General Chemistry Question Thread

[QofQuimica]

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Unacceptable topics:
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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.

 

[brownie786]

First off let me say that I'm using EK and this material is not in our books.

Second, foil makes the most sense. If you have 1 gram of substance, how will you make it have the most surface area? Most likely flatten it out as much as you can, hopefully to 1 e- thick, but that's not really possible. The only one that is as flat as can be with the 1g mass requirement is the foil.

Thanks!!!! 😳
A lot simpler than i was making it out to be.

 

[Mutt]

Solubility Question:

When I see problems involving Ksp, the equations are confusing me. For instance, when Pb(OH)2 dissolves, does is the Ksp= [Pb][OH]^2 or [Pb][2OH]^2; when does the subscript appear as both a coefficient and a exponent or just as an exponent?

I am missing something fundamentally?

 

[deleted106503]

Solubility Question:

When I see problems involving Ksp, the equations are confusing me. For instance, when Pb(OH)2 dissolves, does is the Ksp= [Pb][OH]^2 or [Pb][2OH]^2; when does the subscript appear as both a coefficient and a exponent or just as an exponent?

I am missing something fundamentally?

Ksp is just like any other kind of K. Products/Reactants for elementary equations. Reactants are solid and liquid (not aq), so you don't include it. Ksp = [Pb][OH]^2 for Pb(OH)2 being dissolved.

Now, if you want to find the SOLUBILITY (not solubility product), you use the second equation you have there substituting x for each ion. so Ksp = (x)(2x)^2, Ksp = 4x^3. Solve for x and that's the number of mol/L that can be dissolved.

 

[Mutt]

Ksp is just like any other kind of K. Products/Reactants for elementary equations. Reactants are solid and liquid (not aq), so you don't include it. Ksp = [Pb][OH]^2 for Pb(OH)2 being dissolved.

Now, if you want to find the SOLUBILITY (not solubility product), you use the second equation you have there substituting x for each ion. so Ksp = (x)(2x)^2, Ksp = 4x^3. Solve for x and that's the number of mol/L that can be dissolved.

So molar solubility also uses the notation "Ksp," then?

What doesn't make sense to me, is the former Ksp (solubility "product") where products over reactants should be notated - like other equilibrium constants. If (aq) are not included, then the products should not be included and it should be 1/([Pb][OH]^2) - but that is obviously wrong.

 

[deleted106503]

So molar solubility also uses the notation "Ksp," then?

What doesn't make sense to me, is the former Ksp (solubility "product") where products over reactants should be notated - like other equilibrium constants. If (aq) are not included, then the products should not be included and it should be 1/([Pb][OH]^2) - but that is obviously wrong.

I think I confused you a little. I said "not aq" referring to liquid, which aren't aqueous. Water is a liquid and wouldn't be included in the Ksp.

Pb(OH)2 (s) + H2O (l) --> Pb+2 (aq) + 2OH- (aq)

It's just like anyother constant with products over reactants for only aqueous ions and gases if the gas is not in its natural state (I think).

So then Ksp = ([Pb+2][OH-]^2)/([Pb(OH)2][H2O])

Since Pb(OH)2 is a solid in the reactants, it's concentration is 1. H2O liquid is also given a concentration of 1 when a reactant. So substitute 1 in for each of the concentrations I gave in the Ksp formula and the demoninator is 1, so the numerator is all that matters.

 

[QofQuimica]

I think I confused you a little. I said "not aq" referring to liquid, which aren't aqueous. Water is a liquid and wouldn't be included in the Ksp.

Pb(OH)2 (s) + H2O (l) --> Pb+2 (aq) + 2OH- (aq)

It's just like anyother constant with products over reactants for only aqueous ions and gases if the gas is not in its natural state (I think).

So then Ksp = ([Pb+2][OH-]^2)/([Pb(OH)2][H2O])

Since Pb(OH)2 is a solid in the reactants, it's concentration is 1. H2O liquid is also given a concentration of 1 when a reactant. So substitute 1 in for each of the concentrations I gave in the Ksp formula and the demoninator is 1, so the numerator is all that matters.

The concentrations of solids and liquids aren't one. Pure water actually has a concentration of 55.4 M. But it doesn't get included in an equilibrium expression, because we are making the reasonable assumption that the concentration of pure solids and pure liquids like water remain more or less constant.

Molar solubility is basically what you are solving for when you solve for x. It tells you how much of a particular substance is dissolved in the liquid.

Gases should always be included in the equilibrium expression. If you have every species in your equation in the gas form, you can find the equilibrium constant in terms of partial pressure instead of molarity. You should not do that, however, if your gases are aqueous.

 

[ssa915]

circle the acid which is the weakest acid of the group:

Al3+, Cu2+, Na+, H+

Answer Na+

My problem - basicity follows metallic character, meaning it increases going to the left and down on the periodic table, right?

So Na+ would be the stronger base (weaker acid) than H+ (which is the weaker base, and stronger acid), right?

So this trend is true for all elements, but when you are looking at compounds, like NaH and LiH, this differs?

I just need some clarification, i know i'm missing something very basic. Thanks.

 

[t2oo5]

circle the acid which is the weakest acid of the group:

Al3+, Cu2+, Na+, H+

Answer Na+

My problem - basicity follows metallic character, meaning it increases going to the left and down on the periodic table, right?

So Na+ would be the stronger base (weaker acid) than H+ (which is the weaker base, and stronger acid), right?

So this trend is true for all elements, but when you are looking at compounds, like NaH and LiH, this differs?

I just need some clarification, i know i'm missing something very basic. Thanks.

I may certainly be wrong, so someone please correct me, but heres my idea:

First: Look at things in terms Lewis Acids and Bases, I think it would be the only way to evaluate this problem. Ok, a Lewis acid is an electron receiver. Which ones of these Ions has the highest affinity to attract electrons? A 3+ Ion, or a +1 Ion? I would guess the +3 Ion because it has the highest positive charge, so Aluminum (III) has the highest affinity , next Copper (II).
Now, you have left H+ and Na+. Look in terms of Bronsted/Lowry Acids and Bases. A B.L. Acid is a proton donor, and a base is a proton receiver. H+ is a single proton, so I would presume it to be the stronger Bronsted/Lowry acid. You are left with Na+, with the weakest affinity for electrons among Al3+,Cu2+, and itself, and the weaker proton donor between it and H+.

I never really answered a question like this, so I don't know if my logic is correct. I might be completely wrong, so someone please correct me if I am.

 

[QofQuimica]

circle the acid which is the weakest acid of the group:

Al3+, Cu2+, Na+, H+

Answer Na+

My problem - basicity follows metallic character, meaning it increases going to the left and down on the periodic table, right?

So Na+ would be the stronger base (weaker acid) than H+ (which is the weaker base, and stronger acid), right?

So this trend is true for all elements, but when you are looking at compounds, like NaH and LiH, this differs?

I just need some clarification, i know i'm missing something very basic. Thanks.

I think the "basic" thing you are missing is that NaH is H-, not H+ 😉 The conjugate acid of H- is H2, not H+

 

[Mutt]

Molar solubility is basically what you are solving for when you solve for x. It tells you how much of a particular substance is dissolved in the liquid.

So, why then, does one include "2" as both a coefficient and an exponent for molar solubility? Isn't the "2" accounted for in one or the other?

 

[harrypotter]

I just sat through the dreaded Electrochemistry lecture. Ugh! Some quick questions. If a passage shows you a picture of a generic cell, (with salt bridge, the 2 metal pieces (?), etc) how are you supposed to know if it's galvanic or electrolytic? After answering that, how do you figure out which side is the anode and cathode? Do you have to look at reduction potentials to figure it out?

Also, what are the + and - for anodes and cathodes used for? Is it for the Nernst equation?

 

[ssa915]

I think the "basic" thing you are missing is that NaH is H-, not H+ 😉 The conjugate acid of H- is H2, not H+

Ok i get it. So NaH would be the stronger base comapred to LiH. Right?

 

[QofQuimica]

So, why then, does one include "2" as both a coefficient and an exponent for molar solubility? Isn't the "2" accounted for in one or the other?

No, you need to have both of them. Let's talk about CaF2 just so we have a salt to reference.

You include the coefficient 2 for the F ions because an ion like F that appears twice will be formed in twice the amount that the starting salt dissociates. In other words, if x mol of the salt dissolves, you will form x mol of Ca2+ and 2x mol of F-, and the concentration of F- will be twice the concentration of Ca2+. The exponent is because Keq is defined as the product of all of the concentrations of all of the ions. So since you are forming three ions here: Ca2+, F-, and F-, you must multiply all of their concentrations together to get Ksp. In this case, Ksp = [Ca2+][F-][F-], and when you substitute in the ion concentrations, you wind up getting Ksp = (x)(2x)(2x), giving you the 4x^3 that you are familiar with.

 

[QofQuimica]

I just sat through the dreaded Electrochemistry lecture. Ugh! Some quick questions. If a passage shows you a picture of a generic cell, (with salt bridge, the 2 metal pieces (?), etc) how are you supposed to know if it's galvanic or electrolytic? After answering that, how do you figure out which side is the anode and cathode? Do you have to look at reduction potentials to figure it out?

Also, what are the + and - for anodes and cathodes used for? Is it for the Nernst equation?

I think this post will help you: http://forums.studentdoctor.net/showpost.php?p=3777346&postcount=12

In general, any time you have separate cells, that is going to be galvanic, because a nonspontaneous cell wouldn't need to be separated.

 

[QofQuimica]

Ok i get it. So NaH would be the stronger base comapred to LiH. Right?

Just so that we're on the same page, the base here is the hydride (H-), not the alkali cations. I don't think it would be accurate to say that sodium hydride is a "stronger" base than lithium hydride, because they are both the same base with different counterions. Now whether one hydride would work better in practice than the other in the lab is a complicated issue, because the counterions can definitely affect how the base reacts. I was able to get a reaction to go using LiOH that wouldn't work with NaOH, and I can't really explain why. But I wouldn't say that it's a difference in basicity; I think it's probably because the lithium atom itself might have been chelated by my compound. Oxygen is known to be attracted to lithium, and I was trying to hydrolyze an ester, so maybe it worked because the ester oxygen could interact better with the lithium than it could with the sodium and that helped to activate it. But the base was the same in both cases. Does that make sense, or am I confusing you more?

 

[ssa915]

Just so that we're on the same page, the base here is the hydride (H-), not the alkali cations. I don't think it would be accurate to say that sodium hydride is a "stronger" base than lithium hydride, because they are both the same base with different counterions. Now whether one hydride would work better in practice than the other in the lab is a complicated issue, because the counterions can definitely affect how the base reacts. I was able to get a reaction to go using LiOH that wouldn't work with NaOH, and I can't really explain why. But I wouldn't say that it's a difference in basicity; I think it's probably because the lithium atom itself might have been chelated by my compound. Oxygen is known to be attracted to lithium, and I was trying to hydrolyze an ester, so maybe it worked because the ester oxygen could interact better with the lithium than it could with the sodium and that helped to activate it. But the base was the same in both cases. Does that make sense, or am I confusing you more?

Nope, not confusing. Thanks!

 

[pezzang]

I am reading EK chem and I have trouble understanding this in chpter 1 under the heading "1-17 Energy levels of Electrons" on pg 18. It says "Like the energy between bonding atoms, the energy between the electron and the nucleus increaes from a negative to zero as the electron moves to an infinite distance away from the nucleus."
I have been having trouble understanding "electrostatic potential energy" created due to the bonding between electrons and nucleus. I have trouble envision it even though I know what potential energy is in physics. Can you please explain how this electrostatic potential energy is created and how its signs and magnitude change in respect to the bond length? Also, I hope to understand the quote above; especially, how the energy changes from negative to zero as the electron moves to an infinite distance away from the nucleus (thus increasing). Thanks, QofQuimica and anybody else who can answer.

 

[QofQuimica]

I am reading EK chem and I have trouble understanding this in chpter 1 under the heading "1-17 Energy levels of Electrons" on pg 18. It says "Like the energy between bonding atoms, the energy between the electron and the nucleus increaes from a negative to zero as the electron moves to an infinite distance away from the nucleus."
I have been having trouble understanding "electrostatic potential energy" created due to the bonding between electrons and nucleus. I have trouble envision it even though I know what potential energy is in physics. Can you please explain how this electrostatic potential energy is created and how its signs and magnitude change in respect to the bond length? Also, I hope to understand the quote above; especially, how the energy changes from negative to zero as the electron moves to an infinite distance away from the nucleus (thus increasing). Thanks, QofQuimica and anybody else who can answer.

This is really more of a physics question, but basically electrical potential energy stems from the fact that unlike charges (nucleus versus electrons) attract one another. Ideally, they would like to meet (distance of zero); this is the position of minimum potential energy. However, the electrons are not able to come in to the nucleus for various reasons (such as repulsion by other electrons), and so they are at some distance of d away from the nucleus. Now, here is the tricky part. The maximum distance between the nucleus and the electron is infinity, and at this point, there is no interaction between the electron and the nucleus. This is defined as a potential of zero. In other words, zero is the maximum value that the electrical potential energy can have. As the electron starts moving in toward the nucleus, the electrical potential energy decreases. Since we are starting at zero, there is only one way for it to go down, and that is for it to take a negative value (less than zero).

Bond lengths are a different issue. I think that here you are talking about van der Waals radii: in other words, how close can two atoms get together to form the "ideal" bond length. If that's what you're asking about, take a look at the image I've pasted below. You can see that there is an energy "well" that corresponds to the lowest potential energy, and this is the bond length. Push the atoms closer, and the potential goes up because you are forcing the electron clouds to overlap. Pull the atoms further apart, and the potential goes up because the electrons are being pulled further away from the other nucleus.

 

[Creightonite]

These subject threads have been really helpful.

One more question though. When looking for a half reduction and oxidation reactions, how exactly do you know what is being oxidized or reduced from looking at the Ev ratings. You know especially when they give you a table with a few ions undergoing reduction/oxidation. And the question would ask you whether this or that atom is more likely to be reduced/oxidized based on the Ev rating. Here is my logic to solve those kind of problemns, correct me if I am wrong.

1) convert all the reactions to the reductions, reversing Ev sign if needed
2) greater Ev would mean that a compund is more likely to be reduced than oxidized
3) elliminate the answer choices that are obviously wrong (i.e., like Na+ cant be oxidized anymore)
4) choose the answer: the better reducing agent --> higher Ev; better oxidizing agent --> lower Ev.

As my logic correct? Is there an easier way to solve this kind of problems? Thank you.

 

[QofQuimica]

These subject threads have been really helpful.

One more question though. When looking for a half reduction and oxidation reactions, how exactly do you know what is being oxidized or reduced from looking at the Ev ratings. You know especially when they give you a table with a few ions undergoing reduction/oxidation. And the question would ask you whether this or that atom is more likely to be reduced/oxidized based on the Ev rating. Here is my logic to solve those kind of problemns, correct me if I am wrong.

1) convert all the reactions to the reductions, reversing Ev sign if needed
2) greater Ev would mean that a compund is more likely to be reduced than oxidized
3) elliminate the answer choices that are obviously wrong (i.e., like Na+ cant be oxidized anymore)
4) choose the answer: the better reducing agent --> higher Ev; better oxidizing agent --> lower Ev.

As my logic correct? Is there an easier way to solve this kind of problems? Thank you.

Check out the explanations threads too. Here's a post I wrote about solving redox problems: http://forums.studentdoctor.net/showpost.php?p=2862982&postcount=9

 

[Creightonite]

thank you but that does not really answer my question. I wanted to know how to know whether something is a better reducing or oxidizing agent just when given a table where some of the reactions are written as oxidation and some are reduction with Ev values given.
I had a few questions in EK physics 1001 questions book when they asked which of the element from the table would be a better reducing or oxidizing agent without giving the full reaction.

Thank you.

 

[deleted106503]

thank you but that does not really answer my question. I wanted to know how to know whether something is a better reducing or oxidizing agent just when given a table where some of the reactions are written as oxidation and some are reduction with Ev values given.
I had a few questions in EK physics 1001 questions book when they asked which of the element from the table would be a better reducing or oxidizing agent without giving the full reaction.

Thank you.

First off let me say that if my post is going against rules of this sub-forum, please have it removed. QofQuimica, I received your PM and I understand 👍

For Creightonite, I'm posting this only becaues you mention EK. If you have the EK Gen Chem book, turn to page 113. On the red text, it mentions which are good reducing and oxidizing agents based on the location on reduction potential chart. Maybe this will help QofQuimica answer your question.


I actually have a question that I've been wondering.

Which of these ions is the strongest base: N 3-, OH-, Mg 2+, H+ (all aq). I eliminated the positively charged ones because those would be like acids, yes? As for the first two choices, I picked OH- because I always thought that was the strongest base (as in NaOH aq). The answer was N 3-, why? Is it just because it has a greater negative charge?

I know in previous posts there was something about periodic trends of acids and bases. EK doesn't mention any of this. From what I gathered, is a stronger acid always to the bottom left of the periodic table and stronger bases in the upper right (such as F-?)? How does electronegativity deal with this, too? Thanks in advance.

 

[Creightonite]

so based on that table, I understand that H2O is a better oxidizing agent than Zn2+ (because H2O is a worse reducing agent and has a lower Ev than Zn2+). I still feel confused.

 

[QofQuimica]

thank you but that does not really answer my question. I wanted to know how to know whether something is a better reducing or oxidizing agent just when given a table where some of the reactions are written as oxidation and some are reduction with Ev values given.
I had a few questions in EK physics 1001 questions book when they asked which of the element from the table would be a better reducing or oxidizing agent without giving the full reaction.

Thank you.

I don't have the EK books, but I'm assuming that you're looking at a table of reduction potentials. If so, the larger (i.e., more positive) the reduction potential is, the more likely that species is to get reduced (serve as an oxidizing agent). If you are asked to compare elements, I would assume that they are asking you to compare reduction potentials (ex. Na+ vs. Li+; that's the usual kind of scenario). Of course, if you're being asked to compare neutral Na with neutral Li, you would be looking at their oxidation potentials (just flip the reduction potential backwards) because those species aren't going to take on another electron. All of the information you need is in the table; just be consistent with how you compare the species.

 

[QofQuimica]

First off let me say that if my post is going against rules of this sub-forum, please have it removed. QofQuimica, I received your PM and I understand 👍

For Creightonite, I'm posting this only becaues you mention EK. If you have the EK Gen Chem book, turn to page 113. On the red text, it mentions which are good reducing and oxidizing agents based on the location on reduction potential chart. Maybe this will help QofQuimica answer your question.


I actually have a question that I've been wondering.

Which of these ions is the strongest base: N 3-, OH-, Mg 2+, H+ (all aq). I eliminated the positively charged ones because those would be like acids, yes? As for the first two choices, I picked OH- because I always thought that was the strongest base (as in NaOH aq). The answer was N 3-, why? Is it just because it has a greater negative charge?

I know in previous posts there was something about periodic trends of acids and bases. EK doesn't mention any of this. From what I gathered, is a stronger acid always to the bottom left of the periodic table and stronger bases in the upper right (such as F-?)? How does electronegativity deal with this, too? Thanks in advance.

Ok, just to clarify, is N3- supposed to be azide ion, or is it supposed to be a lone nitrogen atom with a -3 charge on it? Because if it's the former, I'd agree with you that hydroxide would be more basic. If it's the latter, then yes, it has to do with how much charge is on the nitrogen. Even N- species are potent bases. The conjugate base of ammonia (NH2-) has a pKa around 35-40. I don't know what the pKa of N3- would be, but it would be fierce. Don't spill it on yourself. 😉

 

[okcomputer]

Hi, first time poster.

I have been having a mental block in rationally explaining to myself why the Phase shift curve for water (the solid to liquid) shifts to the left (neg slope) while the curve for CO2 is the opposite.

Can you explain what (if anything) that has to do with the idea that water initially decreases in volume as the Temperature increases from 0-4 degress C? And then after, the volume increases with increaseing temperature.

Thanks, and I hope that wasnt too vague.

 

[deleted106503]

Ok, just to clarify, is N3- supposed to be azide ion, or is it supposed to be a lone nitrogen atom with a -3 charge on it? Because if it's the former, I'd agree with you that hydroxide would be more basic. If it's the latter, then yes, it has to do with how much charge is on the nitrogen. Even N- species are potent bases. The conjugate base of ammonia (NH2-) has a pKa around 35-40. I don't know what the pKa of N3- would be, but it would be fierce. Don't spill it on yourself. 😉

Thanks! Yes, it was a negative 3 charge on a lone Nitrogen.

 

[pezzang]

I have two questions regarding thermodynamics:

1. Why is that for an ideal gas, any state function can be described as a function of temp and vol only?

2. For an ideal gas, internal energy is only dependent upon temperature. How come it isn't dependent on volume for an idea gas but for a real gas? How does the behavior of real gas (intermolecular attractions and size of molecules) affect these deviations?

Thank you for the help! 😀

 

[H and D]

I am confused how you figure out if a salt will make a solution acidic or basic. There is only a short description of it in the EK book and I am confused. For example, how do I know NaC2H3O2- when added to pure water will make the solution basic?

Thanks!

 

[t2oo5]

I am confused how you figure out if a salt will make a solution acidic or basic. There is only a short description of it in the EK book and I am confused. For example, how do I know NaC2H3O2- when added to pure water will make the solution basic?

Thanks!

Feel free to correct me if Im wrong:
Sodium Acetate (NaC2H3O2) is conjugate base of a weak acid (Acetic Acid HC2H3O2) and a salt (NaCl).
HC2H3O2+NaCl--->NaC2H3O2+(H+)+(Cl-)

Theres your Sodium Acetate. This, when in an aqeuous environment, will dissociate furthur, and act as a lewis base:

NaC2H3O2+H2O-->HC2H3O2+(OH-)+(Na+)

In this case, water acts as an acid. The strong base, Sodium Acetate, will dissociate and act as a lewis base by accepting the proton from H2O.

The remaining Hydroxide in the solution contributes to the solutions basicity, while the Na+ Ion has a neglibile affect on pH.

 

[scholj]

I have a question about Le'Chatlier's Principle and reaction rates.

I read in EK Chemistry 1001 that adding heat to any reaction, even an exothermic reaction, will increase the rate of the reaction. If you consider Le'Chatlier's principle and take heat to be a product of the exothermic reaction, wouldn't adding heat cause the reverse reaction to occur? How does this increase the rate of the forward reaction?

 

[Creightonite]

^^^

If a reaction is exothermic, addition of heat will shift the equlibrium to the left. Think of heat as one of the products. If you add that product on the right the reaction will tend to go to the left.

I think heat affecting the reaction is a general concept. It works for the reactions where no significant ammount of heat is produced or consumed.

 

[QofQuimica]

Hi, first time poster.

I have been having a mental block in rationally explaining to myself why the Phase shift curve for water (the solid to liquid) shifts to the left (neg slope) while the curve for CO2 is the opposite.

Can you explain what (if anything) that has to do with the idea that water initially decreases in volume as the Temperature increases from 0-4 degress C? And then after, the volume increases with increaseing temperature.

Thanks, and I hope that wasnt too vague.

It's because the solid phase of water, unlike most substances, is less dense than the liquid phase. Nearly every other substance on earth has a denser solid phase, and so the melting point line angles to the right.

 

[QofQuimica]

Feel free to correct me if Im wrong:
Sodium Acetate (NaC2H3O2) is conjugate base of a weak acid (Acetic Acid HC2H3O2) and a salt (NaCl).
HC2H3O2+NaCl--->NaC2H3O2+(H+)+(Cl-)

Theres your Sodium Acetate. This, when in an aqeuous environment, will dissociate furthur, and act as a lewis base:

NaC2H3O2+H2O-->HC2H3O2+(OH-)+(Na+)

In this case, water acts as an acid. The strong base, Sodium Acetate, will dissociate and act as a lewis base by accepting the proton from H2O.

The remaining Hydroxide in the solution contributes to the solutions basicity, while the Na+ Ion has a neglibile affect on pH.

Right.

 

[QofQuimica]

I have a question about Le'Chatlier's Principle and reaction rates.

I read in EK Chemistry 1001 that adding heat to any reaction, even an exothermic reaction, will increase the rate of the reaction. If you consider Le'Chatlier's principle and take heat to be a product of the exothermic reaction, wouldn't adding heat cause the reverse reaction to occur? How does this increase the rate of the forward reaction?

Most reactions you are familiar with in the lab are endothermic, so in most cases adding heat does speed the reaction up. But for an exothermic reaction, it should go backward if you heat it.

Guys, I want to remind everyone that if you haven't taken the MCAT yet, you should not be answering questions in the subforum threads. We don't have any kind of score requirement or anything like that for people to become subforum volunteers, but we do think it's reasonable to ask that you not answer other people's MCAT questions unless you have already completed the pre-reqs and taken the test yourself, or unless you are a grad student answering content questions in your field of graduate study. You can't advise other people very well about the MCAT if you've never taken it yourself, right?

There is a group study thread here in the subforum if you studiers want to work out problems on your own, or you are free to set up new threads out in the main MCAT forum (but no new threads here in the subforum, please!). And for those of you studying now for August, if you want to come back and help answer questions for the next MCAT in Jan., I'd really appreciate it. 🙂

 

[5moreminutes]

Hello!!How are you?


For CH3NH2 and CH3NH3I
Since methyl amine the weak base ,

Then is methylammoonium iodide a conjugate base or a conjugate acid?

This is in a buffer...

Thanks 😀

 

[QofQuimica]

Hello!!How are you?


For CH3NH2 and CH3NH3I
Since methyl amine the weak base ,

Then is methylammoonium iodide a conjugate base or a conjugate acid?

This is in a buffer...

Thanks 😀

Assuming you are using a Bronsted-Lowry definition of acids and bases, the acid is the one with the proton, and the base is the one missing the proton. So the amine is the conjugate base, and the ammonium is the conjugate acid.

 

[5moreminutes]

Assuming you are using a Bronsted-Lowry definition of acids and bases, the acid is the one with the proton, and the base is the one missing the proton. So the amine is the conjugate base, and the ammonium is the conjugate acid.

Hmm, maybe there is a mistake in the Kaplan subject tests. Because that's what I thought it was supposed to be. But I think there's a typo when they say- methyyl; amine is a weak base and the methyl;ammoniumiodide is a conjugate base..

Thanks for your Help QofQ, appreciate it!

 

[QofQuimica]

Hmm, maybe there is a mistake in the Kaplan subject tests. Because that's what I thought it was supposed to be. But I think there's a typo when they say- methyyl; amine is a weak base and the methyl;ammoniumiodide is a conjugate base..

Thanks for your Help QofQ, appreciate it!

Yes, that's a typo. The ammonium should be the conjugate acid.

 

[Creightonite]

Most reactions you are familiar with in the lab are endothermic, so in most cases adding heat does speed the reaction up. But for an exothermic reaction, it should go backward if you heat it.

Guys, I want to remind everyone that if you haven't taken the MCAT yet, you should not be answering questions in the subforum threads. We don't have any kind of score requirement or anything like that for people to become subforum volunteers, but we do think it's reasonable to ask that you not answer other people's MCAT questions unless you have already completed the pre-reqs and taken the test yourself, or unless you are a grad student answering content questions in your field of graduate study. You can't advise other people very well about the MCAT if you've never taken it yourself, right?

There is a group study thread here in the subforum if you studiers want to work out problems on your own, or you are free to set up new threads out in the main MCAT forum (but no new threads here in the subforum, please!). And for those of you studying now for August, if you want to come back and help answer questions for the next MCAT in Jan., I'd really appreciate it. 🙂

ok, sorry. just wated to help.

 

[Bluesmurf314]

How do you know when a reaction takes place at the cathode or the anode of an electrochemical or galvanic cell? I have a question that just gave Aluminum and wanted to know if Al--> Al 3+ happened at the cathode or anode of each type of cell. How do you figure this out?

 

[googlinggoogler]

Hi, a question about vocabulary. A NH3 is "tetrahedral" and "trigonal pyramidal." I think the tetrahedral is termed the "molecular geometry." I can't seem to find what the term for trigonal pyramidal is.

 

[deleted106503]

Is Ka (acid dissociation constant) sometimes also called ionization constant??? One of the AAMC exam's solutions said something that I interpreted this way. It was about a weak acid having a very low ionization constant. Thanks in advance.

 

[QofQuimica]

ok, sorry. just wated to help.

I wasn't specifically posting to you; I was posting it as an FYI for everyone. And if you still want to help after you've taken the test in a few weeks, I and the rest of the volunteers would be very grateful. 🙂

 

[QofQuimica]

How do you know when a reaction takes place at the cathode or the anode of an electrochemical or galvanic cell? I have a question that just gave Aluminum and wanted to know if Al--> Al 3+ happened at the cathode or anode of each type of cell. How do you figure this out?

Two things:

1) Please do not open any new threads in the MCAT subforum. You should just post your question in the appropriate question thread. You asked a gen chem question, so I moved your question to the gen chem question thread. There are other threads for the other subjects.

2) You should also look at the explanations threads before posting a question, because your question may have already been answered. The explanations threads are the ones that are stickied at the top of the page. Try this post about redox reactions and see if it helps: http://forums.studentdoctor.net/showpost.php?p=2862982&postcount=9 Since the question you asked about is an oxidation, it will occur at the anode. Oxidations ALWAYS occur at the anode, regardless of cell type.

 

[QofQuimica]

Hi, a question about vocabulary. A NH3 is "tetrahedral" and "trigonal pyramidal." I think the tetrahedral is termed the "molecular geometry." I can't seem to find what the term for trigonal pyramidal is.

No, ammonia has a tetrahedral electronic geometry (bonding + nonbonding electrons) and a trigonal pyramidal molecular geometry (bonding pairs only).

 

[QofQuimica]

Is Ka (acid dissociation constant) sometimes also called ionization constant??? One of the AAMC exam's solutions said something that I interpreted this way. It was about a weak acid having a very low ionization constant. Thanks in advance.

Ka is a type of Keq, so it's fine to think of it that way. Since Ka measures how much the acid dissociates, and dissociation of an acid forms two ions....

 

[harrypotter]

What is the difference between Ksp and Molar Solubility?

When you are trying to answer a question asking which solution is most soluble, do you look at Ksp or Molar Solubility?

How come some of the units for Molar Solubility are M^2 or M^3 or just plain M?

Thank you!

 

[googlinggoogler]

No, ammonia has a tetrahedral electronic geometry (bonding + nonbonding electrons) and a trigonal pyramidal molecular geometry (bonding pairs only).

okay, thanks QofQuimica!

 

[QofQuimica]

What is the difference between Ksp and Molar Solubility?

When you are trying to answer a question asking which solution is most soluble, do you look at Ksp or Molar Solubility?

How come some of the units for Molar Solubility are M^2 or M^3 or just plain M?

Thank you!

Ksp is the salt solubility equilibrium constant; it is a measure of how much of the salt is in the ionized form. Molar solubility is the "x" that you are solving for when you set up a Ksp expression; it tells you how much of that particular ion dissolves in the solvent. The units of molar solubility will depend on how many ions are formed for that salt. Each concentration that you multiply in the Ksp expression contributes an extra "M" to the units.