Amedeo – masculine, Italian; Italian form of Amadeus
Pronunciation: ahm-a-DAY-o or ahm-a-DEE-o
Amadeus – masculine, late Roman: derived from Latin amare “to love” and Deus “God”
Pronunciation: ahm-a-DAY-us or ahm-a-DEE-us
(courtesy of www.behindthename.com)
I chose the Italian chemist Lorenzo Romano Amedeo Carlo Avogadro di Quaregna e di Cerreto (who was also Count of Quaregna), more easily referred to as Amedeo Avogadro, for the namesake of this blog. Holy long name.
Before we go any further, let’s look at some pictures of this gentleman (Figure 10.1). He looks smart, huh? I can’t believe he was put on a postage stamp! How cool.
Avogadro was born on August 9, 1776 (he just missed the Declaration of Independence!) to Count Filippo and Anna Vercellene in Turin, Italy. In 1796, he received his law diploma and worked as a lawyer until the natural sciences started to seduce him. He finally left law in 1804 after presenting two papers to the Academy of Sciences in Turin on the subjects of electricity and metallic salts. After being appointed professor at Lyceum at Vercelli, Avogadro published two memoirs that would place him squarely in the history of chemistry.
Linus Pauling (a tremendous scientist in his own right) feels that Avogadro “was a man with an intense curiosity about nature. He believed that a scientist should try to understand the world, and should not be content … simply to describe the world.” Referred to as a kind, affable, and sincere by Edgar Smith, Pauling also felt that Avogadro was modest about his work, but not to a fault. Avogadro did push the scientific community to understand and adopt his conclusions about atomic and molecular structure. Unfortunately, like many before him, his work wasn’t fully appreciated in his lifetime. It was left to Stanislao Cannizzaro to take up Avogadro’s cause and impress the importance of his work upon the community.
Avogadro, exactly one month shy of his 80th birthday, died in 1856. He continued as a professor until 1830 and studied science until the end of his life. He was a private man and little is known about either his wife, reportedly Felicita Mazzé, or his six children. Following his death, a bust of Avogadro was placed at the University of Turin and the Scuola Professionale at Biella.
So what was his great contribution to chemistry? He figured out molecular formulas. For example, he figured out that hydrochloric acid was always one hydrogen atom bonded to one chlorine atom (written as HCl, see Salty Water post).
How did he do this? Let’s start with his hypothesis.
Avogadro’s hypothesis (as published in 1811): at the same temperature and pressure, equal volumes of different gases contain the same number of particles.
I’m sure to most of you those words are just a mish-mosh of nonsense. What he’s saying is that if you have a corked flask of 1 L that you completely fill with Gas A and, sitting next to it, you have a corked flask of the exact same size filled completely with Gas B, then the number of particles in each flask is exactly the same (Figure 10.2). Particles can mean atoms or molecules*.
This was a bold assertion but immensely helpful to chemists performing experiments with little understanding of atoms or molecules.
Let’s now set the scene to describe what was known about atoms and molecules at this time to show how Avogadro’s hypothesis fit in.
Scientists knew of atoms but had no way of knowing exactly what a compound looked like. This means that they knew water was a molecule composed of hydrogen and oxygen, but exactly how was a little murky.
John Dalton (important chemistry guy) was able to figure out that water (and other known compounds) were always being made with the same proportions. By mass, water was always two parts hydrogen and one part oxygen. But, did that mean that a water molecule was H2O or H4O2 or H6O3 (or even higher)? All of these are viable possibilities because they are all two parts hydrogen to one part oxygen. Molecules were small so they could not be seen or directly measured. A link between the unseen (atoms, molecules) and the seen (measurable volumes, masses, something!!) was desperately needed.
Around the same time, a familiar gentleman by the name of Joseph Gay-Lussac (hello, Absolute Zero post!) was fooling around with volumes of gases. For example, he knew that if he mixed 2 L of hydrogen with 1 L of oxygen, he’d get 2 L of water vapor. Why? No one was too sure.
Enter Amedeo Avogadro and his hypothesis. He figured out a way to link Dalton’s calculations with Gay-Lussac’s experiments and definitively decide a compound’s molecular make up.
I’m going to explain this with simplified examples instead of real atoms. I will show you two fake Gay-Lussac experiments, the results, and what was known (Figure 10.3). Then we’ll apply Avogadro’s hypothesis to it and show how that leads to molecular formulas. The same logic was applied to real data to determine lots of molecular formulas.
Illustrative Experiment, Part 1
Gay Lussac: I mixed 1 L of Gas X with 1 L of Gas Y and got 1 L of Gas U.
Dalton: I know that Gas U is equal parts X and Y.
Avogadro: Equal volumes contain and equal number of particles. 1 L of Gas X and 1 L of Gas Y and 1 L of Gas U all contain the same number of particles. For simplicity’s sake, let’s say 1 L = 100 particles. 100 particles of X + 100 particles of Y gave 100 particles of U. The only way to get 100 particles of U is if one X and one Y come together. That can happen 100 times. Dalton says U is equal parts X and Y, so a molecule of U = XY.
Illustrative Experiment, Part 2
Gay Lussac: I mixed 1 L of Gas X with 1 L of Gas Y and
got 0.5 L of Gas U
Dalton: I know that Gas U is equal parts X and Y
Avogadro: Equal volumes contain equal number of particles. 100 particles of X + 100 particles of Y gave only 50 particles of U. How do you only get 50 particles of U? Well… what if two Xs and two Ys came together to form one molecule? If Gas U is really X2Y2, then you’d only be able to make 50 of them. If you can only make 50 particles, then the volume you’d see if 0.5 L.
See how Avogadro’s hypothesis became the link between the unseen and the measurable quantities? His work allowed scientists to finally determine the molecular formulas of many known compounds. Most introductory chemistry students know Avogadro in another way: Avogadro’s number. Again, this conversion provides a link between a measurable quantity (a weighed out mass of element/compound) and the number of atoms or molecules actually lying there. His work also lead to further development of the Ideal Gas Law, something I’ll cover in a different post.
So, why did I pick him as a namesake for my blog? His contribution to chemistry was enormously important and laid the groundwork for understanding atoms, molecules, chemical reactions, and everything that followed. The meaning of his name also intrigued me. The longer one is a scientist and studies how the natural world works, the more awed one inevitably becomes. We are all tied to this universe and its laws. On some level, we all respect the science that governs our world. One is free to interpret “God” however one wishes, but I don’t have a religious interpretation of it. I define the word as “however this world came to be.” I see it much more as a broad term to refer to things we don’t yet or may never understand. Rest assured, though, that thousands of post docs and graduate students are out there desperate to try and add a little nugget to our understanding every day.
* This assumption only works because the size of a gas is so large compared to the size of an atom or molecule.
Avogadro’s Number = 6.02 x 1023 atoms or molecules per mole of substance. I won’t define a mole right now, but suffice it to say a mole is analogous to a dozen. If you have a dozen cookies, you have twelve cookies. If you have a mole of water molecules, you’ve got 6.02 x1023 molecules of water. That’s a lot of water molecules.
References
Edgar Smith. “Amedeo Avogadro.” Nature (1911) 88 (2196), pgs 142 – 143.
Hinshelwood and Pauling. “Amedeo Avogadro.” Science (1956) 124, pgs 708 – 713.
Peterson. “Avogadro and His Work.” Science (1984) 226, pgs 432 – 433.
Zumdahl, Steven S. “Chemical Principles, 4th Edition” (2002) Houghton Mifflin Company, Boston, MA.
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