Viewing
the Unseen:Physical Methods for Chemists
Viewing
the Unseen:Physical Methods for Chemists
This webpage is about the methods that chemists use to identify the compounds and elements that make up substances. The four methods that will be discussed in this page are GC/Mass Spectrometry, Nuclear Magnetic Resonance Spectroscopy, UV-Visible Spectroscopy, and Fourier Transfrom Infrared Spectroscopy (FTIR). These four methods apply some pretty basic concepts about chemicals, as well as some fairly complicated theory, some of which I myself do not yet grasp. I will attempt in this page to explain things in fairly simple terms, as well as give some links to more advanced information for those of you to whom it might have interest/application. I will also include some spectra and Hyperchem images of the two unknown substances for which I have been using the four methods above to identify. Please link to whatever you find most interesting, or read below to get some quick background in chemistry.
Before we begin to learn about each method, it is good to have some basic knowledge about chemistry. So, here we go with a crash course in general chemisry.
Everything around us is made of chemicals. Our bodies, our clothes, the food we eat, the air we breath, the cars we drive and the bed we sleep in are all made of chemicals. Chemicals are the stuff matter is made of. Chemicals can either exist as elements or compounds. The basic buiding blocks of all chemicals are atoms. Elements have atoms of all the same kind, and compounds are bunches of different kinds of atoms that bond together to form something new. Atoms are made up of electrons, which have very little mass and are negatively charged, protons, which have some mass and are positively charged, and neutrons, which have the same mass as protons and have no charge. Protons and neutrons compose the inner part of the atom, and electrons orbit the atom at different energies and varying paths, depending on the conditions. Every element is made up of atoms which have different numbers of protons and electrons. The number of neutrons can vary in a given element. Elements with differing numbers of neutrons are called isotopes.
Chemistry is mostly concerned with the behavior of electrons. Electrons are what make different substances react-that is, change at a chemical level. The ways in which electrons on one atom or molecule interact with other electrons to form bonds is the heart of what chemists strive to learn about.
What exactly are chemical reactions? Well, I defined chemical reactions as those which change a substance at the chemical level. Changes at the chemical level are changes that make a substance turn into something else, such that you can't turn it back into what is was before. Confused? Think of it this way. If you apply heat to ice, it will melt into something new-water. Chemical change has occcurred, right? Let's think about this. What if you put that water back in the freezer? In a few hours what would you have again? Ice. That means that no chemical change occured. But, what if you took a potato and boiled it on the stove? You end up with a cooked potato, which now looks, feels, and tastes different than the raw one. Can you do anything to make the potato raw again? No. So, you know a chemical change has occured. Chemical reactions are always about electrons. They involve some change in the ways the electrons on an atom arrange themselves.
Electrons which compose various atoms behave differently. One of the ways in which they behave is to absorb energy in the form of light, which makes them excited...Different atoms absorb different amounts of light energy. Light can be thought of as tiny particles of energy called photons. Electrons absorb photons of certain energies only, and so get excited to different levels. They orbit the nucleas differently when they are excited. We can use machines to see this difference. The machines graph the changes and help us to know what atoms make up the substance we are studying. UV-Vis exploits this method.
Molecules themselves vibrate
in different ways. FTIR takes
advantage of this fact to obatin spectra of various compounds.
We can also look at nuclei
spin states when compounds are in a magnetic field. NMR
machines help us to do that.
Mass Spectrometers measure the mass and fragmentation patterns of a compound. This helps us to know what the compound is.
Sound interesting? Pick a machine and read all about it.....
This page gives alot of links to various student web pages as well as to pages on Chemistry in general from the web...
-this page gives information applicable to Physical Chemistry Methods
If
you have comments or suggestions, email me at ajedwin@inst.augie.edu
This page created with Netscape Navigator Gold