Solutions: Like Dissolves Like Solubility and Intermolecular Forces

There are different ways to incorporate this demonstration in the instructors presentation on solutions.

Option A: When hexane is added to a solution of aqueous potassium permanganate, it remains as a clear layer on top of the water solution. When water is added to a solution of iodine in hexane, it remains as a clear layer below the hexane solution. 

Option B: Add solid KMnO4 to a test tube containing water.  The KMnO4 dissolves in water to give a purple color. Add solid KMnO4 to a test tube containing hexane.  The KMnO4 does not dissolves in hexane.  Add solid I2 to a test tube containing water.  The I2 does not dissolves in water.  Add solid I2 to a test tube containing hexane.  The I2 dissolves in hexane to give a purple color.  To a test tube containing hexane (top layer) and water (bottom layer) add solid I2The I2 dissolves in the top hexane layer to give a purple color. To a test tube containing hexane (top layer) and water (bottom layer) add solid KMnO4 Students are surprised and impressed to observe solid KMnO4 passing immediately through the top hexane layer and then immediately dissolve in the bottom water layer.

Prior to performing the demonstration, students should classify the type of substances involved in the demonstration as polar, non-polar or ionic. Students should identify the intermolecular forces in the pure substances. 

Substance

Classification: Polar, non-polar or ionic

  Primary IMFs in the Substance

 hexane (C6H14)

 

 

 water

 

 

 potassium   permanganate (KMnO4)

 

 

 iodine (I2)

 

 

Students should predict whether or not a solution will form prior to mixing the solute in the solvent. After performing the demonstration, students should identify the type of primary intermolecular force of attraction present in the solution (the solute-solvent interaction).

MixturePrimary IMF (in the solution

Prediction (soluble or not soluble)

Observation (soluble or not soluble)

Hexane + water

   

Hexane + KMnO4

   

Hexane +I2

   

Water + KMnO4

   

Water + I2

   

 

Curriculum Notes 

This demo is usually performed when discussing the formation of solutions involving intermolecular forces and solubility. 

Instructional Objectives

1.Identify the major types of IMFs in solutions and their relative strengths.

2. Apply the “likes dissolves likes” rule with identity of IMFs to determine if two substance will form a solution.  Predict the relative solubilities of substances.

Lead Time 
One day of lead time is required for this project.
Discussion 

If focusing the discussion of the formation of solutions on intermolecular forces and โˆ†Hmixing, be sure to emphasize the three types of interactions involved in the formation of solutions: solvent-solvent, solute-solute-solute, and solute-solvent.  Prior to performing the demonstration, students should classify the type of substances involved in the demonstration as polar, non-polar or ionic. Students should identify the intermolecular forces in the pure substances. Students should predict whether or not a solution will form prior to mixing the solute in the solvent.

Non-polar solutes such as I2 do not dissolve in polar solvents like water because the I2, having just London dispersion forces, are unable to compete with the strong attraction that the polar solvent molecules have for each other. Thus neither hexane nor iodine dissolves in water.

Polar and ionic solutes do not dissolve in non-polar solvents because they have a stronger attraction for each other than for the non-polar solvent molecules. Thus neither water nor potassium permanganate dissolves in hexane. Iodine is a non-polar molecule because the iodine-iodine bond is a pure covalent bond. The difference in electronegativity between the two Iodine atoms is zero. In solid I2I2 molecules are attracted to other I2 molecules by developing temporary partial positive and partial negative areas around the molecule and are attracted to the temporary partial positive and partial negative areas around other I2  molecules, i.e. London dispersion forces.

Hexane is non-polar because the electronegativity difference between the hydrogen and carbon atoms is very small, โˆ†EN = 0.4 and โˆ†EN between the carbon and carbon atoms is zero.  Liquid hexane molecules are held together by London dispersion forces.

Water is a polar molecule because the electronegativity difference between the hydrogen and oxygen atoms is medium, 0.9. The hydrogen bonded to the oxygen atom develops a partial positive charge because the oxygen being more electronegative is pulling electron density away from hydrogen. This partial positive chare is attracted to the partial negative charge on an oxygen atom of a neighboring water molecule. Water is capable of forming hydrogen bonds between water molecules. Hydrogen bonds are a relative strong intermolecular force of attraction.

Potassium permanganate is an ionic compound (ion-ion).

After performing the demonstration, students should identify the type of primary intermolecular force of attraction present in the solution (the solute-solvent interaction).

Potassium permanganate separates into cations and anions when mixed with a polar solvent, such as water.  During the dissolving process water molecules surround the cations and anions using ion-dipole IMFs.

Materials 
  • 4 - 500 mL Erlenmeyer flasks, containing pure hexane, iodine dissolved in hexane, pure water, and potassium permanganate dissolved in water
  • 2 1L separatory funnels mounted in ring stands
  • 2 large funnels
  • Alternate method (minimizing the amounts of solvents and solutes)
  • label six test tubes: water, water, hexane, hexane, hexane - water, hexane- water.
  • Soild I2 ; solid KMnO4
  • Test tube holder and white background
Procedure 
  • Allow about 10 minutes for this demo.
  • One day of lead time is required for this project.
  • Be sure that the stopcocks on the separatory funnels are closed.
  • Using a funnel, pour the purple aqueous solution of potassium permanganate into one of the separatory funnels.
  • Pour the purple iodine/hexane solution into the other separatory funnel.
  • Pour the pure hexane into the funnel containing the aqueous solution of potassium permanganate.
  • Pour the pure water into the funnel containing the iodine/hexane solution.
  • Place the glass stoppers in the separatory funnels and shake them. Vent them to prevent the buildup of hexane vapor pressure. Be sure that the funnel is not pointing at any one when you vent it. The layers will separate back out.

 

Safety Precautions 
  • Hexane is very volatile and very flammable. Do not perform this demo near a possible source of ignition. Keep a fire extinguisher handy.
  • Iodine is an oxidizer and corrosive. Potassium permanganate is a strong oxidizer. Avoid contact with skin. Wear safety goggles.
  • Be sure that the separatory funnels are not pointing at any one when you vent them.
  • Alternate approach is to use stoppered test tubes.
Footnotes 

References

Montes, I.; Lai, C.; Sanabria, D. Like Dissolves Like: A Classroom Demonstration and a Guided-Inquiry Experiment for Organic Chemistry. J. Chem. Educ. 2003, 80 (4), 447449.

 

Prep. Notes 

Do not try to transport these chemicals in the separatory funnels when you retrieve this demo.They are top-heavy and likely to tip over. Take a few moments after class to separate the fractions back into their respective flasks.

© Copyright 2012 Email: Randy Sullivan, University of Oregon Chemistry Department and UO Libraries Interactive Media Group