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.

 

[modelslashactor]

Good morning,

I had a question about Galvanic cells. Since oxidation happens at the anode, that means electrons are leaving the anode and therefore anions should migrate to it to make up for the lost charge, right? I recently ran across a practice problem that contradicted this (possibly their error), so I wanted to ask if my logic was correct. Thanks!

 

[QofQuimica]

Good morning,

I had a question about Galvanic cells. Since oxidation happens at the anode, that means electrons are leaving the anode and therefore anions should migrate to it to make up for the lost charge, right? I recently ran across a practice problem that contradicted this (possibly their error), so I wanted to ask if my logic was correct. Thanks!

Yes, anions should flow from the salt bridge into the oxidation half-cell.

 

[Lests55]

(1)In my textbook's explanation of deviation from PV=nRT bliss, they state that "it is expected PV/RT=1 for one mole of ideal gas at any temperature and pressure". They go on to show V's + deviation and P's negative deviation. Why is the ratio equal to 1?

(2) When a reaction is shown as broken down into 2 or 3 steps, can I assume that the steps are elementary steps (and thus can use the coefficients of the reaction in the slow step to calculate the rate constant and rate)? Or do they have to be explicity labeled as an "elemantary step".

If so, I never made this distinction while taking gen chem. It was always "rate law must be determined by experiment" and the "stupid answer" on the tests were always with reaction orders equal to the coefficients. So for instance, if asked for the rate law I would mark "can't be determined" unless they explicity told me elementary step.

(3) Did you find the PS of the AAMC practice exams to be relevant difficulty-wise to the real deal? I am most concerned about physics of any of the sciences, but PS has been my highest score on AAMC practices. Also, did you find yourself "educatedly guessing" on a lot of the questions rather than knowing for sure (obviously more likely on VR and the why? science questions as to the calculation questions, but...). Thanks so much.

Q, I am seriously going to send you a check for as much work as you have done for me. Drop me a PM with your address.

 

[QofQuimica]

(1)In my textbook's explanation of deviation from PV=nRT bliss, they state that "it is expected PV/RT=1 for one mole of ideal gas at any temperature and pressure". They go on to show V's + deviation and P's negative deviation. Why is the ratio equal to 1?

(2) When a reaction is shown as broken down into 2 or 3 steps, can I assume that the steps are elementary steps (and thus can use the coefficients of the reaction in the slow step to calculate the rate constant and rate)? Or do they have to be explicity labeled as an "elemantary step".

If so, I never made this distinction while taking gen chem. It was always "rate law must be determined by experiment" and the "stupid answer" on the tests were always with reaction orders equal to the coefficients. So for instance, if asked for the rate law I would mark "can't be determined" unless they explicity told me elementary step.

(3) Did you find the PS of the AAMC practice exams to be relevant difficulty-wise to the real deal? I am most concerned about physics of any of the sciences, but PS has been my highest score on AAMC practices. Also, did you find yourself "educatedly guessing" on a lot of the questions rather than knowing for sure (obviously more likely on VR and the why? science questions as to the calculation questions, but...). Thanks so much.

Q, I am seriously going to send you a check for as much work as you have done for me. Drop me a PM with your address.

1) You may not want to pay me after I answer this one for you....that equation equals one because you have one mole of gas. In other words, PV/RT = n, and you have one mole of gas, so.... 😉

2) By elementary step, do you mean RDS? If so, you need to have some way to know which one is rate-determining. You can't figure that out from the individual steps alone, but if you know the overall reaction equation, you can figure out which one is the RDS. Rate law exponents have to be determined experimentally as you said.

3) I actually didn't take any of the AAMC tests, just the five Kaplan tests, so I can't answer that question. I would say that I did not have to make an educated guess on most questions, but of course there were some that I didn't know, and those I did guess on. If you didn't know this already, don't leave any questions blank on the MCAT; there is no penalty for guessing like there is on the SAT.

 

[kevin86]

hi this is just a simple question. But I recently ran into a question involving determines the color of a bunch ionic solutions. I never really learned how the D oribital affected color. Can you explain it to me. Is the D orbital the only thing that determines the color in a transitional metal + some nonmetal solution. So does the nonmetal molecule have no effect on color then?

 

[Lests55]

If I am calculating equilibrium for gases, I can just use pressures in Kp, right? What if the reaction contains different phases?

 

[QofQuimica]

hi this is just a simple question. But I recently ran into a question involving determines the color of a bunch ionic solutions. I never really learned how the D oribital affected color. Can you explain it to me. Is the D orbital the only thing that determines the color in a transitional metal + some nonmetal solution. So does the nonmetal molecule have no effect on color then?

This question is definitely beyond the scope of the MCAT; if you take inorganic chemistry, you will learn about ligand-metal interactions. But the simple answer to your question is that yes, the ligands present DO affect the color of the complex. This happens because the ligands affect the energy of the metal's d-orbitals. In a lone atom, the five d-orbitals are normally all degenerate (have the same energy), but the presence of ligands will "split" the energies of the d-orbitals. The amount of difference in energy between the d-orbitals will affect what color the complex becomes. Again, this is beyond the scope of freshman gen chem. If you got asked about something like this on the MCAT, they would have to give you a passage explaining it all.

 

[QofQuimica]

If I am calculating equilibrium for gases, I can just use pressures in Kp, right? What if the reaction contains different phases?

In that case, use Kc. You should only use Kp if all species present are gases.

 

[Lests55]

Hello Q,

Can you shed some light on this equation that I found on AAMC's topic list under ionic bonding:

"Force of att=R(n+e)(n-e)/d^squared"

It kind of looks like a constant (R), two charges, and the inverse square conservative force form. I don't think I have ever seen anything like that in my life....

Any last words of advice one week out?

 

[QofQuimica]

Hello Q,

Can you shed some light on this equation that I found on AAMC's topic list under ionic bonding:

"Force of att=R(n+e)(n-e)/d^squared"

It kind of looks like a constant (R), two charges, and the inverse square conservative force form. I don't think I have ever seen anything like that in my life....

Any last words of advice one week out?

I promise you that this is an equation you are (or should be!) intimately familiar with from electrostatics in physics. They changed the letters they used to denote the variables and constants, but the relationship is identical. If you still can't recognize it, take a look at the physics explanations thread; I think I wrote about it in the very last post. 😉 Electrostatics applies to chemistry, too. Don't forget about your physics just because you're doing a chemistry problem.

 

[BoyGenius]

Hi

When asked to find molar solubility do they want the X variable in the Ksp eqn. If you have Ba(OH)2 for example will the molar solubility of Ba be X and OH be 2x or (2x)^2?

Also, I have a question about flow speed. Let's say you have a test tube and you put a funnel in. The flow speed is fastest in the neck of the funnel since it has the smallest area, but according to Bernoulli's equation it would be fastest in the test tube since the elevation head (H) is lowest Pressure is constant since everything is at atmospheric pressure and so the speed v must increase right?

 

[kevin86]

oh hey can you explain the difference between ferromagnetic, paramagnetic and dimagnetic. And would a simple definition be sufficient, should we know the maths too

 

[Saluki]

oh hey can you explain the difference between ferromagnetic, paramagnetic and dimagnetic. And would a simple definition be sufficient, should we know the maths too

Diamagnetic: the individual atoms have no net magnetic field
-repelled from the pole of a strong bar magnet
-sometimes called "weakly antimagnetic"

Paramagnetic: can be a degree of alignment to create a net magnetic field, attracted towards the pole of a strong bar magnet- sometimes called "weakly magnetic"

Ferromagnetic: below a certain temperature (dependent on the material) there is a high degree of alignment Ex: iron, nickel

I think you should know the basic math of magnetic fields, if that's what you're referring to... Otherwise, Q will know....

 

[QofQuimica]

Hi

When asked to find molar solubility do they want the X variable in the Ksp eqn. If you have Ba(OH)2 for example will the molar solubility of Ba be X and OH be 2x or (2x)^2?

Also, I have a question about flow speed. Let's say you have a test tube and you put a funnel in. The flow speed is fastest in the neck of the funnel since it has the smallest area, but according to Bernoulli's equation it would be fastest in the test tube since the elevation head (H) is lowest Pressure is constant since everything is at atmospheric pressure and so the speed v must increase right?

Yes, molar solubility is how much of that stuff you have dissolved, which is usually represented as "X."

Ok, so the continuity equation would predict that the tube with the smaller cross-sectional area (the funnel stem) should have a faster velocity, like you said. Bernoulli's equation says that when Pi is the same everywhere (atmospheric pressure), you get an inverse relationship between v and P. So since the velocity is slower in the test tube, the pressure there should be higher.

 

[jsong812]

Hey Q,

I had a real simple question; how does the reaction between HCl and a carbonate ion produce carbon dioxide gas? Thanks!

 

[QofQuimica]

Hey Q,

I had a real simple question; how does the reaction between HCl and a carbonate ion produce carbon dioxide gas? Thanks!

You would form carbonic acid, which decomposes to water and carbon dioxide. It isn't easy for me to write the equations using text, but I'll do my best. This assumes you are using sodium carbonate:

2HCl + Na2CO3 -> H2CO3 <-> H2O + CO2

 

[BoyGenius]

Ok this is probably crucial so I'd like to clear it up.


Regarding pka's and Kas in relation to titration curves. When Ka is greater than 1 this means it is strong, less than 1 weak. when taking the -log of ka to get pka this means that a positive pka indicates a weak acid and a negative pka a strong acid. But on the titration curve of a strong acid, 1/2 way to the endpt. pH=pka and this means pka is positive for a strong acid! Please explain based on the titration curve. How can I figure out based on initial pH on the curve whether an acid/base is strong or weak.

 

[QofQuimica]

Ok this is probably crucial so I'd like to clear it up.


Regarding pka's and Kas in relation to titration curves. When Ka is greater than 1 this means it is strong, less than 1 weak. when taking the -log of ka to get pka this means that a positive pka indicates a weak acid and a negative pka a strong acid. But on the titration curve of a strong acid, 1/2 way to the endpt. pH=pka and this means pka is positive for a strong acid! Please explain based on the titration curve. How can I figure out based on initial pH on the curve whether an acid/base is strong or weak.

Ok, I think what is confusing you is that you're looking for a buffering region. But strong acids do not ever function as buffers, because their conjugate bases are too weak. You can tell very easily from the titration graph whether your acid is strong or weak just by the shape of it. If it is weak, there will be an initial buffering region (flat part of the curve) before you get much change in pH. It is in this buffering region that pH = pKa. In addition, the equivalence point will occur at a basic pH. On the other hand, if the acid is strong, there will be no buffering region (flat area), and the pH will not ever be equal to the pKa. (The pH range only goes from 0-14 in water at 25 C, so it can't be equal to a negative pKa!) The equivalence point will occur at a neutral pH (7 if you're at 25 C) for strong acids.

 

[cstar_24]

Hi
I have a question about superheated liquids, where the pressure is lowered while keeping the temperature constant so as to avoid a phase transition, resulting in a liquid with a temperature higher than the boiling point for the new low pressure. How is this possible on a phase diagram? With constant temp and lowered pressure, doesn't it just look like a straight line down on the diagram, so how is it possible to avoid a transition into gas?

Thanks so much!

 

[Lests55]

Hey Q,

Just watned to clear this up...

Covalent bonds are stronger than ionic bonds, but ionic bonded compounds have stronger intermolecular forces (salts are solids). True?

 

[QofQuimica]

Hi
I have a question about superheated liquids, where the pressure is lowered while keeping the temperature constant so as to avoid a phase transition, resulting in a liquid with a temperature higher than the boiling point for the new low pressure. How is this possible on a phase diagram? With constant temp and lowered pressure, doesn't it just look like a straight line down on the diagram, so how is it possible to avoid a transition into gas?

Thanks so much!

If you look at the phase diagrams, you will see that the lines separating the phases are not perfectly vertical. In fact, they usually slope in the positive direction. (The MP line for water is a notable exception to this rule.) Since the line between the liquid and gas phases represents the boiling point, that sloping takes into account how the boiling point changes with pressure. (BP will decrease as pressure decreases, as you said.) Here's a phase diagram of water so you can see what I mean. Note that the line from O to P (representing BP) slopes in the positive direction. Thus, a decrease in pressure will lead to a corresponding decrease in the temperature where the BP line is reached as well:

 

[QofQuimica]

Hey Q,

Just watned to clear this up...

Covalent bonds are stronger than ionic bonds, but ionic bonded compounds have stronger intermolecular forces (salts are solids). True?

Covalent bonds are not necessarily stronger than ionic bonds. The bond energies of both types of bonds actually overlap, and so you will have to evaluate each particular case individually instead of relying on a rule like this. Ionic compounds are held together by electrostatic forces, and these ARE stronger than any of the inTERmolecular interactions that hold covalent molecules together. This is in contrast to the inTRAmolecular covalent bonds, which, as I said, are of comparable strength to the ionic bonds.

 

[poppytart]

What is the difference between saying that a solution is saturated with 1.0 g of a salt left undissolved and saying that the solution is supersaturated? thanks 🙂

 

[don_Genaro_27]

Hi! Thanks in advance for the reply. I was wondering whether the ionic configuration for the Cobalt-3+ ion is

a. [Ar]3d6 OR
b. [Ar]3d5 4s1

Kaplan test says it is a, but my hunch following Hund's rule says it is b.

Also, which electron comes off first, is it the 4s2 electron or the 3d7.

Thanks a lot!

 

[QofQuimica]

What is the difference between saying that a solution is saturated with 1.0 g of a salt left undissolved and saying that the solution is supersaturated? thanks 🙂

A supersaturated solution has more than the normal amount of solute dissolved in it (i.e., beyond the saturation point). If you raise the temperature of the solution, you can force more salt to dissolve. If you then cool it back to the original temperature, the extra salt may not precipitate out immediately, giving rise to a supersaturated solution. Eventually the extra salt will precipitate out until the solution reaches the saturation point.

 

[QofQuimica]

Hi! Thanks in advance for the reply. I was wondering whether the ionic configuration for the Cobalt-3+ ion is

a. [Ar]3d6 OR
b. [Ar]3d5 4s1

Kaplan test says it is a, but my hunch following Hund's rule says it is b.

Also, which electron comes off first, is it the 4s2 electron or the 3d7.

Thanks a lot!

Kaplan's answer is actually correct. The reason why is that when the 4s and 3d orbitals are empty, the 4s orbital is lower in energy, and therefore it fills first. Once all of these orbitals have electrons in them, the 4s orbital is now higher in energy than the 3d orbitals, and it therefore empties first. In general, you should fill the 4s orbital before the 3d, and you should also empty the 4s orbital before the 3d. Thus, the two 4s electrons would be removed first, followed by the 3d7 electron.

 

[don_Genaro_27]

Thank you Q of Q! I also received a reply from Kaplan saying that their answer was correct. I appreciate your response as well. It clarified the issue nicely.

Best of luck to everyone on the real one!

 

[st.exupery]

On AAMC 6R I noticed that they asked a question about geometry. However, in the solutions, they use the words geometry and shape interchangably saying just to look at the bonds to atoms. (essentially, the shape). For example, XeF4 would be square planar (as opposed to octahedral). I've learned from g-chem classes that geometry and shape were two separate things and would have called the geometry of XeF4 as octahedral. Would it be safe to assume they are asking for shape only in these types of questions? Another example was SO2, was 'bent' as opposed to trigonal planar. Hmmm...

 

[QofQuimica]

On AAMC 6R I noticed that they asked a question about geometry. However, in the solutions, they use the words geometry and shape interchangably saying just to look at the bonds to atoms. (essentially, the shape). For example, XeF4 would be square planar (as opposed to octahedral). I've learned from g-chem classes that geometry and shape were two separate things and would have called the geometry of XeF4 as octahedral. Would it be safe to assume they are asking for shape only in these types of questions? Another example was SO2, was 'bent' as opposed to trigonal planar. Hmmm...

It sounds like you are thinking of electronic geometry and they are describing molecular geometry. In general, I would recommend always picking molecular geometry unless you are specifically asked for the electronic geometry. In other words, do not include the lone pairs when you pick the "shape" of the molecule.

 

[st.exupery]

It sounds like you are thinking of electronic geometry and they are describing molecular geometry. In general, I would recommend always picking molecular geometry unless you are specifically asked for the electronic geometry. In other words, do not include the lone pairs when you pick the "shape" of the molecule.

Got it. Thank you. I appreciate your help in the midst of all my flustered knowledge I'm trying to organize.

 

[toesock]

Hey,

would someone please give me a nice, comprehensive explaination as to how to calculate bond orders?
I know the formula, I just can't grasp it all.

Thanks a lot.

 

[QofQuimica]

Hey,

would someone please give me a nice, comprehensive explaination as to how to calculate bond orders?
I know the formula, I just can't grasp it all.

Thanks a lot.

The easiest thing to do is to count each sigma and pi bond as contributing one to the bond order, and dispense with the formula altogether. So a single bond has a bond order of 1, a double bond has a bond order of 2, and a triple bond has a bond order of 3. That's pretty simple, right? So what do you do when you have multiple resonance structures? You have to average the bond order over each resonance contributor, giving you a fractional bond order. For example, benzene has a bond order of 1.5, because there are two equal resonance contributors. Carbonate ion has a bond order of 1.33 because there are three equal resonance contributors. And so on.

 

[jsong812]

Hey Q,

Is the melting point or decomposition rate related to the strength of a compound's IMF? If so, which one?

 

[QofQuimica]

Hey Q,

Is the melting point or decomposition rate related to the strength of a compound's IMF? If so, which one?

I'm assuming IMF = intermolecular forces? If so, mp does depend on them in part, but it is more complex than that. This is b/c mp also depends heavily on packing ability (how well the molecules can fit together into a lattice), so the pattern isn't as consistently based on intermolecular forces as it is for, say, bp. I would say that decomposition is probably most closely correlated with temperature, and it would depend on mp too. Organic compounds with very high melting points are going to tend to want to decompose rather than melt when you heat them up to high temperatures.

Does that answer your question?

 

[jsong812]

I'm assuming IMF = intermolecular forces? If so, mp does depend on them in part, but it is more complex than that. This is b/c mp also depends heavily on packing ability (how well the molecules can fit together into a lattice), so the pattern isn't as consistently based on intermolecular forces as it is for, say, bp. I would say that decomposition is probably most closely correlated with temperature, and it would depend on mp too. Organic compounds with very high melting points are going to tend to want to decompose rather than melt when you heat them up to high temperatures.

Does that answer your question?

Kinda...

 

[QofQuimica]

bump

 

[pezzang]

Don't blindly memorize formulas, because that is bound to get you into trouble if you are asked a question that is even the slightest bit different than one you've seen before. Instead, the best way to solve Ksp problems IMHO is to make up one of those little charts, with the starting concentrations, change in concentrations, and final concentrations for each species. (Technically, you can ignore the salt, since it's a pure solid and it won't appear in the equilibrium expression.) You might be asked to calculate Ksp using the molar solubility, or you might be given Ksp and asked to calculate Qsp to determine whether your salt will precipate or not. If Ksp<Qsp, the salt will precipitate (supersaturated solution), but if Ksp>Qsp (unsaturated solution), it will not.

Again, I would like to emphasize to everyone: to score well on these tests (MCAT, PCAT, DAT, or OAT), you must understand what you are doing; don't blindly memorize formulas.

Is the reason that the sal will precipiate when Ksp<Qsp is because Qsp =(product^coefficient) /(reactant^coefficient) and that there must be a lot more product than reactant as compared to Ksp. And higher concentration of product in Qsp than that in Ksp indicates superaturated solution? Am I on the right track? Thank you moderators. You are awesome!!!

 

[pezzang]

pKa is the equilibrium constant for an acid dissociating into its conjugate base and H+. Thus, you could get asked to calculate pKa given the molar solubility of an acid, or you could get asked to calculate the molar solubility given the pKa. pKa values are also useful to gauge relative strengths of acids and bases: stronger acids have lower pKas, while stronger bases have higher pKas.

Isn't it Ka that is teh equilibrium constant for an acid dissciating into its conj base and H+, not pKa? I thought pKa was -log[Ka] like pH or pOH? Am I wrong?

 

[osjx-82]

can anyone explain this for me?

how come when you 'fill up' electrons (i.e. the Aufbau principle) you fill up orbitals/subshells in the order determined by the (n+l) values

ie. 4s fills before 3d because the (n+l) values are 4 and 5 respectively

yet when you take electrons away (for example, forming cations) the order is determined by only the principle quantum number

ie. 4s electron gets removed first, then the 3d, then 3p, then 3s etc etc.

and also, when forming Anions, which rule is followed? the former or the latter?

Thanks.

 

[pezzang]

I think there is a typo in your book; check the EK website to see if anyone has reported an error for that question. The cathode is positive, not negative, for a galvanic cell, and the cathode is negative for electrolytic cells. In all types of cells, the electrons always move to the cathode, which is where reduction occurs. If you have a spontaneous cell (galvanic), then your cathode will be positive. This makes intuitive sense; the negatively charged electrons "want" to go to a positively charged electrode if they can. Cations in the salt bridge must move to balance the electrons so that you do not have a buildup of charge in the cathode. Thus, they will also move to the cathode, following the electrons. The salt bridge anions will go toward the anode to balance out the electrode cations left behind after the electrons leave.

I am not getting the concept of cations in the salt bridge. I am aware that electrons always move from anode to cathode through salt bridge. This occurs because of the oxidation allows electrons to be removed from anode (in galvanic cell) to cathode. Do cations actually move too (just like electrons)? In physics, I was taught that it's electrons that generate electric field and etc..always electrons move and positively charged ions (=protons?) are stationary. So, in galvanic cell, cations (protons?) are generated by cathode and move to anode? But I thought due to the movement of electrons in anode, oxidation occurs, and if cations move to anode, further oxication should take place, right? Please correct me.

 

[QofQuimica]

Is the reason that the sal will precipiate when Ksp<Qsp is because Qsp =(product^coefficient) /(reactant^coefficient) and that there must be a lot more product than reactant as compared to Ksp. And higher concentration of product in Qsp than that in Ksp indicates superaturated solution? Am I on the right track? Thank you moderators. You are awesome!!!

Sounds like you've got it.

 

[QofQuimica]

Isn't it Ka that is teh equilibrium constant for an acid dissciating into its conj base and H+, not pKa? I thought pKa was -log[Ka] like pH or pOH? Am I wrong?

No, you're right. pKa is the -log of Ka, and Ka is the dissociation constant for an acid.

 

[QofQuimica]

can anyone explain this for me?

how come when you 'fill up' electrons (i.e. the Aufbau principle) you fill up orbitals/subshells in the order determined by the (n+l) values

ie. 4s fills before 3d because the (n+l) values are 4 and 5 respectively

yet when you take electrons away (for example, forming cations) the order is determined by only the principle quantum number

ie. 4s electron gets removed first, then the 3d, then 3p, then 3s etc etc.

and also, when forming Anions, which rule is followed? the former or the latter?

Thanks.

If you look up at the top of this page, I answered that exact same question for someone else. 🙂

 

[QofQuimica]

I am not getting the concept of cations in the salt bridge. I am aware that electrons always move from anode to cathode through salt bridge. This occurs because of the oxidation allows electrons to be removed from anode (in galvanic cell) to cathode. Do cations actually move too (just like electrons)? In physics, I was taught that it's electrons that generate electric field and etc..always electrons move and positively charged ions (=protons?) are stationary. So, in galvanic cell, cations (protons?) are generated by cathode and move to anode? But I thought due to the movement of electrons in anode, oxidation occurs, and if cations move to anode, further oxication should take place, right? Please correct me.

The electrons don't pass through the salt bridge. They go through the wire. The salt bridge is there to complete the circuit. It contains an inert salt like KCl that dissociates to counter the charges being gained and lost by motion of the electrons. The Cl anions go to the anode cell to counter the cations released from the anode after the electrons leave, and the K cations go to the cathode cell to replace the lost cations from solution that deposit on the cathode.

 

[pezzang]

I read in my gen chem text book that
"Provided that the reaction is sufficiently exothermic, K will decrease so rapidly with increasing temporature."
Can you briefly explain how this can occur? I thought le Chantiler principle said that reaction will proceed in a direction that will alleviate the heat. So I thought the reaction will forward direction since heat is released and thus heat appears in the product. I think my reasoning is incorrect. Correct me if I am wrong. Thanks.

 

[QofQuimica]

I read in my gen chem text book that
"Provided that the reaction is sufficiently exothermic, K will decrease so rapidly with increasing temporature."
Can you briefly explain how this can occur? I thought le Chantiler principle said that reaction will proceed in a direction that will alleviate the heat. So I thought the reaction will forward direction since heat is released and thus heat appears in the product. I think my reasoning is incorrect. Correct me if I am wrong. Thanks.

It's not simple to apply Le Chatelier's Principle with regard to heat, because the value of K actually changes with temperature. So you have two things going on at one time: the reaction does shift according to Le Chatelier's Principle when you change temperature, and the equilibrium position also changes with temperature (i.e., you have a different value of K when you heat or cool the reaction). That is what they are trying to tell you there.

 

[pezzang]

Most reactions speed up if you heat them because most reactions that you are familiar with are endothermic. Heat can be considered a reactant in an endothermic reaction, so adding more of it pushes the reaction toward the products according to LeChatelier's Principle. If the reaction is exothermic, where heat is like a product, you will push it in the reverse direction by heating it up.

According to your model of treating endo/exothermic as addition of heat in reactants and products, respectively, increasing heat (adding heat) moves the reaction to the right for endothermic and to the left for the exothermic. It is contrary to my last question about Le Chantelier's principle that alleviates the heat added. In other words, my last posting said that endothermic will shift the reaction to the left to move toward where heat is added.
Secondly, since K changes with temperature, should we assume that the temperature drop/rise for exo/endothermic will cause the change in value of K? In that case, how do we balance the two factors you mentioned above?
"So you have two things going on at one time: the reaction does shift according to Le Chatelier's Principle when you change temperature, and the equilibrium position also changes with temperature (i.e., you have a different value of K when you heat or cool the reaction). "
It is a long post and I really appreciate your help. I may not have been to clear. Please let me know if anything is too ambiguous. Thanks!

 

[pezzang]

2) I think that would work, because the metals at the bottom left of the periodic table (say Cs) will be the most amenable to giving up their valence electrons, and will have the most metallic character. It would certainly be exciting if you dropped some Cs metal in water. 😉

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... 😱

 

[QofQuimica]

According to your model of treating endo/exothermic as addition of heat in reactants and products, respectively, increasing heat (adding heat) moves the reaction to the right for endothermic and to the left for the exothermic. It is contrary to my last question about Le Chantelier's principle that alleviates the heat added. In other words, my last posting said that endothermic will shift the reaction to the left to move toward where heat is added.
Secondly, since K changes with temperature, should we assume that the temperature drop/rise for exo/endothermic will cause the change in value of K? In that case, how do we balance the two factors you mentioned above?
"So you have two things going on at one time: the reaction does shift according to Le Chatelier's Principle when you change temperature, and the equilibrium position also changes with temperature (i.e., you have a different value of K when you heat or cool the reaction). "
It is a long post and I really appreciate your help. I may not have been to clear. Please let me know if anything is too ambiguous. Thanks!

Ok, I think I misunderstood your question. If you have an exothermic reaction, heat is considered to be a "product." Therefore, we would predict according to Le Chatelier's Principle that heating an exothermic reaction should cause it to go in the reverse direction (back toward reactants.) The reverse would be true for endothermic reactions, where heat is a "reactant."

Knowing how to balance the change in K with temperature with the effect of Le Chatelier's Principle is definitely beyond the scope of the MCAT. And it's also beyond the scope of my knowledge; sorry. 😛

 

[pezzang]

Okay, in this TPR science workbook, they have this question:

Of the following, which is the strongest base?

A) HO-
B) NH3
C) NH2-
D) CH3-

Answer is D. How's that? I thought OH- was about as strong a base as you can get? 😕

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!