14 Org Chem/Physics Questions

[oohpsjin]

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I'm having trouble with these questions that I came up with during studying. Could anyone please help out?

Organic Chemistry
1. How does a base catalyze certain reactions such as hydrolysis of peptide bonds or the reaction between aromatic acid chlorides and alcohol to produce esters? Many reactions occur with the help of aq. acid or aq. base - I understand how acids may protonate atoms to make their bonds unstable, but how does aqueous base (OH/H2O) catalyze reactions?

2. Given methyl acetate (methyl ethanoate) and butanone (methyl ethyl ketone), shouldn't the ester be more reactive than the ketone, because the ester has two oxygens that both cause dipole moment in the carbonyl carbon? But how is the ketone more stable?

3. There are three intermolecular forces I know of: H-bond, dipole-dipole, and van der Waals. However, in wikipedia http://en.wikipedia.org/wiki/Van_der_Waals_force dipole-dipole, hydrogen bonding, london, and induced dipole-dipole are all considered as van der Waals forces. Which is right? How many distinct intermolecular forces are there and what are they?

4. Cellulose, a 1,4'-beta-D-glucose polymer, is comprised of D-glucose alternatingly facing upwards and downwards. Why the alteration? Why wouldn't cellulose have monosaccharide subunits all in the same orientation?

Physics
5. Static Friction Fs is the force that must be overcome to set an object in motion. Its equation is 0<Fs<usN where us is the coefficient for static friction and N is the normal force. Why does static friction have a maximum and a minimum instead of an absolute number, and what does maximum usN mean?

6. The moon is traveling in a uniform circular motion around Earth, and thus has acceleration that is always directed towards the centre (centripetal force). However, then, why doesn't the moon just crash towards Earth? What other force is acting outwards to push the moon away from earth?

7. For reversible isothermal processes, the change in entropy of the system or of the environment can be found from: delta S = delta Q/T. But why is this only for reversible isothermal processes?

8. What determines that a positive charge will form on a glass rod when it is rubbed with silk cloth? In other words, is there some kind of quantitative property intrinsic to distinct materials that decides the transfer of charge?

9. What in the world is permitivity of free space??? Can anyone please explain this in a simpler manner than in wikipedia?

10. Current flows from higher potential (+ terminal) to lower potential (- terminal) because by convention, current is the flow of positive charges. But in a conductor, only negatively charged electrons are free to move. So this means that the electrons are flowing from low to high potential. Are they able to do this "work" because of the voltage difference set up by batteries?

11. Blackbody radiation - why was this of interest? What is its purpose in the study of physics? I just had trouble understanding how blackbody radiation is related to anything that I was learning in MCAT physics.

12. For the MCAT, do we need to memorize certain conversion factors or constants? And how in depth do we need to memorize the formulas? (this must have been asked before. I apologize!)

13. Could anyone clarify the physics of a laboratory centrifuge? How do the spinning motion result in denser particles accumulating to the bottom of the centrifuge tube? This must mean that the centrifuge is not spinning in uniform circular motion, since the centripetal force would have particles accelerate towards the centre?

oops forgot...one more org chem Q!
14. Nitrogen lone pair electrons in rings often contribute to conjugation. This is a basic concept that confuses me, but how do we know that the lone pair is in the p orbital (and thus contribute to aromaticity) and not any other orbital, such as s, d, or f?

Thanks in advance for everyone who could help out with any one of these questions.

 

[GCT]

I'm having trouble with these questions that I came up with during studying. Could anyone please help out?

Organic Chemistry
1. How does a base catalyze certain reactions such as hydrolysis of peptide bonds or the reaction between aromatic acid chlorides and alcohol to produce esters? Many reactions occur with the help of aq. acid or aq. base - I understand how acids may protonate atoms to make their bonds unstable, but how does aqueous base (OH/H2O) catalyze reactions?

2. Given methyl acetate (methyl ethanoate) and butanone (methyl ethyl ketone), shouldn't the ester be more reactive than the ketone, because the ester has two oxygens that both cause dipole moment in the carbonyl carbon? But how is the ketone more stable?

3. There are three intermolecular forces I know of: H-bond, dipole-dipole, and van der Waals. However, in wikipedia http://en.wikipedia.org/wiki/Van_der_Waals_force dipole-dipole, hydrogen bonding, london, and induced dipole-dipole are all considered as van der Waals forces. Which is right? How many distinct intermolecular forces are there and what are they?

4. Cellulose, a 1,4'-beta-D-glucose polymer, is comprised of D-glucose alternatingly facing upwards and downwards. Why the alteration? Why wouldn't cellulose have monosaccharide subunits all in the same orientation?

Physics
5. Static Friction Fs is the force that must be overcome to set an object in motion. Its equation is 0<Fs<usN where us is the coefficient for static friction and N is the normal force. Why does static friction have a maximum and a minimum instead of an absolute number, and what does maximum usN mean?

6. The moon is traveling in a uniform circular motion around Earth, and thus has acceleration that is always directed towards the centre (centripetal force). However, then, why doesn't the moon just crash towards Earth? What other force is acting outwards to push the moon away from earth?

7. For reversible isothermal processes, the change in entropy of the system or of the environment can be found from: delta S = delta Q/T. But why is this only for reversible isothermal processes?

8. What determines that a positive charge will form on a glass rod when it is rubbed with silk cloth? In other words, is there some kind of quantitative property intrinsic to distinct materials that decides the transfer of charge?

9. What in the world is permitivity of free space??? Can anyone please explain this in a simpler manner than in wikipedia?

10. Current flows from higher potential (+ terminal) to lower potential (- terminal) because by convention, current is the flow of positive charges. But in a conductor, only negatively charged electrons are free to move. So this means that the electrons are flowing from low to high potential. Are they able to do this "work" because of the voltage difference set up by batteries?

11. Blackbody radiation - why was this of interest? What is its purpose in the study of physics? I just had trouble understanding how blackbody radiation is related to anything that I was learning in MCAT physics.

12. For the MCAT, do we need to memorize certain conversion factors or constants? And how in depth do we need to memorize the formulas? (this must have been asked before. I apologize!)

13. Could anyone clarify the physics of a laboratory centrifuge? How do the spinning motion result in denser particles accumulating to the bottom of the centrifuge tube? This must mean that the centrifuge is not spinning in uniform circular motion, since the centripetal force would have particles accelerate towards the centre?

oops forgot...one more org chem Q!
14. Nitrogen lone pair electrons in rings often contribute to conjugation. This is a basic concept that confuses me, but how do we know that the lone pair is in the p orbital (and thus contribute to aromaticity) and not any other orbital, such as s, d, or f?

Thanks in advance for everyone who could help out with any one of these questions.

1) Acids and bases can catalyze reactions by creating a more favorable intermediate e.g. by deprotonating an alcohol which can attack a carbonyl carbon or by protonating the carbonyl oxygen which creates a more positive charge on the carbonyl for a nucleophilic attack .

2) Esters are more susceptible to nucleophilic attacks because the carbon has two proximal oxygens ; there's more positive charge on the carbonyl carbon of an ester then a corresponding ketone .

3) Van der waals are a separate phenomena however the first source does not include induced dipole which counts as an intermolecular attraction .

4) The carbon substituents on glucose alternate in and out of plane .

 

[HawkeyePostOp]

Physics
5. Static Friction Fs is the force that must be overcome to set an object in motion. Its equation is 0<Fs<usN where us is the coefficient for static friction and N is the normal force. Why does static friction have a maximum and a minimum instead of an absolute number, and what does maximum usN mean?

6. The moon is traveling in a uniform circular motion around Earth, and thus has acceleration that is always directed towards the centre (centripetal force). However, then, why doesn't the moon just crash towards Earth? What other force is acting outwards to push the moon away from earth?

7. For reversible isothermal processes, the change in entropy of the system or of the environment can be found from: delta S = delta Q/T. But why is this only for reversible isothermal processes?

8. What determines that a positive charge will form on a glass rod when it is rubbed with silk cloth? In other words, is there some kind of quantitative property intrinsic to distinct materials that decides the transfer of charge?

9. What in the world is permitivity of free space??? Can anyone please explain this in a simpler manner than in wikipedia?

10. Current flows from higher potential (+ terminal) to lower potential (- terminal) because by convention, current is the flow of positive charges. But in a conductor, only negatively charged electrons are free to move. So this means that the electrons are flowing from low to high potential. Are they able to do this "work" because of the voltage difference set up by batteries?

11. Blackbody radiation - why was this of interest? What is its purpose in the study of physics? I just had trouble understanding how blackbody radiation is related to anything that I was learning in MCAT physics.

12. For the MCAT, do we need to memorize certain conversion factors or constants? And how in depth do we need to memorize the formulas? (this must have been asked before. I apologize!)

13. Could anyone clarify the physics of a laboratory centrifuge? How do the spinning motion result in denser particles accumulating to the bottom of the centrifuge tube? This must mean that the centrifuge is not spinning in uniform circular motion, since the centripetal force would have particles accelerate towards the centre?

oops forgot...one more org chem Q!
14. Nitrogen lone pair electrons in rings often contribute to conjugation. This is a basic concept that confuses me, but how do we know that the lone pair is in the p orbital (and thus contribute to aromaticity) and not any other orbital, such as s, d, or f?

Thanks in advance for everyone who could help out with any one of these questions.

5. Explanation: Because when you finally reach Fs, the contact with the other surface "breaks," and the system switches to using Fk, kinetic friction. Small imperfections on the surface that used to be largely interlocking have broken free from each other and are now mostly sliding.
Rant:
All coefficients are soft applications of hard physics and thus in a way imaginary. If you really want to get dirty, there is a continuum of coefficients - considering that the surface imperfections are constantly being deformed, then deforming each other, generation heat at variable rates, and creating air pockets.

6. Explanation: The centrifugal force. Consult your college textbook.

7. Explanation: Entropy is a state function. If the process is irreversible, you could get two different values for the same equation(you'd have to use a second equation.)

8. Explanation: The better the insulator, the better at conducting, keeping, and suddenly releasing charge. If you want more, google "triboelectric effect". But I bet you it's not on the MCAT.

9. I think wikipedia does a wonderful job and it would be hard to simplify it more. I would try again.

10. http://forums.studentdoctor.net/showthread.php?t=541520

11. Explanation: blackbody radiation is of interest because it just might show up in a big passage and ruin your life. And it's not that hard.

12. Memorize as much as you can, but remember it's more about thinking "in the moment" than remembering stuff. Know g ~10. Know water density. Don't bother with planck's constant or capital G or things like that.

13. Explanation: For our purposes, the circular motion is uniform. All the particles want to get out of the ring and go in a line tangent to the circular path. The reason why centrifugation works is that it's so much stronger than gravity that centrifugal force does in a short time exactly what gravity takes a while to do.

14. Just know they have p orbitals in the common aromatic N-containing rings. Also the N doesn't always contribute to aromaticity, only some aromatic N p orbitals actually delocalize into the ring. Pyrole does, pyrolidine doesn't. Both are still aromatic.

 

[not respect]

centrifugal force isn't a real force, there's only one force needed to keep something in circular motion. the reason it doesn't crash into the earth is because it has velocity. it's like if you threw something really hard, so it keeps going really straight but when it starts to fall, it falls around the earth's curvature and just keeps going like that forever.

 

[HawkeyePostOp]

centrifugal force isn't a real force, there's only one force needed to keep something in circular motion. the reason it doesn't crash into the earth is because it has velocity. it's like if you threw something really hard, so it keeps going really straight but when it starts to fall, it falls around the earth's curvature and just keeps going like that forever.

The fact that's it's "fake" is kind of in the definition of centrifugal force. If it keeps people from thinking that the moon will crash into the earth it did its job, and deserves to have business cards with Force after its name.

Another way to think of why the moon doesn't crash into the earth in orbit is a differential geometry approach: Imagine how an orbiting moon could crash into the earth and why a certain speed keeps it from doing so. Or just think of it as the moon is always falling toward the earth but the land immediately under it keeps "falling" (veering off curvature) at the exact rate. Push the velocity too far and you reach escape velocity. Go too low and the orbit will degenerate and the satellite will crash.

 

[AZFutureDoc]

Right. In one of the cheesiest Jordan and Jon monents "It just keeps missing."

And don't spend another second thinking about a centrifugal force. It will only get you into trouble. You will never see a situation on the MCAT involving centrifugal force because the AAMC works hard to keep their test content up do date. Seeing as centrifugal force is just as incorrect as matter being made of earth, air, wind and fire, it would only ever show up as a seductive wrong answer.

 

[oohpsjin]

thanks for ur help guys.
I'm confused with orbitals, if i could get some help with that too:

E.g. OF5 cannot exist because there are no empty d orbitals for O. However, compounds such as IBr, UF6, NaLiCO3 can exist. Is it because they have empty d orbitals? but where? For instance aren't the d orbitals filled in IBr for both I and Br?

 

[161927]

I'm having trouble with these questions that I came up with during studying. Could anyone please help out?

Sure I'll try. I like that you didn't split this up into 14 different threads, because some people would do that, and it's annoying, lol.

Organic Chemistry
1. How does a base catalyze certain reactions such as hydrolysis of peptide bonds or the reaction between aromatic acid chlorides and alcohol to produce esters? Many reactions occur with the help of aq. acid or aq. base - I understand how acids may protonate atoms to make their bonds unstable, but how does aqueous base (OH/H2O) catalyze reactions?

In general, acid solutions stabilize negative charges that develop. Basic solutions, conversely, knock off protons that shouldn't be on your intended product.

2. Given methyl acetate (methyl ethanoate) and butanone (methyl ethyl ketone), shouldn't the ester be more reactive than the ketone, because the ester has two oxygens that both cause dipole moment in the carbonyl carbon? But how is the ketone more stable?

The ester is more reactive than the ketone, which means that the ketone is more stable.

3. There are three intermolecular forces I know of: H-bond, dipole-dipole, and van der Waals. However, in wikipedia http://en.wikipedia.org/wiki/Van_der_Waals_force dipole-dipole, hydrogen bonding, london, and induced dipole-dipole are all considered as van der Waals forces. Which is right? How many distinct intermolecular forces are there and what are they?

A good way to think about this is to conceptually group all the short range forces together. This can be your working definition of Van der Waals forces. The details of classification can be left to your chemistry professor, because there is some subjectivity to it depending on the context.

4. Cellulose, a 1,4'-beta-D-glucose polymer, is comprised of D-glucose alternatingly facing upwards and downwards. Why the alteration? Why wouldn't cellulose have monosaccharide subunits all in the same orientation?

Molecular models might help you answer this question.

Physics
5. Static Friction Fs is the force that must be overcome to set an object in motion. Its equation is 0<Fs<usN where us is the coefficient for static friction and N is the normal force. Why does static friction have a maximum and a minimum instead of an absolute number, and what does maximum usN mean?

Beyond the cutoff, kinetic friction is operative. At the maximum value, the object starts to slide.

6. The moon is traveling in a uniform circular motion around Earth, and thus has acceleration that is always directed towards the centre (centripetal force). However, then, why doesn't the moon just crash towards Earth? What other force is acting outwards to push the moon away from earth?

Space-time is curved, and celestial bodies follow tracks through the gravitational field. The moon is attracted to other things besides the Earth.

7. For reversible isothermal processes, the change in entropy of the system or of the environment can be found from: delta S = delta Q/T. But why is this only for reversible isothermal processes?

For irreversible isothermal processes, your system has changed (i.e., it is no longer isolated from the environment), so you can longer make the same kinds of accurate predictions about it as you did before.

8. What determines that a positive charge will form on a glass rod when it is rubbed with silk cloth? In other words, is there some kind of quantitative property intrinsic to distinct materials that decides the transfer of charge?

I asked this of my Physics instructor, and he gave me a "I don't know." Certain materials just develop charge in particular ways.

9. What in the world is permitivity of free space??? Can anyone please explain this in a simpler manner than in wikipedia?

Think of it just as a dimensional constant, so that units cancel appropriately.

10. Current flows from higher potential (+ terminal) to lower potential (- terminal) because by convention, current is the flow of positive charges. But in a conductor, only negatively charged electrons are free to move. So this means that the electrons are flowing from low to high potential. Are they able to do this "work" because of the voltage difference set up by batteries?

In all cases of current, it is the negative charges that are moving, yet it is common convention to talk about positive charges moving. The difference between a voltaic and an electrolytic cell is that one does work to move against the gradient, whereas the other moves naturally across the electric potential.

11. Blackbody radiation - why was this of interest? What is its purpose in the study of physics? I just had trouble understanding how blackbody radiation is related to anything that I was learning in MCAT physics.

We know the temperature of many stars and objects of astronomic interest due to their radiation spectrum. It allows us to know more about the Universe and make very accurate predictions about cosmic phenomena without actually going to measure the objects ourselves.

We know, for example, that the Universe is expanding, due to the color of the blackbody radiation we receive (red-shifted).

12. For the MCAT, do we need to memorize certain conversion factors or constants? And how in depth do we need to memorize the formulas? (this must have been asked before. I apologize!)

Equations, if needed, are presented in passages. Simple equations should be memorized.

13. Could anyone clarify the physics of a laboratory centrifuge? How do the spinning motion result in denser particles accumulating to the bottom of the centrifuge tube? This must mean that the centrifuge is not spinning in uniform circular motion, since the centripetal force would have particles accelerate towards the centre?

Heavier objects, when submitted to the same accelerating force, move faster, because their KE is greater. This causes them to reach the bottom and sediment first.

oops forgot...one more org chem Q!
14. Nitrogen lone pair electrons in rings often contribute to conjugation. This is a basic concept that confuses me, but how do we know that the lone pair is in the p orbital (and thus contribute to aromaticity) and not any other orbital, such as s, d, or f?

Nitrogen's electrons cannot occupy d or f subshells, because N's principle quantum number is 2.

Thanks in advance for everyone who could help out with any one of these questions.

👍