words;
•Write the equilibrium constant expression for any reversible reaction;
•Calculate equilibrium composition from initial data and the numerical value of the equilibrium
constant;
•Calculate molar solubility from kspor vise versa;
•Use lechatelier’s principle to explain the response of an equilibrium system to applied stresses;
•Calculate the new equilibrium composition of a system after an applied stress; and
•Explain the importance of both kinetic and equilibrium considerations in the design of industrial
chemical process
...
• Modern concrete includes admixtures, which are additives that
manipulate concrete into having desired properties
...
• This process accounts for an estimated 5% of CO2 released
into the atmosphere annually
...
• The combination is hydrated (water is added) when
concrete is mixed
...
6
7
Concrete Production and Weathering
Energy liberated from concrete hydration as a function of time
...
• Fly ash is generated when coal is burned in power plants
...
• The average composition of fly ash is similar to Portland cement
with the main components being SiO2, Al2O3, Fe2O3, and CaO
...
8
9
Concrete Production and Weathering
• Uses of admixtures
• Water reducers: lower the amount of water in the concrete
without affecting the ability to work with it
...
• Waterproofers: combat effects of moisture
...
9
10
Concrete Production and Weathering
• Accelerators or
retardants: affect the
speed of the
hardening process
...
• Interior of concrete appears pink with phenolphthalein indicator,
which is pink under basic conditions
...
11
12
Chemical Equilibrium
• For complex chemical reactions, there are several variables that
must be considered
...
• Water in an open system, such as a glass, will slowly evaporate,
decreasing the amount of liquid water over time
...
12
13
Forward and Reverse Reactions
This photo sequence shows the water level in two glasses over the
course of 17 days
...
13
14
Forward and Reverse Reactions
• The equilibrium between
liquid and vapor in a
closed container is
governed by the kinetics
of evaporation and
condensation
...
14
15
Forward and Reverse Reactions
• A chemical system reaches equilibrium when the rate of the forward reaction
equals the rate of the reverse reaction
...
15
16
Mathematical Relationships
• For any reaction involving reactants, R, and products, P, the
chemical reaction is written at equilibrium with a double
arrow
...
16
17
Mathematical Relationships
• At equilibrium Ratefor = Raterev
...
• At equilibrium, Q = K, the equilibrium constant, and this ratio becomes
the equilibrium expression
...
Sulfur oxides in particular are
produced in some cases, and they would establish equilibrium if
they did not disperse:
Write the equilibrium expression for this reaction
...
KC
• Equilibrium expressions can be written for gas phase reactions
using partial pressures
...
21
22
Gas Phase Equilibria: KP vs
...
The
relationship between Kc and Kp is:
• Δngas is moles of product gas minus the moles of reactant gas
...
• All equilibrium constants in this text are based on molar
concentrations
...
22
23
Homogeneous and Heterogeneous Equilibria
• Homogeneous equilibria - the reactants and products are in the
same phase, either gaseous or aqueous
...
• Heterogeneous equilibrium expressions do not contain terms for
solids and liquids
...
23
24
Homogeneous and Heterogeneous Equilibria
• The rate of a chemical reaction depends on a number of
factors
...
• The dependence of reaction rate on concentration often
follows relatively simple mathematical relationships
...
• There are two useful forms of the rate law
...
24
25
Homogeneous and Heterogeneous Equilibria
• For the decomposition reaction of CaCO3(s) forming CaO(s) and CO2(g), the
equilibrium constant only depends on the CO2 concentration because CaCO3 and
CaO are solids
...
SOLUTION:
26
27
Numerical Importance of the Equilibrium Expression
• The size of the equilibrium constant indicates the direction a
chemical reaction will likely proceed
...
• For small values of K, K << 1, reactants are favored
...
Magnesium ions show similar behavior
...
Thus magnesium hydroxide has a much
greater tendency to form and will be the more likely product
...
29
30
Adjusting the Stoichiometry of the Chemical Reaction
• When reversing a chemical reaction by switching the reactants and products,
the value of the equilibrium constant for the new reaction is the inverse of the
value of the original equilibrium constant
...
• K’ = Kn or K’ = K1/2
30
31
Example Problem 4:
PROBLEM:
Ammonia is an important starting material for several industrial
processes, including the production of fertilizers, polymers, and
admixture components for cement
...
Write equilibrium expressions for: (a) the reaction as written; (b)
the reverse reaction; and (c) the reaction as written but with all
coefficients in the equation halved
...
• Where K1 and K2 are equilibrium constants for the two
chemical reactions being combined and K3 is the equilibrium
constant for the new combined chemical reaction
...
34
35
Example Problem 5:
SOLUTION:
(1)
CO2 (g) ⇄ CO(g) + ½ O2(g)
(2) H2 (g) + ½ O2(g) ⇄ H2O(g)
___________________________
(3)
H2 (g) + CO2 (g) ⇄ H2O(g)
35
36
Units and the Equilibrium Constant
• The equilibrium constant K is dimensionless
...
• A dimensionless K is required when K is used as the
argument in a natural log function
...
• There are three basic features for the strategy used in any
equilibrium calculation
...
• Write the corresponding equilibrium expression or expressions
...
37
38
Equilibrium Concentrations
• Equilibrium concentrations can be determined from initial concentrations
by realizing:
• The first row contains the initial concentrations
...
• The third row contains the final equilibrium concentrations
...
38
39
Example Problem 6:
PROBLEM:
When hydrogen gas reacts with iodine gas at elevated
temperatures, the following equilibrium is established:
A student measured the equilibrium constant as 59
...
If
one trial begins with a mixture that includes 0
...
050 M iodine, what will be the equilibrium concentrations of
reactants and products?
39
40
Example Problem 6:
• The concentration of HI =
0 initially, so the reaction
will shift to the right to
achieve equilibrium
...
40
41
Example Problem 6:
SOLUTION:
• The final concentrations are expressed in terms of the initial concentration
minus x for the reactants and initial concentration plus 2x for the
products
...
41
42
Example Problem 6:
SOLUTION:
[H2] = [I2] = 0
...
050 – 0
...
010M
[HI] = 2x
[HI] = 2(0
...
080M
42
43
Example Problem 7:
PROBLEM:
The equilibrium constant for the reaction of chlorine gas with
phosphorus trichloride to form phosphorus pentachloride is 33 at
250°C
...
050 M
PCl3 and 0
...
050 M; [Cl2]=0
...
015 – x
[Cl2] = 0
...
0088
[Cl2] = 0
...
050 – x
[PCl3] = 0
...
0088
[PCl3] = 0
...
0088M
46
47
Mathematical Techniques for Equilibrium Calculations
• For complex equilibria that cannot be solved using the
quadratic equation, software packages such as Maple®, and
Mathematica® must be used
...
47
48
Le Châtelier’s Principle
• Le Châtelier’s principle - When a system at equilibrium is
stressed, it responds by reestablishing equilibrium to reduce
the stress
...
• Changes in concentration
• Changes in pressure
• Changes in temperature
48
49
Effect of a Change in Concentration
• For a reaction at equilibrium, a change in concentration for one or
more of the reactants and/or products will disturb the equilibrium
...
• For a reaction at equilibrium, increasing the concentration of one of
the reactants will shift the equilibrium toward the products
...
49
50
Effect of a Change in Concentration
• The increase in product concentration for a reactant concentration
increase can be rationalized by examining the reaction quotient
...
• For a reaction at equilibrium, K = Q
...
• To re-establish equilibrium, Q must equal K
...
51
52
Effect of a Change in Concentration
• Placed in an empty flask, NO2 achieves equilibrium by reacting to form N2O4
...
The system responds to this stress
by shifting the equilibrium toward N2O4
...
A solution of acetic acid in
equilibrium with hydrogen ions and acetate ion, CH3COO-, can
be disturbed in several ways
...
Then use those
predictions to explain how the equilibrium shifts in response to
each stress
...
The sodium ion is not involved in
this equilibrium, but by adding CH3COO- ions, we increase
the concentration of a product
...
Equilibrium must shift toward reactants
so additional CH3COOH will form
...
Because CH3COOH is a
reactant, the increase in concentration of the acid
decreases the value of Q
...
Some of the added
CH3COOH will dissociate, and the concentration of both H+
and CH3COO- will increase
...
Again the sodium ion does not
play a significant role in this perturbation, and at first
glance, it might seem like the hydroxide ion doesn’t either
...
This will decrease the concentration of H+
...
Thus the equilibrium
shifts to form more products
...
• For an increase in pressure, the equilibrium will shift toward
the side of the equation with fewer moles of gas
...
58
59
Effect of a Change in Pressure
• For the equilibrium between NO2 and N2O4, the increase in pressure is offset
by reducing the moles of gas present by forming N2O4
...
0 atm
...
6 atm by
reacting 2 NO2 to form N2O4
...
It can be produced in several ways, including by the reactions
shown below
...
(a)
; pressure is increased
(b)
;
pressure is decreased
60
61
Example Problem 9:
SOLUTION:
(a)
; pressure is increased
There are two moles of gas on the left and four moles of gas on
the right
...
(b) ;
pressure is decreased
There are seven moles of gas on the left and eight moles of gas
on the right
...
61
62
Effect of a Change in Temperature on Equilibrium
• During a temperature change, heat flows in or out of the reacting
system
...
• Heat is treated as a reactant for an endothermic reaction
...
• Decrease the temperature, equilibrium shifts toward the side with
the heat
...
• As temperature increases, the
amount of NO2 increases, as
indicated by the deepening color
of the NO2 gas in the 50oC water
bath (right) compared to the ice
bath (left)
...
64
65
Effect of a Change in Temperature on Equilibrium
• When a catalyst is added to a system at equilibrium, there is
no impact on the equilibrium position
...
• The equilibrium concentrations of products and reactants
do not change
...
• ΔG = 0 at equilibrium
...
• When equilibrium is reached, the change in free
energy is zero
...
67
68
Free Energy and Nonstandard Conditions
• For reactions with negative free energy changes, the equilibrium
is product-favored, or the value of K is greater than 1
...
• The value of the equilibrium constant can be calculated from the
Gibbs free energy change, or vice versa
...
Still, this process is not yet economically
viable
...
69
70
Example Problem 10:
SOLUTION:
ΔG° = -115
...
73 x 1020
70
71
Example Problem 10:
SOLUTION:
71
72
Borates and Boric Acid
• Borax is used as a cross-linker during polymer synthesis
...
• Borates are used in a number of industrial applications
...
• Borates are used to control the temperature at which glass
melts, allowing melted glass to be pulled into fibers
...
• The heat resistance of borosilicate glass is applied to the
production of halogen headlights and the cathode ray tubes found
in traditional television sets and computer monitors
...
• Zinc borates have the important property of retaining their waters
of hydration at high temperatures, which retards fires
...
73
REFERENCES
PRIMARY REFERENCE
L
...
Brown & T
...
Holme
...
OTHER REFERENCES
W
...
Masterton & C
...
Hurley
...
M
...
Silberberg
...
T
...
Brown, et
...
Chemistry: T h e C e n t r a l S c i e n c e
...
S
...
Principles of General Chemistry
...
Chang
...
74