Lab

Summary

In this lesson, students witness a chemical reaction that involves chemicals of various states of matter. A quantitative piece could be added to the lesson that has students carry out a stoichiometric problem.

Grade Level

High school

Objectives

By the end of this lesson, students will

• Recognize that a chemical change can take place via multiple step reactions.
• Witness a combustion reaction

Chemistry Topics

This lesson supports students’ understanding of

• Stoichiometry
• Combustion

Time

Teacher Preparation: 15 minutes

Lesson: 30 minutes

Materials

• Goggles
• Large test tube
• 6–8 M NaOH
• Ring stand
• Test tube clamp
• 2-hold stopper
• Right angle bends (2)
• Pneumatic trough
• Wate
• Gas supply
• Clamp
• Rubber tubing (2)
• Bunsen burner
• Yellow phosphorous
• Spanish moss (for dramatic effect)
•  Night sounds (for dramatic effect)
•  “The Will o’ the Wisp” poem (for dramatic effect)

Safety

• This reaction must be done in a hood.
• To clean up, let the test tube cool. Sweep the system with the gas supply to push any remaining PH₃ out of the system. Unstopper the test tube and pour its contents into a beaker of cool water for dilution. Neutralize with vinegar before discarding. Rinse all delivery hoses and glass bends with cool water.
• When working with base, if any solution gets on your skin, thoroughly flush your skin with water immediately.
• An operational fire extinguisher should be in the classroom.

Teacher Notes

• It takes some heating time to initiate the reaction and to have PH₃ bubbles break through the water’s surface in the trough. Be patient!
• I use this same set up all day long and make sure that the phosphorus is consumed by the end of my last class.

Downloads

• Teacher Guide.doc
• Teacher Guide.pdf
  
  

Submitted by
Linda Ford
The Seven Hills School
Cincinnati, Ohio

Thanks to
Ward’s Science

For the Student

Lesson

The Setup

Place your apparatus in a ventilation hood. Half fill a large test tube with 6–8 M NaOH solution and clamp the test tube to a ring stand. Add a pea-size piece of yellow phosphorus to the test tube and stopper tightly with a 2-hole stopper. There should be two right-angle bends placed into the stopper. One delivery hose to the stopper is connected to a nonoxygen gas supply. You could use nitrogen; I use a table gas outlet. The other delivery hose is connected to the bottom hole of a pneumatic trough; the trough should be filled about two-thirds with tap water. Initially, the system is cleared of air by sweeping the gas through until several bubbles emerge from the water’s surface in the trough. Turn off the gas and clamp the delivery hose from the gas source tightly. Now the phosphorous is ready to be gently heated with a Bunsen burner. I use this same set up all day long and make sure that the phosphorus is consumed by the end of my last class. It takes some heating time to initiate the reaction and to have PH₃ bubbles break through the water’s surface in the trough. Be patient!

     To clean up, let the test tube cool. Sweep the system with your nonair gas supply to push any remaining PH₃ out of the system. Unstopper the test tube and pour its contents into a beaker of cool water for dilution. Neutralize with vinegar before discarding. Rinse all delivery hoses and glass bends with cool water.

The Setting

The students enter a darkened room and gather in front of the ventilation hood (turned off during the demo) draped in Spanish moss. I ask them to envision they are on the edge of a swamp on a warm spring evening. As I gently heat the large test tube, I play night sounds such as chirping crickets and rustling wind through leaves. Then I recite Jack Prelutsky’s poem, “The Will o’ the Wisp” (see below for poem). Phosphine (PH₃) gas from the test tube is delivered over to the pneumatic trough filled with water. A flicker of flame accompanies the bursting of each gas bubble at the water’s surface. White smoke rings circle up and away from the trough.

This is my favorite demonstration because it combines interesting chemistry with the serenity of Mother Nature. Postdemonstration discussion can include the source of phosphorus in the swamp, handling of hazardous laboratory chemicals, balancing reactions, gas laws, and stoichiometry.

“The Will o’ the Wisp” by Jack Prelutsky

You are lost in the desolate forest
Where the stars give a pitiful light,
But the faraway glow of the will’o the wisp
Offers hope in the menacing night.
It is lonely and cold in the forest,
And you shiver with fear in the damp,
As you follow the way of the will o’ the wisp
And the dance of its flickering lamp.

But you know as you drudge through the forest
Toward the blistering torch in the gloom,
That the errie allure of the will o’ the wisp
Summons you down to your doom.
It will lead you astray in the forest
Over ways never traveled before.
If ever you follow the will o’ the wisp,
You’ll never be seen anymore.

The Chemistry

In the test tube

3 NaOH (aq)  +   P₄ (s)  +  3 H₂O (l)  ⇾  3 NaH₂PO₂ (aq)  +  PH₃ (g)

Combustion

PH₃ (g)  +  2 O₂ (g)  ⇾  H₃PO₄ (s) 

white smoke rings = H₃PO₄

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