General Chemistry Question Thread

[QofQuimica]

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All users may post questions about MCAT, DAT, OAT, or PCAT general chemistry here. We will answer the questions as soon as we reasonably can. If you would like to know what general chemistry topics appear on the MCAT, you should check the MCAT Student Manual (http://www.aamc.org/students/mcat/studentmanual/start.htm)

Acceptable topics:
-general, MCAT-level gen chem.
-particular MCAT-level gen chem problems, whether your own or from study material
-what you need to know about gen chem for the MCAT
-how best to approach to MCAT gen chem passages
-how best to study MCAT gen chem
-how best to tackle the MCAT physical sciences section

Unacceptable topics:
-actual MCAT questions or passages, or close paraphrasings thereof
-anything you know to be beyond the scope of the MCAT

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If you really know your gen chem, I can use your help. If you are willing to help answer questions on this thread, please let me know. Here are the current members of the General Chemistry Team:

-QofQuimica (thread moderator): I have my M.S. in organic chemistry and I'm currently finishing my Ph.D., also in organic chemistry. I have several years of university general chemistry TA teaching experience. In addition, I teach general chemistry classes through Kaplan for their MCAT, DAT, OAT, and PCAT courses. On the MCAT, I scored 14 on PS, 43 overall.

-Learfan: Learfan has his Ph.D. in organic chemistry and several years worth of industrial chemistry experience. He scored 13 on the PS section of the MCAT, and 36 overall.

-Sparky Man: Sparky Man has his Ph.D. in physical chemistry. He scored 14 on the PS section of the MCAT, and 36 overall.

-GCT: GCT scored in the 99th percentile on the PCAT. He has also taught introductory physics and general chemistry.

 

[QofQuimica]

I came across one of the questions/answers in this thread and was curious about this:
"if I were asked which of the following metals was most reactive would that mean that I have to choose the one that has the smallest electron affinity/electroneg? Can I predict reactivity with these trends?"
You said that it's the one with the most metallic character, which I think means high conductivity of electrons (and thus heat+electricity). Why would it be the metals with most metallic character but not something like Al (13)? Does it have to do with the number of valence electrons? Because Al has three while Cs has one valence electron? Also, because Cs has higher n than Al, meaning that Cs has higherPE and more unstable (=more likely to lose electrons)? Increasing n (principal number) increase PE and consequently decrease stability?
Lastly, if i were to choose which one is more reactive bw Li and Al (both of the same n but different orbit), i should choose Li because it's in s orbit, which is more stable? How is s more stable if s is in lower energy state than p, where electrons fill in after s orbit is filled?
i am confused... 😱

Sorry, I should have been more clear about what I meant by "metallic character." I was actually referring to the periodic trends. Aluminum is a fairly electronegative metal, and thus would not have as much metallic character compared to highly electropositive metals like the alkalis. The periodic trends would predict that the most metallic elements (low electronegativity, low electron affinity, large radius, and low ionization energy) would be found in the bottom left of the table. Hence why I specifically mentioned Cs, which is the most metallic naturally occurring element.

 

[QofQuimica]

How are we to know that CH3- is weaker acid than the rest of the choices? I can't see why. I tried to make sense with the defitions of acid (h donor and e acceptor) and inverse relationship between the strength of conj. acid and its base but I can't tell how each choice will be weaker/stronger conj. acid in relation to other choices. Thanks for your help!

LMAO, you're really going through this entire thread, aren't you???

Ok, here you are mixing up conjugate bases and acids. Remember that if CH3- is the base, then its conjugate acid is CH4. The others are NH3, H2O, and NH4+. Of those four conjugate acids, CH4 definitely has the highest pKa, estimated to be around 50. Ammonia is around 35. Water is about 16. I forget exactly what ammonium is, but I think it's about 9 or so. So ammonium would be the strongest acid, and methane would be the weakest, exactly as you'd expect based on the relative conjugate base strengths.

 

[pezzang]

LMAO, you're really going through this entire thread, aren't you???

Ok, here you are mixing up conjugate bases and acids. Remember that if CH3- is the base, then its conjugate acid is CH4. The others are NH3, H2O, and NH4+. Of those four conjugate acids, CH4 definitely has the highest pKa, estimated to be around 50. Ammonia is around 35. Water is about 16. I forget exactly what ammonium is, but I think it's about 9 or so. So ammonium would be the strongest acid, and methane would be the weakest, exactly as you'd expect based on the relative conjugate base strengths.

So are we supposed to have memorized the relative pKa's (strength) of different molecules in order to tell the order of strength? If that is the case, generally, are there any general molecules that appear frequenly in MCAT? Are there any easy way to know the relative pKa's of different complex ions? Thanks.

 

[pezzang]

This is a question that I asked before but I am still not clear about reactivity.

If i were to choose which one is more reactive between Li and Al (both of the same n but different orbit), is Li less reactive because it's in s orbit (lower PE orbit than p), which is more stable?

 

[QofQuimica]

So are we supposed to have memorized the relative pKa's (strength) of different molecules in order to tell the order of strength? If that is the case, generally, are there any general molecules that appear frequenly in MCAT? Are there any easy way to know the relative pKa's of different complex ions? Thanks.

No, you don't need to memorize pKa values. I just cited them so that you can see how far apart they are. (Remember that since pKa is a log scale, every unit is 10x different than the one above or below it, so these differences in acidity are huge.) You should learn to have a feel though for where things lie relative to one another on the acidity or basicity scale. More electronegative atoms like O make better (weaker) conjugate bases versus less electronegative atoms like C. So CH4 will make a weaker acid and a stronger base versus NH3, which is a weaker acid than H2O, and that's weaker than HF.

 

[QofQuimica]

This is a question that I asked before but I am still not clear about reactivity.

If i were to choose which one is more reactive between Li and Al (both of the same n but different orbit), is Li less reactive because it's in s orbit (lower PE orbit than p), which is more stable?

I don't understand this question. They don't have the same n-value. Li's valence shell is the 2s, while Al's is the 3p. My interpretation of the previous poster's question was that the question was asking which elements behave most like metals in terms of the periodic trends (i.e., give up electrons most readily). Those elements are found at the bottom left of the periodic table. All of the alkali metals are very reactive in this sense. You generally don't find them as the neutral metals in nature; in fact, we have to keep our metallic sodium in oil, because it's so humid here in FL that the sodium will react with the water in the air and release hydrogen gas, which can actually catch fire. Don't ever stick a chunk of sodium in your pocket. 😉

 

[Ultimeaciax]

For this question Calculate the percent ionization in 0.250 M HCrO4-.

I don't understand how to get the HCrO4- disassociate into. The Ka value for it is 3.0x10^-7.

Anyway, I tried and this is what I got.

Ax^2+Bx+C=0
1x^2 + 3.0E-7x - 7.5E-8 = 0
[H+] = 2.737113198E-4 M
Percent Ionization = [H+]/[Acid concentration]*100
2.7371E-4/0.250 * 100 = .109%

 

[QofQuimica]

For this question Calculate the percent ionization in 0.250 M HCrO4-.

I don't understand how to get the HCrO4- disassociate into. The Ka value for it is 3.0x10^-7.

Anyway, I tried and this is what I got.

Ax^2+Bx+C=0
1x^2 + 3.0E-7x - 7.5E-8 = 0
[H+] = 2.737113198E-4 M
Percent Ionization = [H+]/[Acid concentration]*100
2.7371E-4/0.250 * 100 = .109%

If that's the acid you're supposed to use, it must be dissociating into CrO4^2- and H+. In that case, you wind up having Ka = [H+][A-]/[HA] ~ x^2/0.25, so x is about 2.74 ^-4 as you said. Then divide by .25 and multiply by 100 gives me 0.11%, same as you. Looks like you did it fine to me. Does your book say the answer is different?

 

[pezzang]

MCAT prep books say that increase in temp in exothermic reaction causes the reaction to shift left. is the reverse true as well? for example, decrease in exo. reaction shifts the reaction to the right. similarly, increase/decrease in temp in endo. reaction causes teh reaction to shift right/left, respectively?

As for the volume of the container in which gases reside, does increase in volume shift the reaction toward the side that contains more molecules? (just opposite of decreasing vol.(threby increasing pressure) causes the reaction to shift from the side with more molecules to the side with less molecules)?

I understand the logic of how decreasing vol. causes the reaction to shift toward the side with less molecules but not the reverse. Similary, with the increase/decrease in temp. in endothermic reaction and decrease in temp in exothermic reaction. Thanks for your help in advance.

 

[Dave_D]

MCAT prep books say that increase in temp in exothermic reaction causes the reaction to shift left. is the reverse true as well? for example, decrease in exo. reaction shifts the reaction to the right. similarly, increase/decrease in temp in endo. reaction causes teh reaction to shift right/left, respectively?

As for the volume of the container in which gases reside, does increase in volume shift the reaction toward the side that contains more molecules? (just opposite of decreasing vol.(threby increasing pressure) causes the reaction to shift from the side with more molecules to the side with less molecules)?

I understand the logic of how decreasing vol. causes the reaction to shift toward the side with less molecules but not the reverse. Similary, with the increase/decrease in temp. in endothermic reaction and decrease in temp in exothermic reaction. Thanks for your help in advance.

Ok, I'll give you the explaination my professor gave us about this. The answer uses Le Châtelier's principle. Basically you can think of heat as either a reagent or product. So in an exothermic reaction one of the products is heat. So write out the equation

A + B <->AB + heat

So remember the principle? If you increase the temperature you basically have more heat. Since the system tries to undo the change it shifts left. Reduce the temperature and it shifts right.(If I remember right this is why cars have cooling systems. You have to get rid of the heat otherwise it builds up and stops the reaction.)

Similarly an endothermic reaction is one that requires heat
A + B + heat <->AB

So increasing temperature drives this one right, reducing drives it left.(Actually once our professor told us to just consider heat to be a reagent or product it was so much easier to understand.)

 

[pezzang]

Ok, I'll give you the explaination my professor gave us about this. The answer uses Le Châtelier's principle. Basically you can think of heat as either a reagent or product. So in an exothermic reaction one of the products is heat. So write out the equation

A + B <->AB + heat

So remember the principle? If you increase the temperature you basically have more heat. Since the system tries to undo the change it shifts left. Reduce the temperature and it shifts right.(If I remember right this is why cars have cooling systems. You have to get rid of the heat otherwise it builds up and stops the reaction.)

Similarly an endothermic reaction is one that requires heat
A + B + heat <->AB

So increasing temperature drives this one right, reducing drives it left.(Actually once our professor told us to just consider heat to be a reagent or product it was so much easier to understand.)

Thanks, Dave. How about volume change?

 

[QofQuimica]

Thanks, Dave. How about volume change?

Volume is inversely proportional to pressure. If volume is increased, pressure is decreased, and the reaction moves to the side with more moles of gas. If volume is decreased, pressure is increased, and the reaction moves to the side with fewer moles of gas.

 

[pezzang]

Volume is inversely proportional to pressure. If volume is increased, pressure is decreased, and the reaction moves to the side with more moles of gas. If volume is decreased, pressure is increased, and the reaction moves to the side with fewer moles of gas.

Yes you are right. But my intuition says that even when volume is increased (pressure decreased), the reaction should proceed from the side with more moles of gas to that with less moles of gas because of diffusion. What's wrong with this?

 

[QofQuimica]

Yes you are right. But my intuition says that even when volume is increased (pressure decreased), the reaction should proceed from the side with more moles of gas to that with less moles of gas because of diffusion. What's wrong with this?

This isn't a case of diffusion. Diffusion occurs when you have a substance spreading out down its concentration gradient. You've probably had your chem teacher open a bottle of perfume and over time, the entire class can smell it, even the people in the back of the room. That's diffusion. You are dealing with an equilibrium here. If you perturb your system away from equilibrium by changing its pressure, you will cause the reaction to shift to restore equilibrium. But it isn't an issue of substances moving down their concentration gradients.

 

[pezzang]

I realize that E (or U) = q + w according to the 1st law of thermo. Is w = p(delta v) or w = fd + p(delta v)?

At constant volume, E = q because delta v = and hence w = p(delta ) = 0. Why at constant pressure, E = q as well if only delta p is 0 and delta v is not necessarily 0 and thus w = p(deltav) does not have to be equal to 0? I think E = q + w where w = p(delta v) and p is constant but not zero.

Lastly, books claim that at only constant pressure q = delta H. But because H = E (or U) + P(delta v) and P is constant but not necessarily zero, i think P(delta V) doesn't necessarily have to be zero, which makes the eq. at const pressure H = E + P(delta v). On the other hand, books claim that at const volume, H is not equal to E. How is P(delta V) not equal to zero at const volume is not changing and therefore delta V is equal to zero?

I am so confused about these equations involving E, q, H, P and V. Please help me. Thank you,

 

[pezzang]

Examkracker Gen chem book says that weak acids does not necessarily have to have strong conjugate base. For the sake of MCAT, should we neglect this fact and assume that weak acids generally have strong conjugate base?
And if we shouldn't neglect this fact and want to know the relative strength of conjugate base of a weak acid, what factors should we take account? Thank you for your help.

 

[pezzang]

Lol another question...

in a reaction,

NaH + H2O -> Na+ + OH- + H2

After a careful review I said that H2O is an acid because it donates H+ and OH- is an obvious conjugate acid, accepting H+ (proton). However, NaH in the left side looks like acid because it donates H+ as well. But I thought because it actually dissociates into Na+ and H- instead of donating H+, it is not an acid. But how do we explain H2? How is this made and is my reasoning correct? What would you add to solidify my understanding?

Also, is the reason that NO3- is neutral is because HNO3 is a strong acid and hence NO3- is a weak conjugate base, which is more or less equal to neutral? Examkracker says NO3- is neutral.

Thanks.

 

[QofQuimica]

I realize that E (or U) = q + w according to the 1st law of thermo. Is w = p(delta v) or w = fd + p(delta v)?

At constant volume, E = q because delta v = and hence w = p(delta ) = 0. Why at constant pressure, E = q as well if only delta p is 0 and delta v is not necessarily 0 and thus w = p(deltav) does not have to be equal to 0? I think E = q + w where w = p(delta v) and p is constant but not zero.

Lastly, books claim that at only constant pressure q = delta H. But because H = E (or U) + P(delta v) and P is constant but not necessarily zero, i think P(delta V) doesn't necessarily have to be zero, which makes the eq. at const pressure H = E + P(delta v). On the other hand, books claim that at const volume, H is not equal to E. How is P(delta V) not equal to zero at const volume is not changing and therefore delta V is equal to zero?

I am so confused about these equations involving E, q, H, P and V. Please help me. Thank you,

After reading this post, I'm confused myself. 😛 My advice is to not rely on memorizing equations. Work on actually understanding what is going on. A lot of your confusion is probably because the equations you're using are actually derivatives that must be integrated. If you try to solve them algebraically without taking that into account, you will get some strange results like you did.

 

[QofQuimica]

Examkracker Gen chem book says that weak acids does not necessarily have to have strong conjugate base. For the sake of MCAT, should we neglect this fact and assume that weak acids generally have strong conjugate base?
And if we shouldn't neglect this fact and want to know the relative strength of conjugate base of a weak acid, what factors should we take account? Thank you for your help.

Strength of conjugate bases is relative and correlated with the strength of the corresponding weak acids. A very weak acid, like a hydrocarbon (pKa around 50) will have a very strong conjugate base. A medium weak acid, like an alcohol (pKa around 17) will have a strong conjugate base. A slightly weak acid, like acetic acid (pKa around 5) will have a weak conjugate base as well. Understand that there are different degrees of being weak or strong for bases and acids, and it's a continuum from extremely strong to extremely weak, with everything else in between.

 

[QofQuimica]

Lol another question...

in a reaction,

NaH + H2O -> Na+ + OH- + H2

After a careful review I said that H2O is an acid because it donates H+ and OH- is an obvious conjugate acid, accepting H+ (proton). However, NaH in the left side looks like acid because it donates H+ as well. But I thought because it actually dissociates into Na+ and H- instead of donating H+, it is not an acid. But how do we explain H2? How is this made and is my reasoning correct? What would you add to solidify my understanding?

Also, is the reason that NO3- is neutral is because HNO3 is a strong acid and hence NO3- is a weak conjugate base, which is more or less equal to neutral? Examkracker says NO3- is neutral.

Thanks.

NaH is a strong base, not an acid. The hydrogen is a hydride, H-, not a proton.

Nitrate ion is too weak of a base to be able to dissociate water. An aqueous solution of nitrate ion would not be basic because of this.

 

[pezzang]

After reading this post, I'm confused myself. 😛 My advice is to not rely on memorizing equations. Work on actually understanding what is going on. A lot of your confusion is probably because the equations you're using are actually derivatives that must be integrated. If you try to solve them algebraically without taking that into account, you will get some strange results like you did.

Okay, so why is it that when P is constant, q = E? Also, why is it that when P is constant, H = q?

Is it only when P and V are both constant that q = E? As for the equation for heat of reaction (H), do both P and V have to be kept constant so as to make H = q? or just P?

 

[pezzang]

In the Nernst equation, E = E(standard) - (0.06/n)logQ. Here Q is product/reactant. For galvanic cell, which concentration should be the product and reactnat? Is reactant the conc. of anode and product = conc. of cathode because e- travel from anode to cathode? Similarly, in electrolytic cell, anode is the conc. of product and cathod the concentration of reactant? Thankx

 

[pnoybballin]

for S: Ca, Ba, Sr.....just remember the tv network CBS
for H: Ca, Ba, Sr + Happy...whats happy? Hb Ag Pb ...mercury, silvr and lead...add a py to the end and all the first letters spell HAPPY

I read this part in the soluubility rules and i just want to clarify. So those that are insolube for S, are CaSO4, BaSO4 etc etc. And for H, it would be CaCl2, BaF2, etc etc. Am I correct? Sorry, I just have a test on friday for general chem and I happen to run into this and I figured it would be very useful 😎. Thanks again.

 

[pezzang]

I need to stop writing here today but yet I have one more question.
Electrolytic runs by an external battery so it does not (or can't) use its chemical energy to work. And that's why electrons run from cathode to anode unlike galvanic cell where electrons run from anode to cathode toward the positive end. I read the Gen chem thread about EMF and electro/galvanic cell and I encountered this problem that said that electrons flow to the cathode in an electrolytic cell, which seems just too contrary to what I thought about electrolytic cell. The explanation says that because things are reduced in cathode, electrons flow to the cathode in electrolytic cell. But cathode in electrolytic cell is negative so electrons would be repelled if they move to cathode.....
IS this true? And if so, what was it that was flowing from cathode to anode in electrolytic cell? Thanks

 

[pezzang]

Is it ever possible that such intermolecular forces as covalent is used also as intramolecular interaction? If so, can other intermolecular forces as ionic and others used as intramolecular interaction forces as well? Thank you.

 

[QofQuimica]

Is it ever possible that such intermolecular forces as covalent is used also as intramolecular interaction? If so, can other intermolecular forces as ionic and others used as intramolecular interaction forces as well? Thank you.

I think I *am* going to limit you to one question per day. 😉 Ok, this is a lot of questions, and I don't have time to get to them all now, but I'll work on them when I get the chance. In answer to this one:

No, covalent bonds are inTRAmolecular (within a single molecule), not inTERmolecular (among several molecules). Ionic bonds are also inTRAmolecular. However, it is correct to say that the inTERmolecular forces (H bonds, dipole-dipole, and dispersion forces) are all held together by electrostatic forces, as are ionic bonds. I wrote two very extensive explanations posts about intramolecular bonds and intermolecular interactions in the gen chem explanations thread that you might want to read through if you haven't already.

 

[QofQuimica]

I need to stop writing here today but yet I have one more question.
Electrolytic runs by an external battery so it does not (or can't) use its chemical energy to work. And that's why electrons run from cathode to anode unlike galvanic cell where electrons run from anode to cathode toward the positive end. I read the Gen chem thread about EMF and electro/galvanic cell and I encountered this problem that said that electrons flow to the cathode in an electrolytic cell, which seems just too contrary to what I thought about electrolytic cell. The explanation says that because things are reduced in cathode, electrons flow to the cathode in electrolytic cell. But cathode in electrolytic cell is negative so electrons would be repelled if they move to cathode.....
IS this true? And if so, what was it that was flowing from cathode to anode in electrolytic cell? Thanks

Ok, you're kind of mixed up here. Electrons always go from the anode to the cathode. ALWAYS, no matter what kind of cell you have. This is because the cathode is defined as the reducing electrode, while the anode is being oxidized. Again, this is always the case, regardless of what kind of cell it is. What changes for galvanic vs. electrolytic cells is the SIGN of the electrodes. In a spontaneous (galvanic) cell, the negative electrons go from a negatively charged anode down their electrical gradient to a positively charged cathode. Makes sense, right? Negative charges would like to go to a positively charged electrode, given their druthers. On the other hand, in an electrolytic cell, you are forcing the negative electrons to go backward, against their electrical gradient, to a negative electrode. Thus, in electrolytic cells, the anode is now positive while the cathode is negative.

As for what flows to the anode, that would be the current. Recall that current is defined by physicists as the direction that imaginary positive charges would travel (though of course it is the electrons we are concerned with in chemistry, not the current). It is also possible if you were reading about a galvanic cell that the problem might mention ions from the salt bridge moving to the cathode and anode. (Since the electrodes are in separate cells for a galvanic cell, you need the salt bridge to complete the circuit or you won't get any electron flow.) What happens? Well, when the electrons move from the anode, they leave behind positive charges. So negative charges from the salt bridge (anions) travel to the anode cell to counterbalance them. Conversely, the electrons going to the cathode side would build up a net negative charge if positive ions (cations) from the salt bridge didn't travel to the cathode side to balance them. So salt bridge anions go to the anode side, and cations go to the cathode side.

 

[QofQuimica]

for S: Ca, Ba, Sr.....just remember the tv network CBS
for H: Ca, Ba, Sr + Happy...whats happy? Hb Ag Pb ...mercury, silvr and lead...add a py to the end and all the first letters spell HAPPY

I read this part in the soluubility rules and i just want to clarify. So those that are insolube for S, are CaSO4, BaSO4 etc etc. And for H, it would be CaCl2, BaF2, etc etc. Am I correct? Sorry, I just have a test on friday for general chem and I happen to run into this and I figured it would be very useful 😎. Thanks again.

Right, you've got the idea. Hope your test went/goes well.

 

[pezzang]

Are there ever a case where the triple point has lower (triple point) pressure and temperature than the temperature and the pressure of any melting point (the equilibrium line between gas and liquid)?
Is it a rule for the sake of MCAT that the triple point will always has higher P and T than melting point P and T?
Thanks, QofQuimica!!!!! 👍

 

[QofQuimica]

Are there ever a case where the triple point has lower (triple point) pressure and temperature than the temperature and the pressure of any melting point (the equilibrium line between gas and liquid)?
Is it a rule for the sake of MCAT that the triple point will always has higher P and T than melting point P and T?
Thanks, QofQuimica!!!!! 👍

Ok, first of all, the melting point is the line dividing the liquid phase from the SOLID phase, not the gas phase. Second, the triple point is at a LOWER pressure than either the MP or the BP. Below the triple point, you don't have any liquid phase at all any more. Instead, you have a sublimation/deposition line where the solid and the gas phases interconvert directly to one another without going through a liquid. (Think of dry ice; the reason why it sublimates instead of melting is because the triple point of carbon dioxide is at about 5.2 atm.) So the triple point pressure will always be lower than any melting point pressure. If you mean the vaporization/condensation line (separating the gas and liquid phases), then again, the triple point pressure will be lower than the vaporization pressure, because you can't have vaporization below the triple point.

If you look at the image that I've pasted below, the triple point is point B. Line BD is the melting point line, and line BC is the vaporization line. Pressure is on the Y-axis. Note that both lines BD and BC are at HIGHER pressure than point B is. The only line that is at a lower pressure than B is line AB, which is the sublimation line. Thus, the triple point is, by definition, the lowest pressure at which a liquid can exist. Below the triple point pressure, you can still have solid or gas phases, but no more liquid.

 

[pezzang]

Ok, first of all, the melting point is the line dividing the liquid phase from the SOLID phase, not the gas phase. Second, the triple point is at a LOWER pressure than either the MP or the BP. Below the triple point, you don't have any liquid phase at all any more. Instead, you have a sublimation/deposition line where the solid and the gas phases interconvert directly to one another without going through a liquid. (Think of dry ice; the reason why it sublimates instead of melting is because the triple point of carbon dioxide is at about 5.2 atm.) So the triple point pressure will always be lower than any melting point pressure. If you mean the vaporization/condensation line (separating the gas and liquid phases), then again, the triple point pressure will be lower than the vaporization pressure, because you can't have vaporization below the triple point.

If you look at the image that I've pasted below, the triple point is point B. Line BD is the melting point line, and line BC is the vaporization line. Pressure is on the Y-axis. Note that both lines BD and BC are at HIGHER pressure than point B is. The only line that is at a lower pressure than B is line AB, which is the sublimation line. Thus, the triple point is, by definition, the lowest pressure at which a liquid can exist. Below the triple point pressure, you can still have solid or gas phases, but no more liquid.

Oh, thank you, QofQuimica. Yes, I meant vap/condensation line. Looking at the phase diagram, the boliling temp will always be higher than that of the temp at the triple point. Can there be any exceptions in the relative location of the triple point in relation to boiling temp? In other words, are there any exceptions where vap/condensation temp will be lower of equal to that of the triple point temperature at different pressure? (like the uniqueness of water in phase change from solid to liquid or liquid to solid as compared to other substances) So it would mean that from the triple point, the BC is parallel to x-axis for some pressure greater than the pressure of the triple point. Do I make sense?... Thanks again! 😀

 

[QofQuimica]

Oh, thank you, QofQuimica. Yes, I meant vap/condensation line. Looking at the phase diagram, the boliling temp will always be higher than that of the temp at the triple point. Can there be any exceptions in the relative location of the triple point in relation to boiling temp? In other words, are there any exceptions where vap/condensation temp will be lower of equal to that of the triple point temperature at different pressure? (like the uniqueness of water in phase change from solid to liquid or liquid to solid as compared to other substances) So it would mean that from the triple point, the BC is parallel to x-axis for some pressure greater than the pressure of the triple point. Do I make sense?... Thanks again! 😀

Ok, so I think what you are asking is whether the boiling point line ever slants to the left the way the melting point line does for water? I can't think of any example where it would. Remember that the reason why the melting point line slants to the left for water is because unlike for most substances, the solid phase of water (ice) is less dense than the liquid phase. But I can't think of any substance that has a denser gas phase in comparison to its liquid phase. If there is such a substance, you certainly wouldn't be expected to know that for the MCAT. 😛

 

[pezzang]

"The lewis acids include all simple cations except teh alkali and the heavier alkaline earth metal cations."

Are the alkali and alkaline earth metal cations not a part of the lewis acids because they are basically conjugate bases of some of the strong acids? If so, wouldn't that leave such cations as Rb and Be because they, when reacted with H2O, don't form a strong acid? Or, is it because, regardless of the strength, the alkali and alkaline earth metal cations are weak/really weak conjugate bases of weak/strong acids?

I have reviewed EK Gen chem and now I am going to start working on physics. Thanks for answering so many questions everyday.. 🙂

 

[QofQuimica]

"The lewis acids include all simple cations except teh alkali and the heavier alkaline earth metal cations."

Are the alkali and alkaline earth metal cations not a part of the lewis acids because they are basically conjugate bases of some of the strong acids? If so, wouldn't that leave such cations as Rb and Be because they, when reacted with H2O, don't form a strong acid? Or, is it because, regardless of the strength, the alkali and alkaline earth metal cations are weak/really weak conjugate bases of weak/strong acids?

I have reviewed EK Gen chem and now I am going to start working on physics. Thanks for answering so many questions everyday.. 🙂

A Lewis acid is defined as a species that can accept an electron pair. Alkali metal cations have only lost one electron, so they won't want to accept an electron pair; if they did, they'd go from a +1 to a -1 oxidation number. Metallic character increases as you go down a group on the periodic table, following the periodic trends. The larger alkaline earth metals have more metallic character than the smaller ones do, and apparently that's enough to prevent them from being able to accept an electron pair.

 

[Lady Victoria]

Here's a question I've been struggling with and I'd appreciate it if someone could talk me through it like I'm a small child because no matter how I try to solve it, I just don't understand.

I'm probably just overlooking something basic, but I had a friend go over it with me and we figured out a way to get to the right answer, but we don't know why, if that makes sense. In other words, we don't know if we can apply the concept on a test. It's been bugging me for days.

It's a two-parter. OK, here it is:

The gage pressure in a tire is 35.0 lb/inches-squared in January at 25 degrees F. In July at 96 degrees F, the gage pressure is at 26.0 lb/inches-squared. The atm pressure is at 14.7 lb/inches-squared for both.

(a) What precentage of air leaked out of the tire?

(b) If half the air leaked out, what would the gage pressure then be?

 

[gridiron]

Here's a question I've been struggling with and I'd appreciate it if someone could talk me through it like I'm a small child because no matter how I try to solve it, I just don't understand.

I'm probably just overlooking something basic, but I had a friend go over it with me and we figured out a way to get to the right answer, but we don't know why, if that makes sense. In other words, we don't know if we can apply the concept on a test. It's been bugging me for days.

It's a two-parter. OK, here it is:

The gage pressure in a tire is 35.0 lb/inches-squared in January at 25 degrees F. In July at 96 degrees F, the gage pressure is at 26.0 lb/inches-squared. The atm pressure is at 14.7 lb/inches-squared for both.

(a) What precentage of air leaked out of the tire?

(b) If half the air leaked out, what would the gage pressure then be?

I have an idea on how to solve this bad boy....but I am not 100% sure...here it is....I would use the ideal gas law (P1V1/T1) = (P2V2/T2). First, make sure to convert F to K---as is the case when using the ideal gas law. Since P1, P2, T1, and T2 are known and since this occurs at constant atm. pressure set up a flow chart. What goes in is a cool gas and what comes out is a hot gas. And remember one more thing, total pressure is equal to gage pressure plus atmospheric pressure. So calculate the total pressure for each case first. Then, Divide both sides by V1 and then multiply each side by (T2 over P2). You should then get an answer that represents the ratio of V2/V1. This is the ratio of the final volume over the final volume. Convert the ratio to a percentage by multiplying the ratio by a 100 and subtract this from 100%. That should be how much air leaked out. For the second question, are you what the final gage pressure will be. Use the same formula (P1V1/T2)=(P2V2/T1). If the all the values are same as part (a), this time set V2=(1/2)V1. You should be able to cancel V1 from both sides of the equation because of this and than solve for P2. Hope this helps....if I am wrong please correct me....good luck.

 

[Lady Victoria]

I did most of what you said, but I used n2/n1 instead of V because I was trying to find the number of moles. My first thought was to find volume as well, but it doesn't work out to the right answer that way. I got 29% when I did it with moles, which is supposedly the right answer.

Now on part B, I'm having one hell of a time because the gage pressure is whatever the overall pressure is minus atmospheric pressure and for some reason, I get stuck.

Thank you so much for your help!

 

[gridiron]

I did most of what you said, but I used n2/n1 instead of V because I was trying to find the number of moles. My first thought was to find volume as well, but it doesn't work out to the right answer that way. I got 29% when I did it with moles, which is supposedly the right answer.

Now on part B, I'm having one hell of a time because the gage pressure is whatever the overall pressure is minus atmospheric pressure and for some reason, I get stuck.

Thank you so much for your help!

For part a, that was a mistake on my behalf. I thought it was asking for the volume of air leaked out, but looking back at your question, it is moles. For part B, remember, total pressure is the sum of the atmospheric pressure plus the gauge pressure. It is very unusual to see c.g.s units on the MCAT---if I remember correctly, the MCAT uses SI units only. 14.7 pounds per square inch is equivalent to 760 mmHg, which is 1 atm. For part b, set n2=0.5n1. This way, you eliminate n1 from the overall equation. I assume you can use the initial parameters (P1, and T1) from part a of the question and T2 from part a. This time, you will get a value of P2 which should represent the overall pressure. I assume this part builds off part a, so you can use the value of T2, T1 and P1. Now, assuming it builds of part a, subtract the overall pressure you get from your calculation from the atmospheric pressure(14.7 lb per square inch) [overall pressure - atmoshperic pressure] and that value should be your gauge pressure. I hope this helps!

 

[Lady Victoria]

For part a, that was a mistake on my behalf. I thought it was asking for the volume of air leaked out, but looking back at your question, it is moles. For part B, remember, total pressure is the sum of the atmospheric pressure plus the gauge pressure. It is very unusual to see c.g.s units on the MCAT---if I remember correctly, the MCAT uses SI units only. 14.7 pounds per square inch is equivalent to 760 mmHg, which is 1 atm. For part b, set n2=0.5n1. This way, you eliminate n1 from the overall equation. I assume you can use the initial parameters (P1, and T1) from part a of the question and T2 from part a. This time, you will get a value of P2 which should represent the overall pressure. I assume this part builds off part a, so you can use the value of T2, T1 and P1. Now, assuming it builds of part a, subtract the overall pressure you get from your calculation from the atmospheric pressure(14.7 lb per square inch) [overall pressure - atmoshperic pressure] and that value should be your gauge pressure. I hope this helps!

I can't believe how many careless mistakes I make. I got the cross-multiplying wrong when I was trying to isolate P2, which is what threw off my answer. Thank you so much for your help! The gage pressure is 13.8, if you care. LOL!

One last question. I know this is basic, but I've never had chemistry in my life and I'm taking Chemistry I this summer and I'm overwhelmed with the material, so please help me understand how you knew it wanted moles instead of volume? I immediately thought it was asking for volume when I initially read it and only changed the unknown to moles when the answer wasn't coming out the way it was supposed to.

 

[gridiron]

I can't believe how many careless mistakes I make. I got the cross-multiplying wrong when I was trying to isolate P2, which is what threw off my answer. Thank you so much for your help! The gage pressure is 13.8, if you care. LOL!

One last question. I know this is basic, but I've never had chemistry in my life and I'm taking Chemistry I this summer and I'm overwhelmed with the material, so please help me understand how you knew it wanted moles instead of volume? I immediately thought it was asking for volume when I initially read it and only changed the unknown to moles when the answer wasn't coming out the way it was supposed to.

Hey, no problem---that is why I am here to help. I figured it was moles and not volume (my mistake earlier) because I assumed the gas we were dealing with was ideal. Since the atmospheric pressure is the same for the input and output of the process, the only thing changing then, the temperature and pressure, is the internal energy of the gas. From the numbers, the temperature is decreasing as is the gage pressure. This means the total pressure is also decreasing. So if we think of the gas as concealed in the container, some amount escapes as it expands. Since the internal energy of the system is changing, that means some amount is leaving---and this is moles. With ideal gas laws, a frequent topic on the MCAT, if the internal energy is changing than volume changes are negligent. Volume changes take into effect with a change in phase--something not indicated in the problem. Best of luck....feel free to post any other questions/issues you might have 🙂

 

[Gabby]

I've got a thermochemistry question if anyone's up for it. I've gone over it a number of times and I can't figure out why the solutions manual did what it did for this problem.

I'm working on Hess's Law.

We have to calculate the enthaply change for the reaction:

N2H4 (l) + 2H2O2 (l) ----> N2 (g) +4H2O

From these equations:

N2H4 (l) + O2 (g) -----> N2 (g) + 2H2O (l) H= -622.2

H2 (g) + 1/2 O2 (g) ----> H2O (l) H = -285.8

H2 (g) + O2 (g) -----> H2O2 (l) H = -187.8


So I kept the first equation as is because the N2H4 is on the left in both the first equation and the original. I didn't need to multiply or anything because it's just one mole in the original equation.

I changed the third equation so that the H2O2 was on the left since it's on the left in the original. I also multiplied by two since it's 2H2O2 in the original. I also changed the sign.

Now for the second equation -- I kept the second equation as is, but multiplied by four because in the original equation, it's 4H2O on the right. However, the solutions guide only multiplied it by two. Why is that? I thought you use the original equation as a guide.

P.S. If anyone can clarify something for me, I'd be forever grateful. Is the answer always negative if the question is asking for how much heat was absorbed? Is the sign always positive if the question is asking for how much heat was evolved or released? I'm having trouble keeping the signs straight and my professor has contradicted the solutions manual a few times. He said the solutions manual is wrong and that just confuses me more because that's what I depend on to make sure I'm doing the problems correctly.

 

[gridiron]

I've got a thermochemistry question if anyone's up for it. I've gone over it a number of times and I can't figure out why the solutions manual did what it did for this problem.

I'm working on Hess's Law.

We have to calculate the enthaply change for the reaction:

N2H4 (l) + 2H2O2 (l) ----> N2 (g) +4H2O

From these equations:

N2H4 (l) + O2 (g) -----> N2 (g) + 2H2O (l) H= -622.2

H2 (g) + 1/2 O2 (g) ----> H2O (l) H = -285.8

H2 (g) + O2 (g) -----> H2O2 (l) H = -187.8


So I kept the first equation as is because the N2H4 is on the left in both the first equation and the original. I didn't need to multiply or anything because it's just one mole in the original equation.

I changed the third equation so that the H2O2 was on the left since it's on the left in the original. I also multiplied by two since it's 2H2O2 in the original. I also changed the sign.

Now for the second equation -- I kept the second equation as is, but multiplied by four because in the original equation, it's 4H2O on the right. However, the solutions guide only multiplied it by two. Why is that? I thought you use the original equation as a guide.

P.S. If anyone can clarify something for me, I'd be forever grateful. Is the answer always negative if the question is asking for how much heat was absorbed? Is the sign always positive if the question is asking for how much heat was evolved or released? I'm having trouble keeping the signs straight and my professor has contradicted the solutions manual a few times. He said the solutions manual is wrong and that just confuses me more because that's what I depend on to make sure I'm doing the problems correctly.

Hey. For the first part, you have done all the correct steps until the second equation. You are right to keep the first equation untouched, and to reverse the second equation and to multiply it by a factor of two. However, the third equation, you multiply by 2 and not 4. The first equation already has 2H20. That means, the second equation also needs to have 2H20. Therefore, you multiply the second equation by a factor of 2. You also multiply the change in enthalpy by two since it a state function. You do the same for the third equation. When you solve all three, you should get a change in enthalpy of -818.2 units. For the second question--it all depends on how you view the system and surroundings. My college chemical engineering professor gave me a good analogy---When you take out of the bank it is negative and when you put into the bank it is positive. Therefore, when heat has to be put in than the sign is positive, and when heat is released the sign is negative. If you use this convention, make sure you keeps the signs straight or you will get a different answer. Most endothermic reactions have a positive change in enthalpy and exothermic reactions have a negative change in enthalpy. Hope this helps!! Good luck

 

[Gabby]

Hey. For the first part, you have done all the correct steps until the second equation. You are right to keep the first equation untouched, and to reverse the second equation and to multiply it by a factor of two. However, the third equation, you multiply by 2 and not 4. The first equation already has 2H20. That means, the second equation also needs to have 2H20. Therefore, you multiply the second equation by a factor of 2. You also multiply the change in enthalpy by two since it a state function. You do the same for the third equation.

Oh, so you don't always have to go by the original equation? The equations just have to add up to the original? So if the original equation had 4H20s and the first equation had just one H20, then I'd multiply the second equation by 3? That way the first and second equation together would give me 4H20s? Is that right or did I misunderstand?

My college chemical engineering professor gave me a good analogy---When you take out of the bank it is negative and when you put into the bank it is positive. Therefore, when heat has to be put in than the sign is positive, and when heat is released the sign is negative. If you use this convention, make sure you keeps the signs straight or you will get a different answer. Most endothermic reactions have a positive change in enthalpy and exothermic reactions have a negative change in enthalpy. Hope this helps!! Good luck

I'm sorry to be such a pain, but I'm confused because that's exactly the opposite of what my professor said. He said if heat is coming out (exothermic), it'll have a positive charge, but endothermic will have a negative charge. Does it not matter in the final answer as long as you choose a way and stick to it? What happens if my answer is correct, but I make it a negative instead of a positive like the solutions manual did on a couple of problems?

 

[gridiron]

Oh, so you don't always have to go by the original equation? The equations just have to add up to the original? So if the original equation had 4H20s and the first equation had just one H20, then I'd multiply the second equation by 3? That way the first and second equation together would give me 4H20s? Is that right or did I misunderstand?[/QUOTE]

Problems with Hess Law involve manipulation of the reactions given to you. You have to make sure you understand the original equation before you try to manipulate the reactions given to you. Some people see how to manipulate the reactions right away and for some it involves practice. What I did was I wrote all the reactions down one by one. First, I try to match the reactants side of the original equation. Usually, when I figure that out, I can tackle the products side. But, you always have to make sure the reactions add up to the original. It is like solving a puzzle, once you get some to fit everything else falls into place.



I'm sorry to be such a pain, but I'm confused because that's exactly the opposite of what my professor said. He said if heat is coming out (exothermic), it'll have a positive charge, but endothermic will have a negative charge. Does it not matter in the final answer as long as you choose a way and stick to it? What happens if my answer is correct, but I make it a negative instead of a positive like the solutions manual did on a couple of problems?[/QUOTE]

Different text books give different signs but it all depends on how you view the system and the surroundings. The final answer doens't matter as long as you stick with one system--the numbers will comply accordingly. If you say exothermic is negative then whenever you see negative or release heat put a negative sign. The same holds true for endothermic reactions. The whole sign issue comes into play when you have to choose a way and stick with it. My professor told us that you will get a answer but you can change the sign accordingly depending on what your sign convention is. Just make sure to stick with only one sign convention---otherwise it can get tricky and troublesome. Good luck

 

[Gabby]

Great! Thanks for all your help!

 

[Gabby]

You know, it just clicked. It isn't dependent on that first equation but on the reactions side of the second equation and since there's an H2 and a 1/2 O2 there, that's why you multiply by two. I misunderstood you the first time. Duh. Thank you again! You saved me on this problem!

 

[gridiron]

Great! Thanks for all your help!

Hey, its no big deal. I love to help others with questions they might have in physics or chemistry because I know how stressful studying for the exam is

 

[skitzoe]

What bonds are the strongest and would have the highest boiling point? What are the order of the bonds from the strongest to the weakest?

 

[PBandJ]

What bonds are the strongest and would have the highest boiling point? What are the order of the bonds from the strongest to the weakest?

From strongest to weakest (and correspondingly highest BP to lowest BP):

Triple bonds > Double bonds > Single bonds

And if you're looking at bond length from longest to shortest:

Single bonds > Double bonds > Triple bonds

 

[skitzoe]

From strongest to weakest (and correspondingly highest BP to lowest BP):

Triple bonds > Double bonds > Single bonds

And if you're looking at bond length from longest to shortest:

Single bonds > Double bonds > Triple bonds

Sorry, I wasn't more specific, I meant between intramolecular bonds.
Polar, nonpolar, ionic, dipole-dipole, hydrogen bonds. I don't know the order of strongest to weakest. Do you?