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Welcome to chemistry class for yet another gripping, action-packed review of what we’ve been learning during the past month. We’ve all settled in for the long haul of finishing up this first semester, and because we’ve gotten into the swing of things by now, lab reports, tests, and homework assignments are coming to us a little bit easier. As always, I’d like to highlight a few of the interesting, thought-provoking concepts we’ve learned in chemistry.

Primarily, we've studied the concepts of stoichiometry, wave theory, and atomic structure. You will remember from last month that stoichiometry is simply the process of relating quantities of substances reacted or produced in a chemical reaction. Wave theory is the study of waves, how they operate, and how to determine their wavelength, frequency, and energy using a few powerful tools and equations. Our study of atomic structure introduced us to the component parts of atoms, the history behind their discovery, and how atoms operate.

Saying that stoichiometry has been one of the hardest concepts in chemistry to learn so far would not be a stretch of the imagination, by any means. Though we studied numerous other concepts in the sixth module, they all tied back directly to the concept of stoichiometry in one way or another. The mole concept that was learned previously has now come back on stage with stoichiometry, and we’ve seen that they make a powerful team. The reason why they are so powerful when used together is that if you know how much of one substance you have, with the help of stoichiometry, the amount of another substance in the same reaction can be determined. Using this process along with the density of these substance, you can then figure out the exact mass of the substance. This is an extremely dynamic concept since it allows chemists to know precisely what they will get when they perform a reaction in the laboratory.

One problem that can often present itself in stoichiometry, however, is the concept of the limiting reactant. In a chemical reaction, most often two or more substances are used to make other substances. When two substances are used, however, they must be used in the right amounts. If the right amounts are not used, one of the substances will be used up completely and the other will still have some leftover, even after the reaction has been completed. Because of this, we say that the substance that is used up entirely limited how much of the product(s) could be produced and was therefore, the limiting reactant.

The reason why the concept of limiting reactants is tricky when applied to stoichiometry is that using a reactant other than the limiting reactant to determine how much product was produced yields an incorrect amount. Thus, the limiting reactant must always be used in coordination with stoichiometry to figure out other things about the reaction. Doing this though, presents the problem of figuring out which of the reactants was the limiting reactant in the first place, which takes several more steps. As you can see then, the seemingly harmless stoichiometric chemistry problem has now turned into a monster requiring a great amount of time and concentration to analyze.

That’s not to say that everything we’ve been learning has been so complicated, though. Waves and how they travel through a medium has been a complete change of focus from chemical reactions to more physics, and the change is greatly welcomed since the types of problems done shifts away from the ten-part monsters of stoichiometry to just a few, easy steps. Through our study of waves, we learned that the wavelength of a wave is the distance between two of its adjacent crests or two of its adjacent troughs. Applying this to waves at the ocean, notice how each wave is spaced a certain amount of distance apart from the wave following it. This is called the wave’s wavelength and is very helpful in other wave computations. Frequency is another easy concept to grasp, and it shows how many waves pass a certain point over a given amount of time. Because of this, frequency is often measured in waves per second or a specific unit called Hertz named after the German physicists Heinrich Rudolf Hertz who studied light in great detail.

Using the two wave properties, wavelength and frequency, we learned that frequency can be equated to wavelength using an equation where frequency, abbreviated as “f,” equals the speed of light, “c,” divided by the wavelength of the wave, abbreviated with the lowercase Greek letter lambda. Thus, the equation is shown as f = c/lambda. Because the speed of light is a physical constant, meaning its value never changes when traveling through the same medium, frequency and wavelength are inversely proportional to each other, and a direct comparison can be made between the two.  One, final thing that we learned about waves was that how much energy they contain is directly proportional to its frequency. This makes sense because the more waves hit something per second, the more energy is released.

Upcoming material in our chemistry class includes studying the Lewis structures of atoms and molecules, Electronegativity, the ionization potential of atoms, the VSEPR theory of a molecule's three-dimensional structure, and a review of ionic and covalent compounds and how they relate to these new concepts. But we won’t be getting to any of this new stuff until after Christmas Break and until the end of the first semester approaches in 2006. As the semester begins to come to a close, semester exam preparation begins with a furry and the spine of that old chemistry textbook gets a real workout. Since we don’t have to think about any of that until after the holidays are over, though, I’ll bid you all a very merry Christmas and a happy new year!

 

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