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Jonathan K.
Chemist with 9+ Years of Industry Experience & Former Head TA in Organic Chemistry
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Organic Chemistry
TutorMe
Question:

How do you push electrons?

Jonathan K.
Answer:

Well, electrons are really light, so if you can figure out how to deal with all the negative charges repelling each other it shouldn’t be that hard to-- What’s that? You didn’t mean literally? Ah, so you meant pushing electrons as in writing reaction mechanisms. I thought it was an odd question… Knowing how to write reaction mechanisms (AKA, “pushing electrons”) is an important part of organic chemistry because it allows you to predict the outcome of a reaction even if you aren’t familiar with that specific set of conditions. It also helps you avoid a tirade of profanity in the lab when you realize the product of your last reaction is not at all what you planned because you can figure out where things went wrong and fix it for the next trial. So, let’s start with the basics. Reaction mechanisms are written by diagramming the flow of electrons from one molecule to another or from one atom to another within the same molecule. Electrons almost always flow from areas of higher electron density to areas of lower electron density or from less electronegative atoms to more electronegative atoms. “Almost” is the important word in that sentence; there are always exceptions! The majority of the time you will be writing reaction mechanisms with “double-barbed” arrows (like this: →) to represent pairs of electrons moving around. “Single-barbed” arrows (like this: ⇀) represent single electrons, and all the rules go out the window when those guys show up to the party. That’s free radical chemistry and weird stuff starts to happen: bonds that would normally not react suddenly do, expected product distributions are nowhere to be found, and generally, things fall apart. Reaction mechanisms are a huge topic and trying to cover all the aspects in one spot would be like trying to dig out from a blizzard using a teaspoon: it might get done, but it would take forever, it wouldn’t be pretty, and crying would probably be involved. So here a few general rules of thumb to start with: 1) Look for carbon atoms with electron withdrawing groups attached (almost anything with oxygen, nitrogen, or a halogen), they’re likely targets. 2) No pentavalent (five-bonded) carbons allowed! 3) If your reaction is acid-catalyzed (meaning there’s H+ in solution) make sure that none of the steps in your mechanism have negatively charged intermediates, and vice versa for base-catalyzed reactions. 4) Mild reaction conditions (lower temperatures, weak acids, and bases, etc.) mean only the most reactive atoms will get involved. Harsh reaction conditions (high heat, high pressure, concentrated acids, and bases, generally beating the hell out of the reactants) means that you’re likely trying to force something to react that otherwise wouldn’t. 5) Any time a gas can be formed and escape (e.g., O2, H2, N2), it will.

Chemistry
TutorMe
Question:

How big is a mole?

Jonathan K.
Answer:

About 50 - 150 g, depending on species. No, the OTHER mole. One mole of something is 6.02 x 10^23 somethings. It could be atoms, molecules, golf balls, squirrels, or even moles. Doesn’t matter. To make it easier, you can just think of it as any other numerical designation like hundreds or thousands or millions. You can have two million squirrels in your backyard, and you can also have two moles of squirrels. Either way, you probably can’t see the grass anymore. Or your house for that matter. You know, since it’s buried under squirrels. But I digress. That’s great and all, but it doesn’t answer the question of how big a mole is. So let’s use something simple like water as a demonstration. If you have one mole of water, you have 6.02 x 10^23 molecules of water. That’s almost exactly 18 grams of water. At standard temperature and pressure, one milliliter of water weighs one gram, so that’s also 18 ml of water. In more everyday terms, that’s about one tablespoon plus a half teaspoon of water. That’s not really much water, is it? Well, I think water molecules are too hard to see, but then again I’m nearsighted so that might have something to do with it. So let’s try using something visible now. Like basketballs. So, how big is a mole of basketballs you ask? Good question, let’s find out! A basketball has a diameter of about 24 centimeters, so that means it has a volume of about 7238 cubic centimeters (or cc). For those of you interested, that comes from the equation volume of sphere = 4/3 x Pi x radius of sphere cubed. So one mole of basketballs has a volume of 7238 x 6.02 x 10^23 cc, or about 43,573 x 10^23 cc. I’m having an issue with that 10^23 number. It’s too big to really appreciate. But, we can shrink it with some unit conversions. There is one million (1,000,000) cc in one cubic meter, so that brings it down to 43,573 x 10^17 cubic meters. That’s still too big for me, so let’s go one more step. There are one billion (1,000,000,000) cubic meters in a cubic kilometer, so now we’re down to 43,573 x 10^8 cubic kilometers. That’s 4,357,300,000,000 cubic kilometers or 4.3573 trillion cubic kilometers, which is just enough basketballs to fill the Earth… four times! And that’s only accounting for the volume inside the basketballs, not taking into consideration that the basketballs will have some space between them! So, the question really shouldn’t be how big a mole is, but what you’re counting instead.

Basic Chemistry
TutorMe
Question:

Why does ice float on water and why does it matter?

Jonathan K.
Answer:

Get it? Matter, as in states of matter! Funny, right? No? Anyway, ice floats on water because water has a somewhat unusual behavior when it freezes. But before we talk about that, we have to talk about temperature and what’s happening on a molecular level. The temperature of something is directly related to how fast its atoms or molecules are moving. In a gas like steam, the water molecules can move freely and are far apart from each other. As the steam cools, it condenses to form liquid water. In a liquid state, the water molecules can still move around, but not as quickly or freely as in the gas phase. Also, the molecules are now in close contact with each other and can’t be compressed anymore. Further cooling brings the water to its freezing point, where the molecules get locked into a crystal structure as ice forms. They can’t move past each other anymore and are limited to just vibrating in the spot. The transition from liquid to solid is the important part here. Remember how the water molecules were basically packed together as close as possible in a liquid state since it can’t be compressed? Well, as the water freezes the crystal structure of ice forces them into a pattern that causes them to take up about 10% more space than the liquid. Since the mass is the same (we didn’t add any water), the ice has a lower density than the surrounding water and it floats. This matters because if the ice didn’t float, it would sink (since that’s the only other choice we have here). Big deal you say? Yes, it IS a big deal! Imagine a lake in winter. Since ice floats, even when the lake is “frozen solid” the ice isn’t usually much more than a foot or two thick and there’s a lot of open water below it for the fish and other animals to live in until the lake thaws. If ice sank, the lake would literally freeze SOLID into one giant block of ice along with anything (formerly) living in the lake.

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