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chemical equilibrium, static and dynamic equilibrium State Le Chatelier’s Principle
Typology: Lab Reports
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Modified from an activity originally created by participating teachers. 2008 Summer Green Chemistry Workshop.
Traditionally, equilibrium experiments and Le Chatelier’s Principle are illustrated using the following experiments. In this experiment, everyday, non-‐toxic materials replace these materials. exo (A) CoCl (^4)
2-‐
2+
-‐
blue endo red
This experiment is used to demonstrate the effects of both temperature changes and concentration changes on an equilibrium mixture.
(B) Cr 2 O (^7)
2-‐
2-‐
2H
orange yellow
This experiment is used to demonstrate the effects of concentration changes on an equilibrium mixture.
(C) Fe
3+
2+
yellow red
This experiment is also used to demonstrate the effects of concentration changes on an equilibrium mixture.
Safety glasses should be used whenever working in the lab. Iodine is a minor eye irritant and vinegar and
H I G H S C H O O L
Materials:
Carry out the Teacher Demonstration and discuss in class as an introduction to Le Chatelier’s Principle and the concept of equilibrium.
moving up. There is no overall change in your position because the two opposing motions are balanced.
Chemical equilibrium is a state of dynamic balance where the rate of the forward reaction is the same as the rate of the backward reaction. Examples of reactions at equilibrium are:
N 2 + 3H 2! 2NH (^3) 2NO 2 (g)! N 2 O 4 (g)
You may have come across the word “equilibrium” in your study of physics. If you look up “equilibrium,” you will find it explained using words like “state of balance.” A meter stick which is suspended at its center of gravity is said to be balanced or in equilibrium. A meter stick suspended at its center of gravity remains stationary or static. Thus, this type of equilibrium is often referred to as static equilibrium. In other words, the entire system is not moving. Consider now the case of a man running on a treadmill. Overall, there is no change in the position of the man. He is running forward at the same speed that the belt is moving in the opposite direction. The two opposing motions balance each other. This is an example of a type of equilibrium called dynamic equilibrium , the word dynamic means “moving.” Another example of a dynamic equilibrium is if you walk down an escalator at the same speed as it is moving up. There is no overall change in your position because the two opposing motions are balanced.
How can Le Chatelier’s Principle be used to predict the direction in which a system at equilibrium will shift when conditions are altered? The equilibrium system that we will study in the Pre-‐Lab is:
CuCl 2 (aq)! Cu +2^ (aq) + 2Cl -‐1^ (aq) + heat green blue colorless
Stress
Direction of Shift (!; "; or no change )
Raise temperature
Lower temperature
Add Ag +^ NO 3 -‐
Add Na +^ Cl -‐
Activity 1 – Iodine and Starch
Data and Observations:
Iodine + Starch! Starch-‐Iodine Complex colorless blue-‐black
Stress Resulting Color
Control
Raise temperature
Lower temperature
Questions:
Activity 2 – Tea
Data and Observations:
Tea + H +^! TeaH + dark light color
Stress Resulting Color
Control
Vinegar addition
Ammonia addition
Questions: