The Coin operated Demonstration

Materials

500 mL florence flask, ring stand, large ring placed below the flask, small ring that fits over the neck of the flask, one-hole rubber stopper, 60 cm glass tubing, a large container of water, concentrated nitric acid, coil of copper wire.

Procedure

Add about 50 mL of concentrated nitric acid to the 500 mL florence flask. Place the coil of copper wire into the acid and stopper with a one-holed rubber stopper fitted with a long tube. The end of the tube is placed into a large container filled with water. The glass tube is bent in such a way as to connect the top of the flask with the bottom of the water container. The water in the container can be stirred by hand or with a magnetic stirrer. It needs to be stirred, however, or else the NO₂ collects above the liquid.

When the copper wire is added to the colorless nitric acid, the solution turns green and a large amount of red-brown gas is formed. The air being displaced by the gas formation can be seen bubbling through the water. The flask gets VERY warm. When enough gas is formed, it bubbles through the water (keep the liquid stirred so most of it will dissolve. The gas that makes it to the top is noxious.) The gas in the flask begins to cool and therefore contracts. As the pressure inside the flask decreases, the outside air pressure begins to push the water back toward the original flask. In addition, the red-brown gas dissolves in the water. Eventually, the water rushes into the flask, the solution turns characteristic blue, and the red-brown gas disappears as it is dissolved.

The Chemistry

Oxidation of copper metal with a strong oxidizing agent, conc. nitric acid: 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. Although the nitrogen dioxide gas is noxious and toxic, it dissolves readily in water and makes the solution acidic. Adding a little indicator to the water and making the water slightly basic before the copper is added to the acid can show this.

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⁺.

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