The Secret behind the Dissolving Pennies

Stem Demo

Introduction

A myriad of colors is produced by several reactions occurring in one apparatus, and it all starts with two pennies. Penny Operated Nitric Acid Fountain Nitric acid acts upon copper to yield nitric oxide in an exothermic reaction. The expanding gas displaces water from another flask; actually bubbles in this set up. When the gas cools, the water is drawn back into the flask, dissolving the nitric oxide and forming a blue solution with the copper ions still there. This eventually will produce a visually stunning reaction.

Materials

  • Nitric acid,15.8M,500 mL

  • Sodum hydroxide solution,0.1 M,20 mL

  • Universal indicator solution, 35 mL'

  • Stopper,1-hole,#7

  • Stopper,2-hole,#8

  • Glass tubing, 5mm O.D.,3 pcs.

  • Plastic tubing, 3/16" I.D.,1/16" wall thickness, 12 ft. length

  • 1000-ml, Boiling flask

  • 500-mL Erlenmeyer flask

  • Distelled watter,800 mL

  • Pre-1982 pennies,2

  • Medicine dropper

Proccedure

  1. Fill the boiling flask with approximately 800 mL of distilled water and add 10 to 12 drops of universal indicator solution\.

  2. using the medicine dropper, add yje 0.1 M NaOH solution is deep blue color

  3. Tightly stopper the boiling flask with the #8 2-hole stopper of the assembled apparatus.

  4. Add approximately 50 mL of concentrated (15.8 M) nitric acid to the Erlenmeyer flask 50 mL of concentrated (15.8M) nitric acid to the Erlenmeyer flask

  5. Holding the 1-hole stopper in one hand, drop the two pennies into the nitric acid and immediately stopper the Erlenmeyer flask. The red-brown gas given off is nitrogen dioxide and is toxic. Therefore., the stoppers MUST be inserted quickly and tightly. This reaction should be done only in an operating fume hood or well-ventilated area.

  6. Observe the color changes. The indicator solution will change from blue to pink, and the nitric acid solution will become green,and a later a light blue/green. The reaction will last about 10 minutes.

  7. When the reaction is finished, shake the Erlenmeyer flask to dissolve all of the red-brown NO2 gas in solution before unstoppering the flask.

Safety regulations !!! Hazards !!!

*This reaction must be done in a fume hood!*

Nitric acid and Sodium hydroxide sokution is corrosive to eyes,skin,and other tissue;strong oxidant; toxic by inhalation; AVOID contact with readily oxidized substances. Universal indicator solution is an alcohol based solution and therefore flammable. Cupric nitrate solution is toxic. The substance being produced: Nitrogen dioxide is an irritant and toxic by inhalation. Wear chemical splash goggles, chemical resistant gloves, and a chemical-resistant apron at all times!

Scientific principle

The Chemistry...

Oxidation of copper metal with a strong oxidizing agent, conc. nitric acid.

In a classic experiment, copper metal is turned into copper(II) ion while the nitrogen(V) in the nitrate ion becomes nitrogen(IV) in the nitrogen dioxide gas.

Charles' Law

As the temperature from the reaction warms the gas, it expands. Later, as it cools, the gas contracts.

Nonmetal oxides are acid anhydrides (also link to acid rain)

Although the nitrogen dioxide gas is noxious and toxic, it dissolves readily in water and make the solution acidic. This can be shown by adding a little indicator to the water and making the water slightly basic before the copper is added to the acid.

Air pressure

As the pressure in the flask is decreased as it cools, the outside pressure pushes the water up the tubing toward the flask. The nitrogen dioxide gas is not pulling the water in.

Descriptive chemistry--copper solutions are green and blue

The colored solutions come from complexes of copper(II) ion in solution. Aqueous copper ion is blue, Cu(H2O)42+ The green must be copper surrounded by nitrates.

EXPERIMENT PICTURES- Following the procedure carefully:)

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