1. Enthalpy, not happining.
2. Carnot heat pump, which probably has to do with enthalpy
3. Pressure involved with fluid flow. I didn't know the pressure decreased when fluid velocity increases until last week.
4. Hardy-weingberg stuff
5. Viruses(what does that + and - mean again?)
6. Entropy, thought I got it but then I read something to do with reactions being irreversible meant something and reversible meant something else about entropy.
7. Photoelectric effect
8. Why adding water to a buffer doesn't change the pH. They never explain it good enough for me.
9. Most verbal reasoning questions/answers. Well not most but approximately 40% of them
Theres a lot more than that, but those are the ones I'll never understand because the most I'll ever study those things in my life is over.
Jeez, you're in trouble if you have no clue about some of this stuff. Other stuff, not such a big deal.
1. Enthalpy is tricky, but remember it's just a form of energy, specifically heat. Everything always tends to get spread out evenly. In a sense, it wants to get spread out. This sort of thing happens spontaneously. When something LOSES heat to its environment which is cooler (negative deltaH, exothermic) or when something spreads out more (postive deltaS, gain of entropy) the free energy of that thing goes down (deltaG is negative). The amount of useful work that could be had from that thing being hot or concentrated all in one place is lower than it was before; it's given up some of its free energy.
2. Carnot cycle is a thermodynamics concept that I seriously doubt you will ever see on an MCAT. Essentially it tells you the maximum amount of useful work that can be had from something being hot and losing that heat to something that's colder. Yes, it's enthalpy!
3. Pressure decreasing with fluid velocity can be derived from fluid flow equations, or you can think of a vertical drainpipe that gets narrower as it goes down. If a water molecule is moving down at some speed when the pipe is wide, it must be rocketing by where the pipe is narrow, because equal volumes of water are flowing everywhere in equal amounts of time. Otherwise the water would pile up in some parts and there would be vacuums in other parts. (This actually sort of happens in turbulent flow, which is why it's much more complicated to analyze, and you're not expected to know it.)
4. Can't help you there, I'm not a biologist. But essentially it says that in a large, stable, randomly reproducing population, alleles don't disappear or appear over time. If there are 500 D and 380 d alleles of a gene in a given population there will be approximately the same number in each subsequent generation.
5. +/- means exactly what it looks like. lac+ means it has a lac gene.
6. Entropy is, in the simplest sense, how irreversibly spread out something is. (You can take a graduate level physics course where people have trouble understanding what entropy REALLY is, but we don't need to go there.)
If you have a piston with gas inside and you let the gas push the piston, you haven't actually increased the entropy because you can always push the piston back down and compress the gas to its original volume. But if you open the piston and let the gas escape into the room, no amount of scrambling around is going to get all that gas back inside the piston, because it has irreversibly spread out to every corner of the room. The entropy of the universe is always increasing. That's why teacups always break and never unbreak. It's why ice spontaneously melts but doesn't spontaneously freeze in a hot room.
SOME chemical reactions spontaneously lower their entropy (by bringing molecules together and thus reducing how spread out the thing is), but to counteract this reduction in entropy, they must lose an enormous amount of heat energy to their environment (reaction has negative deltaH) in the process.
7. The photoelectric effect is, at its core, a quantum physics phenomenon. Before the experiment was performed, people thought that if you fired some light waves at a metal, the number of electrons kicked off that metal would be proportional to the intensity of the light that hit it at any given frequency. It was believed that energy was infinitely divisible, so if some frequency of light knocked a certain number of electrons off, a slightly lower frequency (lower energy) light wave would knock off slightly fewer electrons. As it turned out, this was not the case. When you lowered the frequency past a certain point, you suddenly got no electrons at all being knocked off. Einstein explained that energy is quantized; it only comes in packets of a certain amount. (E=h*f) Each light wave is really a light particle, called a photon, carrying a balled-up amount of energy proportional to its frequency. Below a certain frequency, none of the photons have enough energy to knock any of the electrons off.
Brian Greene explains it this way:
Imagine an infinite sea of criminals in a prison, and it costs $50 to get out of jail. If you drop $50 bills or $100 or dollar bills from a helicopter, some individuals can get out because they catch a $50 bill and use it, or they catch a $100 bill and there's $50 left over which some other criminal grabs and uses to get out. But if you drop $20 bills, even a huge number of them, no criminal ever gets more than one, because there's an infinite number of them. Thus exactly zero criminals ever get out.
8. Sorry, I've never thought about this.
9. Can't help you with logic. Either you've got it or you don't.
Hope this helps.
