12
General
Chemistry 2
Quarter 4
Week 2 - Module 2
Chemical Equilibrium
_________________________________________________________
NAME
___________________________________________
SECTION
General Chemistry 2
Quarter 4- Module 2
Chemical Equilibrium
ALVIN L. MONTALLA., LPT
MAeD: Major in Science and Technology
AUTHOR
Lesson 13
Chemical Equilibrium
Section 1
INTRODUCTION
What I Need to Know?
Chemical Equilibrium
When changes occur, there comes a time when no further change appears to take place. Chemical
equilibrium, condition in the course of a reversible chemical reaction in which no net change in the
amounts of reactants and products occurs. A reversible chemical reaction is one in which the products,
as soon as they are formed, react to produce the original reactants. At equilibrium, the two opposing
reactions go on at equal rates, or velocities, and hence there is no net change in the amounts of
substances involved. At this point the reaction may be considered to be completed; i.e., for some
specified reaction condition, the maximum conversion of reactants to products has been attained.
The Concept of Equilibrium
Chemical equilibrium occurs when a reaction and its reverse reaction proceed at the same rate.
•
•
As system approaches
equilibrium, both the
forward and reverse
reactions are occurring.
At equilibrium, the
forward and reverse
reactions are proceeding
at the same rate.
Once equilibrium is
achieved, the amount of
each reactant and
product remains
constant.
Once equilibrium is
achieved, the amount of
each reactant and
product remains
constant.
In a system at equilibrium, both the forward and reverse reactions are running simultaneously. We
write the chemical equation with a double arrow:
The Equilibrium Constant
Forward reaction:
Reverse reaction:
Rate law
Rate Law
The ratio of the rate constants is a constant (as long as T is constant).
The expression becomes
To generalize, the reaction:
Has the equilibrium expression:
This expression is true even if you don’t know the elementary reaction mechanism.
The conditions that pertain to equilibrium may be given quantitative formulation. For example, for the
reversible reaction A ⇋ B + C, the velocity of the reaction to the right, r1, is given by the mathematical
expression (based on the law of mass action) r1 = k1(A), where k1 is the reaction-rate constant and the
symbol in parentheses represents the concentration of A. The velocity of the reaction to the left, r2,
is r2 = k2(B)(C). At equilibrium, r1 = r2, therefore:
SAMPLE: Writing Equilibrium-Constant Expressions
Write the equilibrium expression for Kc for the following reactions:
Answer:
What’s New?
Activity 1: Chemical equilibrium
Write the equilibrium-constant expression, Kc for
DEVELOPMENT
What I Know?
Equilibrium Constant
Consider the hypothetical reversible reaction in which reactants AA and BB react to form
products CC and DD. This equilibrium can be shown below, where the lowercase letters represent the
coefficients of each substance.
aA+bB⇌cC+dD
As we have established, the rates of the forward and reverse reactions are the same at equilibrium, and so
the concentrations of all of the substances are constant. Since that is the case, it stands to reason that a
ratio of the concentration for any given reaction at equilibrium maintains a constant value. The equilibrium
constant (Keq) is the ratio of the mathematical product of the products of a reaction to the mathematical
product of the concentrations of the reactants of the reaction. Each concentration is raised to the power of
its coefficient in the balanced chemical equation. For the general reaction above, the equilibrium constant
expression is written as follows:
The concentrations of each substance, indicated by the square brackets around the formula, are measured
in molarity units (mol/L)
The value of the equilibrium constant for any reaction is only determined by experiment. As detailed in the
above section, the position of equilibrium for a given reaction does not depend on the starting
concentrations and so the value of the equilibrium constant is truly constant. It does, however, depend on
the temperature of the reaction. This is because equilibrium is defined as a condition resulting from the
rates of forward and reverse reactions being equal. If the temperature changes, the corresponding change in
those reaction rates will alter the equilibrium constant. For any reaction in which a Keq is given, the
temperature should be specified.
When Keq is greater than 1, the numerator is larger than the denominator so the products are favored,
meaning the concentration of its products are greater than that of the reactants.
If Keq is less than 1, then the reactants are favored because the denominator (reactants) is larger than the
numerator (products).
When Keq is equal to 1, then the concentration of reactants and products are approximately equal.
Reaction Quotient
The reaction quotient, Q, is used when questioning if we are at equilibrium. The calculation
for Q is exactly the same as for KK but we can only use K when we know we are at equilibrium.
Comparing Q and K allows the direction of the reaction to be predicted.
 Q = K equilibrium
 Q < K reaction proceeds to the right to form more products and decrease amount of reactants so
value of QQ will increase
 Q > K reaction proceeds to the left to form more reactants and decrease amount of products so
value of QQ will decrease
What’s In?
Types of Chemical Equilibrium
There are two types of chemical equilibrium:
 Homogeneous Equilibrium
 Heterogeneous Equilibrium
Homogenous Chemical Equilibrium
In this type, the reactants and the products of chemical equilibrium are all in the same phase. Homogenous
equilibrium can be further divided into two types: Reactions in which the number of molecules of the
products is equal to the number of molecules of the reactants. For example,
 H2 (g) + I2 (g) ⇌ 2HI (g)
 N2 (g) + O2 (g) ⇌ 2NO (g)
Reactions in which the number of molecules of the products is not equal to the total number of reactant
molecules. For example,
 2SO2 (g) + O2 (g) ⇌ 2SO3 (g)
 COCl2 (g) ⇌ CO (g) + Cl2 (g)
Heterogeneous Chemical Equilibrium
In this type, the reactants and the products of chemical equilibrium are present in different phases. A few
examples of heterogeneous equilibrium are listed below.
 CO2 (g) + C (s) ⇌ 2CO (g)
 CaCO3 (s) ⇌ CaO (s) + CO2 (g)
Thus, the different types of chemical equilibrium are based on the phase of the reactants and products.
⇒ Check: Ionic Equilibrium
What is It?
Factor Affecting Equilibrium
According to Le-Chatelier’s principle, if there is any change in the factors affecting the equilibrium
conditions, the system will counteract or reduce the effect of the overall transformation. This principle
applies to both chemical and physical equilibrium.
There are several factors like temperature, pressure and concentration of the system which affect
equilibrium. Some important factors affecting chemical equilibrium are discussed below.
Change in Concentration:
 The concentration of the reactants or products added is relieved by the reaction which consumes
the substance which is added.
 The concentration of reactants or products removed is relieved by the reaction which is in the
direction that replenishes the substance which is removed.
 When the concentration of the reactant or product is changed, there is a change in the composition
of the mixture in chemical equilibrium.
Change in Pressure:
Change in pressure happens due to the change in the volume. If there is a change in pressure it can affect
the gaseous reaction as the total number of gaseous reactants and products are now different. According to
Le Chatelier’s principle, in heterogeneous chemical equilibrium, the change of pressure in both liquids and
solids can be ignored because the volume is independent of pressure.
Change in Temperature:
The effect of temperature on chemical equilibrium depends upon the sign of ΔH of the reaction and follows
Le-Chatelier’s Principle.
 As temperature increases the equilibrium constant of an exothermic reaction decreases.
 In an endothermic reaction the equilibrium constant increases with increase in temperature.
Along with equilibrium constant, the rate of reaction is also affected by the change in temperature. As per
Le Chatelier’s principle, the equilibrium shifts towards the reactant side when the temperature increases in
case of exothermic reactions, for endothermic reactions the equilibrium shifts towards the product side with
an increase in temperature.
Effect of a Catalyst:
A catalyst does not affect the chemical equilibrium. It only speeds up a reaction. In fact, catalyst equally
speeds up the forward as well as the reverse reaction. This results in the reaction reaching its equilibrium
faster.
The same amount of reactants and products will be present at equilibrium in a catalysed or a non-catalysed
reaction. The presence of a catalyst only facilitates the reaction to proceed through a lower-energy
transition state of reactants to products.
Effect of Addition of an Inert Gas:
When an inert gas like argon is added to a constant volume it does not take part in the reaction so the
equilibrium remains undisturbed. If the gas added is a reactant or product involved in the reaction then the
reaction quotient will change.
Examples of Chemical Equilibrium
In chemical reactions, reactants are converted into products by the forward reaction and the products may
be converted into the reactants by the backward reaction. The two states, reactants and products are
different in composition.
After some time of the start of the reaction, the rate of the forward and the backward reactions may
become equal. After this, the number of reactants converted will be formed again by the reverse reaction
such that the concentration of reactants and products do not change any more. Hence, the reactants and
products are in chemical equilibrium.
 N2O4 ⇌ 2NO2
 PCl5 ⇌ PCl3 +PCl2
 N2 + H2 ⇌ 2NH3
Importance of Chemical Equilibrium
It is useful in many industrial processes like,


Preparation of ammonia by Haber’s process: In this nitrogen combines with hydrogen to form
ammonia, the yield of ammonia is more at low temperature, high pressure and in the presence of
iron as catalyst.
Preparation of sulfuric acid by contacts process: In this process, the fundamental reaction is the
oxidation of sulfur dioxide into sulfur trioxide. This involves chemical equilibrium.
Problems on Chemical Equilibrium
1. The equilibrium constant KP for the reaction N2 (g) + 3H2 (g) ⇌ 2NH3 (g) is 1.6 × 10-4 atm-2 at 400oC. What
will be the equilibrium constant of the Chemical equilibrium at 500 oC if the heat of the reaction at this
temperature range is -25.14 kcal?
Solution:
Equilibrium constants at different temperature and heat of the reaction are related by the equation,
log KP2 = -25140/2.303 × 2 [773 – 673 / 773 × 673] + log 1.64 × 10-4
log KP2 = -4.835
KP2 = 1.462 × 10-5 atm-2
2. Given the equation, N2 (g) + 3H2 (aq) ⇌ 2NH3 (g), Find Q and determine which direction the reaction will
shift in order to reach the state of chemical equilibrium.
Solution:
Given, [N2] = 0.04M, [H2] = 0.09M, and K = 0.040
Since only nitrogen and hydrogen concentration is given, it can be assumed as the reactants and ammonia
as the product. Since ammonia concentration is not given it can be assumed to be zero.
As q is the ratio of the relative concentration of products to reactants, here Q =0.
Since K = 0.04 is larger than Q, nitrogen and hydrogen will combine to form product ammonia.
ENGAGEMENT
What’s More?
Activity 2: Equilibrium Constant
Write the equilibrium constant expressions for the following reactions and determine the value of
equilibrium constant
ASSIMILATION
What I Have Learned?
Assessment
Multiple Choice. Choose the best answer form the give option.
1. At equilibrium, the concentrations of the reactants and products will be
A. high
B. low
C. equal
C. negative
2. Which of the chemical reaction does not go to completion and is said to be in a state of
A. ionic equilibrium
B. dynamic equilibrium
C. static equilibrium
C. None of the above
3. For a chemical reaction that has reached dynamic equilibrium at a certain temperature, which condition is
incorrect?
A. Continuous formation of products
B. Equal rate of the forward and reverse reaction
C. Concentration of the reactants is constant
D. Reaction comes to a halt
4. The equilibrium constant for a reaction that has a value of ΔGo = -41.8 kJ at 100oC will be
A. 13.5
B. 1.4 x 10-6
C. -5.87
D. 7.1 x 105
A vessel at 1000 K contains CO2 with a pressure of 0.5 atm. Some of the CO2 is converted into CO
on the addition of graphite. If the total pressure at equilibrium is 0.8 atm, the value of K is
A. 0.16 atm
B. 1.8 atm
C. 2 atm
D. 0.4 atm
II. Answer the following question:
1. Why are the concentrations of pure solids and pure liquids not included in the equilibrium
constant expression?
2. How does a reaction quotient differ from equilibrium constant?
Write the expression for Kc and Kp for the following reactions. Indicate whether the equilibrium
system is homogeneous or heterogeneous.
What I Can Do?
Portfolio
Quick Connect!
Write the connection of Chemical Equilibrium in real life situation
REFLECTION
You will write a personal insight using the prompts below:
I understand that _________________________________________________________________________
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I realize that _____________________________________________________________________________
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References:
• https://byjus.com/
• https://www.builtlean.com/
• https://healthfully.com/
Breaking through chemistry p. 336 - 370
General Chemistry 2 pp. 50 - 52
Brown, Theodore. Chemistry: The Central Science. New York: Pearson,2015.
Chang, R. and Goldsby, K. Chemistry. New York: McGraw-Hill Education, 2010.
“Chemical Equilibrium” Brightstorm. Accessed August 28, 2020.
https://www.brightstorm.com/science/chemistry/the-atom/quantumnumbers/.