Friday, November 6, 2015

Nitric Acid Acts Upon Copper

The beautiful blue copper(II) solution is a great contrast to the brown nitrogen dioxide gas.
"Nitric acid acts upon copper," what does that really mean? In this fabulous demonstration, my students got to see it first hand. I brought out the fancy 2 liter round bottom flask to give this beautiful chemical reaction the extra flare it deserves. (Flinn Scientific, ChemFax Publication No. 91624)
The reaction starts right away when the nitric acid touches the penny.
The reaction set up: round bottom flask, tubing, basic solution.
The demonstration starts with a pre-1982 penny, freshly shined by one of my students. These old pennies are mostly copper, unlike the newer pennies that are actually copper-plated zinc. The shiny penny goes into the round bottom flask. I connected the flask to a 500-mL graduated cylinder filled with a slightly basic solution. The reaction starts with the addition of twenty milliliters of concentrated nitric acid into the flask. The brown gas, nitrogen dioxide, is produced immediately; with a vigorous bubbling and frothing. The brown gas spits and bubbles while a beautiful green liquid pools in the bottom of the flask. The gas begins to bubble through the pink solution, and eventually the pink color fades. The nitrogen dioxide reacts with the water to produce nitric acid in the cylinder, which drops the pH of the solution in the cylinder to turn the phenolphthalein indicator to colorless. When the reaction stops, the bubbling ends and the water slowly begins to creep back up the tube. Once the water drops into the flask, the temperature drops dramatically and then the water drains quickly from the cylinder into the flask. The big finale happens when the water gets sucked back into the round bottom flask and makes a loud slurping sound.
The round bottom flask filled with nitrogen dioxide.
I haven't done this demo for several years. I'm not sure why it fell off my radar, but it is one of my favorites. The truth is, I thought that it was too much set up and took too long to do this demonstration. I pulled it out of the "closet" because my students saw it in one of my homework videos and asked for it specifically. With their inspiration, I realized that this demo is the perfect fit for my new block schedule. With the longer classes, I can really enjoy this demo from the apparatus set-up to the final slerp of the water out of the graduated cylinder.

You can see the color change in the graduated cylinder here.

My student photographer took a shot from beneath the flask. So cool.

This reaction has it all: color change, brown gas, blue solution, heat exchange, and gas laws.

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