QUALITATIVE CHANGES IN EQUILIBRIUM-LE CHATELIER'S PRINCIPLE
What is Le Chatelier's Principle?
Le Chatelier's Principle refers to the actions taken by a system to restore equilibrium once it has been disturbed by an outside force. The formal definition for Le Chatelier's Principle is:
When a chemical system at equilibrium is disturbed by a change in a property, the systems adjusts in a way that opposes the change.
Le Chatelier's Principle refers to the actions taken by a system to restore equilibrium once it has been disturbed by an outside force. The formal definition for Le Chatelier's Principle is:
When a chemical system at equilibrium is disturbed by a change in a property, the systems adjusts in a way that opposes the change.
LESSON OVERVIEW (SPECIFIC EXPECTATIONS: E2.2, E3.3)
Lesson Hook:
Le Chatelier's Principle is a hard concept for students to grasp. A common problem is they do not comprehend that Le Chatelier's principle is a reaction to a disturbance. In order to drive that point home, a good lesson hook to introduce the concept is given below. It is a simple thought experiment, asking students to think about their work-life balance and what happens when that balance gets disturbed. Starting the lesson this way allows for students to think about Le Chatelier's principle as a RESPONSE to a disturbance. Please click below for a detailed version of Le Chatelier's Thought Experiment:
Le Chatelier's Principle is a hard concept for students to grasp. A common problem is they do not comprehend that Le Chatelier's principle is a reaction to a disturbance. In order to drive that point home, a good lesson hook to introduce the concept is given below. It is a simple thought experiment, asking students to think about their work-life balance and what happens when that balance gets disturbed. Starting the lesson this way allows for students to think about Le Chatelier's principle as a RESPONSE to a disturbance. Please click below for a detailed version of Le Chatelier's Thought Experiment:
Misconceptions Addressed: Students have difficulty understanding that Le Chatelier's Principle is about perturbing an equilibrium state, driving the system to a new equilibrium state.
Action:
Once the thought experiment has been completed and the formal definition has been introduced, a good idea is to ask students to think back to the "Really Dynamic Equilibrium" activity that was done at the beginning of the unit. In particular, ask students what would happen to the equilibrium if more "paper wads" were added to one side. If time allows, it may be a good idea to repeat the activity but with the emphasis on Le Chatelier's principle by focusing on adding either more reactant or more product.
Note: Before students can fully grasp Le Chatelier's principle they must have a good understanding of exothermic and endothermic reactions as well as a good grasp on the relationship between pressure and volume. It may be a good idea to review these concepts before introducing this topic.
Le Chatelier's Principle can be broken down into three distinct "disturbances to the system"-a change in concentration of either products or reactants, a change in the temperature of the system, and a change in the volume (or pressure) of the system. In all three cases, Le Chatelier's principle states that the system will work to COUNTERACT the disturbance by shifting either left or right to restore equilibrium. This concept is met with some difficulty for some students, and the introduction of a computer simulation or model may aid in driving that fact home. Click below for a good computer simulation courtesy of the University of Colorado (PhET). When running this simulation, have students first start with molecules of "A" only, once equilibrium is established, introduce disturbances by having them add molecules of "B" to the system and observe the resulting system and then add and remove heat and again, observe the resulting system.
Note: You can have students run the simulation first to discover the effects of these disturbances and then introduce Le Chatelier's principle as well for a more inquiry based approach to this concept.
Action:
Once the thought experiment has been completed and the formal definition has been introduced, a good idea is to ask students to think back to the "Really Dynamic Equilibrium" activity that was done at the beginning of the unit. In particular, ask students what would happen to the equilibrium if more "paper wads" were added to one side. If time allows, it may be a good idea to repeat the activity but with the emphasis on Le Chatelier's principle by focusing on adding either more reactant or more product.
Note: Before students can fully grasp Le Chatelier's principle they must have a good understanding of exothermic and endothermic reactions as well as a good grasp on the relationship between pressure and volume. It may be a good idea to review these concepts before introducing this topic.
Le Chatelier's Principle can be broken down into three distinct "disturbances to the system"-a change in concentration of either products or reactants, a change in the temperature of the system, and a change in the volume (or pressure) of the system. In all three cases, Le Chatelier's principle states that the system will work to COUNTERACT the disturbance by shifting either left or right to restore equilibrium. This concept is met with some difficulty for some students, and the introduction of a computer simulation or model may aid in driving that fact home. Click below for a good computer simulation courtesy of the University of Colorado (PhET). When running this simulation, have students first start with molecules of "A" only, once equilibrium is established, introduce disturbances by having them add molecules of "B" to the system and observe the resulting system and then add and remove heat and again, observe the resulting system.
Note: You can have students run the simulation first to discover the effects of these disturbances and then introduce Le Chatelier's principle as well for a more inquiry based approach to this concept.
Possible questions to ask the students during or after the simulation would be:
Misconceptions Addressed: Equilibrium constants are constant under all conditions.
- What happened to the reaction when molecules of "B" were added?
- What happened when heat was added to the system?
- What do you think would happen if we added more of molecule "A" once equilibrium was established?
Misconceptions Addressed: Equilibrium constants are constant under all conditions.
Debrief:
To check for understanding of Le Chatelier's principle, have students write the definition in their own words and provide an example as an exit slip for class. You could also do a Think-Pair-Share activity here, have students individually write down their definitions, then share with a partner and refine if necessary and then share with the class. Either activity allows you to do a formative assessment and review once again if necessary.
FOPP: Commitment to students and student learning, professional knowledge and ongoing professional learning.
To check for understanding of Le Chatelier's principle, have students write the definition in their own words and provide an example as an exit slip for class. You could also do a Think-Pair-Share activity here, have students individually write down their definitions, then share with a partner and refine if necessary and then share with the class. Either activity allows you to do a formative assessment and review once again if necessary.
FOPP: Commitment to students and student learning, professional knowledge and ongoing professional learning.
The concept of Le Chatelier's Principle can be reinforced in a variety of ways to appeal to all forms of multiple intellegences. Click below for a few resources that can be used in conjunction with a lesson to further demonstrate Le Chatelier's Principle