F.
G GIRLS INTER COLLEGE KARACHI CANTT
LECTURE PLAN
Subject: Chemistry Date:
Class: XI Federal Board Lesson No: 04
Chapter: 9 Chemical equilibrium Topic: Le Chatelier's Principle
Instructor Name: Mrs Saakhi Sooraj (SST)
Objective / Goal / SLO’s:
1. At the end of a 45min class,
Students will demonstrate an understanding of Le Chatelier's Principle by
explaining how changes in concentration, pressure, and temperature affect chemical
equilibria.
Students will be able to predict the direction of the equilibrium shift in a chemical
reaction when subjected to changes in concentration, pressure, or temperature.
Students will be able to interpret and explain the observed changes in equilibrium
position in terms of Le Chatelier's Principle.
Given a chemical equilibrium scenario, students will apply Le Chatelier's Principle to
devise strategies for maximizing the yield of desired products.
.
Instructor Aids, Tools or Material needed / Resources
Xi fb text book (KPK), soft board, white board, marker
• Lesson Outline: Prior knowledge quest
• What is chemical equilibrium?
Ans: A state in chemical reaction, when the rates of the forward and reverse reactions are equal
• How do you know when a reaction has reached equilibrium?
Ans: If you observe no visible changes in the system over time, it's likely that equilibrium has
been reached.
• Why chemical equilibrium described as a dynamic state?
Ans: Because even though the concentrations of reactants and products remain constant, the
forward and reverse reactions are still occurring at equal rates
• List the factors that can impact the rate of a chemical reaction.
Ans: temperature, concentration of reactants, surface area, presence of a catalyst and nature of
reactants
• How do these factors contribute to the establishment of equilibrium?
Ans: These factors affect how quickly the reaction reaches equilibrium by influencing how fast the
forward and reverse reactions occur.
• Define equilibrium constant (Kc). How are they utilized to measure the extent
of a reaction at equilibrium?
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Ans: Kc is the ratio of product concentrations to reactant concentrations at equilibrium.
Kc quantifies the extent of a reaction by comparing the concentrations of products to
reactants at equilibrium, providing a numerical value that reflects how far the reaction has
progressed.
Le Chatelier's Principle
If a dynamic equilibrium is disturbed by changing the conditions, the position of equilibrium shifts
to counteract the change to reestablish an equilibrium.
Using Le Chatelier's Principle with a change of concentration
Suppose you have an equilibrium established between four substances A, B, C and D.
What would happen if you changed the conditions by increasing the concentration of A?
According to Le Chatelier, the position of equilibrium will move in such a way as to counteract the
change. That means that the position of equilibrium will move so that the concentration of A
decreases again - by reacting it with B and turning it into C + D. The position of equilibrium moves
to the right.
This is a useful way of converting the maximum possible amount of B into C and D. You might use
it if, for example, B was a relatively expensive material whereas A was cheap and plentiful.
What would happen if you changed the conditions by decreasing the concentration of A?
According to Le Chatelier, the position of equilibrium will move so that the concentration of A
increases again. That means that more C and D will react to replace the A that has been removed.
The position of equilibrium moves to the left.
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This is essentially what happens if you remove one of the products of the reaction as soon as it is
formed. If, for example, you removed C as soon as it was formed, the position of equilibrium
would move to the right to replace it. If you kept on removing it, the equilibrium position would
keep on moving rightwards - turning this into a one-way reaction.
Relationship between equilibrium constants and Le Chatelier's Principle
Students often get confused about how it is possible for the position of equilibrium to change
as you change the conditions of a reaction, although the equilibrium constant may remain
the same.
Changing concentrations
The facts
Equilibrium constants aren't changed if you change the concentrations of things present in the
equilibrium. The only thing that changes an equilibrium constant is a change of temperature.
The position of equilibrium is changed if you change the concentration of something present in the
mixture.
Suppose you have an equilibrium established between four substances A, B, C and D.
According to Le Chatelier's Principle, if you decrease the concentration of C, for example, the
position of equilibrium will move to the right to increase the concentration again.
Explanation in terms of the constancy of the equilibrium constant
The equilibrium constant, Kc for this reaction looks like this:
If you have moved the position of the equilibrium to the right (and so increased the amount of C
and D), why hasn't the equilibrium constant increased?
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This is actually the wrong question to ask! We need to look at it the other way round.
Let's assume that the equilibrium constant mustn't change if you decrease the concentration of C -
because equilibrium constants are constant at constant temperature. Why does the position of
equilibrium move as it does?
If you decrease the concentration of C:
Bell Ringer Activity: Chemical Equilibrium Challenge
Instructions:
1. Display the following scenario on the board or screen:
"Consider the following reversible reaction at equilibrium: A(g) + B(g) ⇌ C(g) + D(g)"
2. Ask students to work individually or in pairs to predict the effect of the following changes
on the equilibrium position, using Le Chatelier's Principle:
a) Increasing the concentration of A(g)
b) Decreasing the concentration of A(g)?
c) Removing one of the products of the reaction as soon as it is formed.
d) What happen to the equilibrium constant if you decrease the concentration
of C (g).
Changing pressure
This only applies to reactions involving gases:
What would happen if you changed the conditions by increasing the pressure?
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According to Le Chatelier, the position of equilibrium will move in such a way
as to counteract the change. That means that the position of equilibrium will
move so that the pressure is reduced again.
Pressure is caused by gas molecules hitting the sides of their container. The more
molecules you have in the container, the higher the pressure will be. The system
can reduce the pressure by reacting in such a way as to produce fewer molecules.
In this case, there are 3 molecules on the left-hand side of the equation, but only
2 on the right. By forming more C and D, the system causes the pressure to
reduce.
Increasing the pressure on a gas reaction shifts the position of equilibrium
towards the side with fewer molecules.
What would happen if you changed the conditions by decreasing the pressure?
The equilibrium will move in such a way that the pressure increases again. It can
do that by producing more molecules. In this case, the position of equilibrium
will move towards the left-hand side of the reaction.
What happens if there are the same number of molecules on both sides of the
equilibrium reaction?
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In this case, increasing the pressure has no effect whatsoever on the position of
the equilibrium. Because you have the same numbers of molecules on both sides,
the equilibrium can't move in any way that will reduce the pressure again.
Activity
3. Instructions
4.
5. Display the following scenario on the board or screen:
"Consider the following reversible reaction at equilibrium
N2(g) + 3H2(g) ⇌ 2NH3(g)
6. Ask students to work individually or in pairs to predict the effect of the following changes
on the equilibrium position, using Le Chatelier's Principle:
a) By Increasing the Pressure
b) By Decreasing the Pressure
c) What would happen to equilibrium constant if we decrease the pressure?
Using Le Chatelier's Principle with a change of temperature
For this, you need to know whether heat is given out or absorbed during
the reaction. Assume that our forward reaction is exothermic (heat is
evolved):
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What would happen if you changed the conditions by increasing the temperature?
According to Le Chatelier, the position of equilibrium will move in such a way as to
counteract the change. That means that the position of equilibrium will move so that the
temperature is reduced again.
Suppose the system is in equilibrium at 300°C, and you increase the temperature to 500°C.
How can the reaction counteract the change you have made? How can it cool itself down
again?
To cool down, it needs to absorb the extra heat that you have just put in. In the case we are
looking at, the back reaction absorbs heat. The position of equilibrium therefore moves to
the left. The new equilibrium mixture contains more A and B, and less C and D.
If you were aiming to make as much C and D as possible, increasing the temperature on a
reversible reaction where the forward reaction is exothermic isn't a good idea!
What would happen if you changed the conditions by decreasing the temperature?
The equilibrium will move in such a way that the temperature increases again.
Suppose the system is in equilibrium at 500°C and you reduce the temperature to 400°C.
The reaction will tend to heat itself up again to return to the original temperature. It can do
that by favouring the exothermic reaction.
The position of equilibrium will move to the right. More A and B are converted into C and
D at the lower temperature.
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Summary
Increasing the temperature of a system in dynamic equilibrium favours the endothermic
reaction. The system counteracts the change you have made by absorbing the extra
heat.
Decreasing the temperature of a system in dynamic equilibrium favours the exothermic
reaction. The system counteracts the change you have made by producing more heat.
Activity
Objective: To engage students in applying Le Chatelier's Principle to predict
the effect of temperature change on a chemical equilibrium quickly.
Instructions:
Display the exothermic reaction: "N2 (g) + 3H2(g) ⇌ 2NH3(g)" on the
board.
Prompt students to predict the equilibrium shift when temperature
increases, using Le Chatelier's Principle.
Give students 1 minute to write down their predictions.
Discuss predictions briefly, emphasizing the principle's application.
Conclude by summarizing the principle's use in predicting equilibrium
shifts due to temperature changes.
.
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Assessment and Evaluation. (for reinforcement of concepts).
1. What does Le Chatelier's Principle predict about the response of a chemical
system at equilibrium to changes in its conditions?
2. How would increasing the concentration of a reactant affect the equilibrium
position of an exothermic reaction?
3. If the volume of a reaction vessel containing gases at equilibrium is decreased,
in which direction will the equilibrium shift, according to Le Chatelier's
Principle?
4. Explain why adding a catalyst to a system at equilibrium does not change the
position of equilibrium.
5. What happens to the equilibrium position of a reaction if the temperature is
increased in an endothermic reaction?
6. In the end, worksheet will be provided to the students to solve in class and
handover to the teacher.
References: https://www.chemguide.co.uk/physical/equilibria/lechatelier.html
H.W. Apply Le Chatelier's Principle conceptually by explaining its role in predicting how
chemical equilibrium systems respond to changes in concentration, pressure, and
temperature. Reflect on real-world examples illustrating its practical significance.
