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Equilibrium and Le Chatelier’s Principle Lab, Lab Reports of Chemistry

Gettysburg CollegeChemistry

chemical equilibrium, static and dynamic equilibrium State Le Chatelier’s Principle

Typology: Lab Reports

2020/2021

Uploaded on 05/12/2021

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ekaram 🇺🇸

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bg1
Equilibrium/Le!Chatelier’s!Principle!
Modified!from!an!activity!originally!created!by!participating!teachers.!2008!Summer!Green!Chemistry!Workshop.
Teacher!Background!Information:!
Traditionally,"equilibrium"experiments"and"Le"Chatelier’s"Principle"are"illustrated"using"the"following"
experiments."In"this"experiment,"everyday,"nonFtoxic"materials"replace"these"materials."
"""exo"
(A) CoCl4
2F""""+""""6H2O""""!""""Co(H2O)6
2+""+""""4ClF
""""""""blue" " ""endo""""""""red"
This"experiment"is"used"to"demonstrate"the"effects"of"both"temperature"changes"and"concentration"
changes"on"an"equilibrium"mixture."
(B) Cr2O7
2F""""+""""H2O""""!""""2CrO4
2F""""+""""2H+
"""""""orange"""""""""""yellow"
This"experiment"is"used"to"demonstrate"the"effects"of"concentration"changes"on"an"equilibrium"mixture."
(C) Fe3+""""+"CNSF""""!""""Fe(CNS)2+
""""""""""yellow"red"
This"experiment"is"also"used"to"demonstrate"the"effects"of"concentration"changes"on"an"equilibrium"
mixture."
Safety!Information:!
Safety"glasses"should"be"used"whenever"working"in"the"lab.!Iodine"is"a"minor"eye"irritant!and!vinegar"and"
ammonia"can"cause"skin"irritation."""
Educational!Goals:!
•Provide"students"with"an"understanding"of"the"concept"of"chemical"equilibrium"and"to"demonstrate
Le"Chatelier’s"Principle,"i.e."if"a"stress"is"applied"to"a"system"at"equilibrium,"the"system"readjusts"to
relieve"the"stress"applied.
Student!Objectives:!Students"will…!
•Explain"the"concept"of"chemical"equilibrium
•Distinguish"between"static"and"dynamic"equilibrium
•State"Le"Chatelier’s"Principle
•Describe"how"to"set"up"an"experiment"that"is"at"chemical"equilibrium
•Predict"the"effect"of"adding"a"stress"to"the"system"at"equilibrium
HIGH!SCHOOL!
Green!Chemistry!
pf3
pf4
pf5
pf8

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Equilibrium/Le Chatelier’s Principle

Modified from an activity originally created by participating teachers. 2008 Summer Green Chemistry Workshop.

Teacher Background Information:

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-­‐

  • 6H 2 O! Co(H 2 O) (^6)

2+

  • 4Cl

-­‐

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-­‐

  • H 2 O! 2CrO (^4)

2-­‐

  • 2H

orange yellow

This experiment is used to demonstrate the effects of concentration changes on an equilibrium mixture.

(C) Fe

3+

  • CNS-­‐^! Fe(CNS)

2+

yellow red

This experiment is also used to demonstrate the effects of concentration changes on an equilibrium mixture.

Safety Information:

Safety glasses should be used whenever working in the lab. Iodine is a minor eye irritant and vinegar and

ammonia can cause skin irritation.

Educational Goals:

  • Provide students with an understanding of the concept of chemical equilibrium and to demonstrate Le Chatelier’s Principle, i.e. if a stress is applied to a system at equilibrium, the system readjusts to relieve the stress applied.

Student Objectives: Students will…

  • Explain the concept of chemical equilibrium
  • Distinguish between static and dynamic equilibrium
  • State Le Chatelier’s Principle
  • Describe how to set up an experiment that is at chemical equilibrium
  • Predict the effect of adding a stress to the system at equilibrium

H I G H S C H O O L

Green Chemistry

Time: 45 minutes

Materials (per group of 2-­‐4 students):

  • Bottle of soda water
  • Balloon
  • Matches
  • Candle
  • Universal indicator
  • 5 Erlenmeyer flasks
  • Black tea
  • Vinegar
    • Ammonia – cleaning solution
    • Tincture of iodine
    • Starch spray
    • Drinking straw
    • Hot plate
    • Ice bath
    • Glass stirring rods
    • Test tube

Teacher Prep:

  • Starch solution: Spray starch into a 400 mL beaker (cover the bottom of the beaker with spray or use 5-­‐ 10 drops of liquid starch). Fill the remainder of the beaker with water and stir.
  • Black tea: Prepare a 1000 mL beaker of black tea the darker the better. Prepare with electric kettle and allow it to cool to room temperature.

Teacher Demonstration Activator (Optional):

Materials:

  • Bottle of soda water
  • 1 balloon
  • Candle
  • Matches
  • Universal indicator

Procedure:

Carry out the Teacher Demonstration and discuss in class as an introduction to Le Chatelier’s Principle and the concept of equilibrium.

  • Introduce to class the idea of a bottle of soda water. Does equilibrium exist inside the bottle?
  • What gas is present in the bottle (dissolved and above the solution)?
  • Remove gas from bottle by shaking and then trapping the gas in the balloon.
  • Test for gas by pouring some of gas over a lighted candle. What can we deduce?
  • Bubble gas into litmus indicator or cabbage juice (or better, limewater). The gas is acidic (or turns limewater milky)– what is the name of the acid in the soda water?
  • Discuss the equilibrium inside the bottle:

CO 2 + H 2 O! H 2 CO 3

Activity 1 – Iodine and Starch

  • Assign pre-­‐lab for homework and review equilibrium
  • Introduce the concept of the starch-­‐iodine complex. Is this an example of an equilibrium reaction?
  • Add a one drop of tincture of iodine to 30 ml of starch solution. Note the formation of a blue-­‐black color.
  • Heat the solution to about 80 °C. Note that the blue-­‐black color disappears. What can we deduce from this?

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 N 2 + 3H 2 2NH (^3) 2NO (^) 2(g) N 2 O (^) 4(g)

Disposal Information:

  • All waste safe for disposal down the drain

Le Chatelier’s Principle

Pre Lab Exercise

Background Information:

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.

Problem:

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

  1. Write out the balanced reaction between AgNO 3 (aq) and CuCl 2 (aq) and identify the precipitate formed.
  2. For each change listed, predict the equilibrium shift, using the reaction from question number one and your knowledge of Le Chatelier’s Principle:

Stress

Direction of Shift (!; "; or no change )

Raise temperature

Lower temperature

Add Ag +^ NO 3 -­‐

Add Na +^ Cl -­‐

Le Chatelier’s Principle

Name: _________________________________________

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:

  1. What effect did heating the test tube have on the concentration of starch-­‐iodine complex? Explain how you know this by using Le Chatelier’s Principle.
  2. What effect did cooling the test tube have on the concentration of starch-­‐iodine complex? Explain how you know this by using Le Chatelier’s Principle.
  3. Which direction is exothermic? Which direction is endothermic? Explain your results.

Activity 2 – Tea

Data and Observations:

Tea + H +^! TeaH + dark light color

Stress Resulting Color

Control

Vinegar addition

Ammonia addition

Questions:

  1. In this activity, how could you determine whether or not a change occurred in equilibrium? Explain.

2. For each reaction in Activity 2, demonstrate how each change can be explained by Le

Chatelier’s Principle. Be specific about where the chemical was added (the stress) and

its impact on the other components of the Tea Equilibrium.

Vinegar (Acetic acid, HC 2 H 3 O 2 ): Tea + H+^ # TeaH+

dark light color

Ammonia (NH 3 ): Tea + H +^ # TeaH+

dark light color

  1. Did these activities help you to understand Le Chatelier’s Principle? Why or why not?
  2. Explain why this is a green chemistry lab.