Expt 022 -- Mixing and Reactions

A colorful, dramatic chemical reaction is observed when two solutions are created in the same puddle of liquid by adding tiny crystals of potassium thiocyanate and iron(III) chloride hexahydrate to opposite sides of a small puddle of dilute hydrochloric acid.

Safety

Potassium thiocyanate (KSCN) is toxic. Wear goggles and apron. Avoid ingestion. Wash up spills immediately. Wash hands after the experiment.

Procedure

1. Place one large drop of the 1 M HNO₃ the size of a nickel ($0.05) on the acetate sheet. Spread the drop with the plastic pipet tip to make it as flat as possible. Obtain one crystal of approximately 1 mm³ of Fe(NO₃)₃ and one comparably sized crystal of KSCN. Place the crystals opposite one another on either side of the drop. Using toothpicks, pull the solution out to the two crystals simultaneously.

   !!!Click here to See Movie.

2. Note and record the observations.
3. Movie is shown at 10 times faster than the actual speed.

   !!!Click here to See Movie.

4. After several minutes, stir the solution with a toothpick. Note and record the observations. Note rate of reaction.

   !!!Click here to See Movie.

5. If turbulence is present, the reaction is not diffusion limited. The movement of the large crystals and uneven heating cause motion of the liquid rather than diffusion of the solute.
6. Movie is shown at 10 times faster than the actual speed.

   !!!Click here to See Movie.

7. This experiment works better with some acetate sheets than others. If the drop of water has a very round surface, rinse the acetate sheet with a detergent solution. Dry and try again. If the drop of water still has a very round surface, dip a toothpick first in a 1% detergent solution and then in the middle of the drop. Repeat until a flat surface is created. Stir and allow the motion to stop.
8. Wash the acetate sheet at the sink with large amounts of water. Wash hands thoroughly.

Questions

1. Cite the evidence for changes and reaction.
2. State the cause for the delay in seeing evidence of reaction.
3. Flat puddles give clearer results than round puddles. Suggest a reason for this.

Handout Makeup

Name ___________________________ Class ________

Teacher__________________________

SmallScale 022 Mixing and Reactions

Watch the movies.

1. Describe the reaction in detail.
2. Answer the questions.

Curriculum-

This experiment may be performed early in the class when chemical reactions are discussed. It also fits in at any of several other places related to equilibrium, complex ion formation, solutions, and solutes.

Safety-

Potassium thiocyanate (KSCN) is toxic. Wear goggles and apron. Avoid ingestion. Wash up spills immediately. Wash hands after the experiment.

Time-

Teacher Preparation: 5-10 minutes

Class Time: 15 minutes

Materials-

• 1.5 mL of 1 M HNO₃ (Add 33 mL of 3 M HNO₃ to enough water to make 100 mL of solution.)
• 0.1 g of Fe(NO₃)₃•9H₂O -- use as solid; provide 1 g in a small container such as a 1.6 mL capped plastic centrifuge tube
• 0.1 g of KSCN -- use as solid; provide 1 g in a small container such as a 1.6 mL capped plastic centrifuge tube
• Acetate sheet.
• toothpicks
• paper towels

Disposal-

The solutions may be disposed of at the sink washing with large volumes of water.

Answers-

Q1. Cite the evidence for changes and reaction.

A1. The crystals appear to dissolve. A deep red color is observed between the site where the KSCN is added and that where the FeCl₃ is added.

Q2. State the cause for the delay in seeing evidence of reaction.

A2. The reagents must dissolve and diffuse through the water before mixing and reaction can take place.

Q3. Flat puddles give clearer results than round puddles. Suggest a reason for this.

A3. It is easier to see boundaries where reaction takes place for two dimensions than for three.

Key Words 1-

mixing, diffusion, solution, solute, complex ion, equilibrium

Elements-

Fe S C N