Wednesday, September 28, 2011

Chemical Proportions and Electrolysis

In preparing for Middle School Chemistry with our co-op our family we are going through Matter and Energy: Principles of Matter and Thermodynamics . This volume has two chapters about chemical reactions and the periodic table so it makes a good transition.

The first chapter is about the Law of Conservation of Matter, and while we just read the chapter I've been inspired by something I found here.  I modified it for homeschoolers, taking out much of the school jargon and requirements, and posted it here.  I hope to try it and blog about it soon.

Meanwhile, the second chapter is "How the Elements Combine."  Proust first figured out that chemicals combine by mass in definite proportions.  Dalton realized that some chemicals combine in several but definite proportions, a fact that fit in well with his proposition of matter being made up of atoms.  Next Gay-Lussac showed that gases combined in the same proportions by volume, leading to his law stating that equal volumes of gases have equal numbers of atoms (Avogadro figured out how many.)

This was all well and good, but I was not sure how much they got from this chapter.  I did not have the materials the book listed to carry out electrolysis.  I did an Internet search and found that some people used pencils with both ends sharpened, making holders from Styrofoam trays or cardboard, all powered with a 9V battery.  (See here and here.)  I wanted to capture the gas as was done in the book so we could test them, so this is what I did:




I filled a measuring cup with water and added a teaspoon of salt.  I attached a piece of aluminum foil to one end of two wires with alligator clips. I filled the test tubes with water and placed them over the foil pieces, using the top of the test tube rack to hold them upright.  Finally I connected the other end of the wires to each terminal of a 9V battery.  As you can see at the top of the test tubes gas began to collect.

My boys knew water was H2O, and that meant 2 molecules of hydrogen and one of oxygen.  I explained that electrolysis was "splitting" water with electricity.  I posed the question, "How do you know which gas is collecting in which tube?"  We went through the chapter again.  Proust and Dalton both worked with mass while our experiment was based on volume.  We discussed more about Gay-Lussac's law and asked how definite proportions relate to the creation of water with H and O.  The wheels started turning and they then knew for certain that the tube with more gas had to be the hydrogen.

I do suggest a lantern battery for this, though, because the 9V ran out when only a fourth of the hydrogen tube was filled.  But I am actually a bit glad.  I pulled it out of the water, let the water fall into the measuring cup, lit a match and put it into the mouth of the tube.  The loud whistle and pop was impressive enough to prove that indeed it was hydrogen.

4 comments:

Alecia said...

Thanks! I have bookmarked this entry for our homeschool group! We will be learning about electricity in the spring. By the way, I nominated you for the Stylish Blogger Award. See my blog for details on how to accept (oct. 10). Blessings, Alecia

Andres Garcia said...

Thank you very much to share this kind of experiments with us!
I'm going to try this with my nephew, to explain him just a bit about chemistry and how fun it can be. But i have a question, how one can be sure that oxygen from water is reacting to form gaseous oxygen instead of chloride to form chlorine? (Did you add sodium chloride, didn't you?)

Greetings,

Andrés García

Andres Garcia said...

Thank you very much to share this kind of experiments with us!
I'm going to try this with my nephew, to explain him just a bit about chemistry and how fun it can be. But i have a question, how one can be sure that oxygen from water is reacting to form gaseous oxygen instead of chloride to form chlorine? (Did you add sodium chloride, didn't you?)

Greetings,

Andrés García

Kris said...

Hi Andres--

Chloride would not cause a match to whistle and pop the way oxygen does. Chlorine gas is yellowish whereas oxygen is colorless. BTW, chlorine gas is highly toxic. My guess is that one would need a far more saltier solution to get chlorine gas with electrolysis.