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Investigating Chemical Equilibrium

Description of Activity

Title: Investigating Chemical Equilibrium
Overview: Working in pairs, students will investigate equilibrium systems and their responses to stress.
Duration: one 50 minute lab period
Materials:

Beaker, 100 mL			0.1 M iron (III) chloride  (FeCl3)
Graduated cylinder, 10 mL	0.1 M potassium thiocyanate  (KSCN)
Test Tubes			0.1 M potassium chloride  (KCl)
Test Tube Rack			distilled water
Dropper pipet

Background

A state of equilibrium is affected by concentration, temperature, and pressure. If a system at equilibrium is subjected to change in one or more of these factors, a stress is placed on the system. When a stress is placed on a system at equilibrium, the equilibrium will shift in the direction that tends to relieve the stress.

Teaching and Learning Strategies

This activity may be modified for micro-scale methods.

Students may have problems drawing the conclusions requested. Suggest a data table which includes the balanced equation and the initial observations (color of each species). Added substances can then be circled and final observations denoted by an arrow which shows the direction of increased concentration.

		Fe3+	+	SCN-	<---->	Fe(SCN)2+

		Light Brown	Colorless	Dark Red



		Fe3+	+	SCN-	<---->	Fe(SCN)2+

		Light Brown	Colorless	Dark Red

• Prerequisite Instruction
  
  Student should be familiar with balanced chemical equations and have an understanding of the components of a reversible reaction.
  

Invitation to Learn

What effect will adding different solutions have on an equilibrium system? In this exercise you will investigate an equilibrium system. As you follow the procedures record your observations carefully.

• Procedure
  
  Procedure A: Investigation of Fe3+---SCN- equilibrium
  

  		Fe3+	+	SCN-	<---->	Fe(SCN)2+
  
  		(light brown)			(red)

  
  1. Measure out 5 mL 0.1 M FeCl3 and pour it into a 100 mL beaker. Add 5 ml of 0.1 M KSCN to the same beaker. Dilute the contents of the beaker with distilled water until the solution is a light reddish-orange color. Divide the solution equally among the four numbered test tubes. Set test tube 1 at one end of the rack to be used for color comparison.
  2. Using a dropper pipet, add 0.1 M FeCl3 drop by drop to the solution in test tube 2 until a color change occurs. Record your observations. Rinse the pipet.
  3. Add KSCN and KCl to the next two test tubes. Rinse the pipet after each use. Record your observations in your data table.
  4. Discard the solutions as instructed by your teacher.

Safe Operating Procedures

Safety glasses and apron should be worn at all times while working in the chemistry laboratory.

Dispose of all chemicals appropriately. See Flinn Chemical Catalog.

Summary of Learning
Conclusions and Questions

1. What color is the Fe3+ ion in solution?
2. What color is the FeSCN2+ ion in solution?
3. Were more product or reactant formed when FeCl3 was added to the equilibrium solution? How do you know? Is FeCl3 a product or reactant?
4. Were more product or reactant formed when KSCN was added to the equilibrium solution? How do you know? Is KSCN a product or reactant?
5. Were more product or reactant formed when KCl was added to the equilibrium solution? How do you know? Is KCl a product or reactant?
6. Based on your results, what happens to a system in equilibrium when one side of the equation is stressed (more solution is added)?
7. Hypothesize what would happen to the amount of a substance if the other side of the reaction had one of its components decreased.

2 multiple choice questions

The following questions refer to a hypothetical reversible chemical reaction in which reactant Y is a bright yellow color, reactant C is colorless, and the product B is a bright blue color.

C + Y <--> 3B

1. Adding a large quantity of C gives a:

a. blue color
b. yellow color
c. green color
d. no color change

2. Removing B from the system give a:

a. blue color
b. yellow color
c. green color
d. no color change

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Updated September 25 1997 by Michelle Dumas