General Chemistry Question Thread

[QofQuimica]

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.

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[xlr8]

On this same topic...

The energy of an e is E = -Rydberg / n^2. According to this eqtn, e gains energy as it is separated from nucleus, ie energy value becomes less negative, but the magnitude of the energy is also reduced. Once ionized (n=infinity) the e's energy approaches 0.

I'm confused by the negative energy value. Could someone explain? For instance, when e moves further out (from n=1 to n=4) it is excited and is said to have gained energy (or lost negative energy). But as e moves from n=4 to n=10 (if 10 were possible), its energy is approaches zero-- ie has virtually NO energy. How can it have gained energy-- excitement-- and end up with no energy??

Thanks...

I believe the energy indicates the amount of energy needed to displace the electron among energy levels. The electron uses the energy to move from one energy level to another. So if an electron is farther outside (higher n), the nucleus exerts a weaker pull, and thus less energy is needed to move it to an even higher energy level. Someone correct me if I'm wrong.

 

[BrokenGlass]

On this same topic...

The energy of an e is E = -Rydberg / n^2. According to this eqtn, e gains energy as it is separated from nucleus, ie energy value becomes less negative, but the magnitude of the energy is also reduced. Once ionized (n=infinity) the e's energy approaches 0.

I'm confused by the negative energy value. Could someone explain? For instance, when e moves further out (from n=1 to n=4) it is excited and is said to have gained energy (or lost negative energy). But as e moves from n=4 to n=10 (if 10 were possible), its energy is approaches zero-- ie has virtually NO energy. How can it have gained energy-- excitement-- and end up with no energy??

Thanks...

It always takes energy to separate objects that exert attractive forces on each other. This energy input is stored as potential energy. In this case, when energy goes towards zero, it's actually increasing, NOT decreasing. Potential energy is always measured relative to some reference value. In this case, this reference value is zero, and zero in this case is the maximum value of potential energy, i.e. any other value is less than zero.

ps: for a similar reason, there is a negative sign in graviational potential energy, E = - GMm/r

 

[axp107]

Are we supposed to know how to do a problem like this?

pKa of H2SO3 = 1.81
pKA of HSO3- = 6.91

What is the pKb for HSO3-

A) 7.09
B) 121.19

I was like.. ok you just do: 14 - 6.91... = 7.09

Apparently thats wrong! Am I missing something here?

 

[BrokenGlass]

Are we supposed to know how to do a problem like this?

pKa of H2SO3 = 1.81
pKA of HSO3- = 6.91

What is the pKb for HSO3-

A) 7.09
B) 121.19

I was like.. ok you just do: 14 - 6.91... = 7.09

Apparently thats wrong! Am I missing something here?

pKa + pKb = 14 applies to a conjuage pair (at 25 degrees celsius), NOT to the same compound.

Post the entire problem statement and all the answer choices.

 

[axp107]

tricky tricky.. lol

or is it really not tricky at all and something we're supposed to know really well haha

 

[free_radical]

ok-

pKa + pKb = 14, for the same conjugate pair.

but then in this case, the pKb of HSO3- should be "14- 1.81", or 12.19.

in which case, is the answer really B, and a decimal point got accidentally moved over?

 

[BrokenGlass]

tricky tricky.. lol

or is it really not tricky at all and something we're supposed to know really well haha

Post all answer choices?

 

[mpatricksweeney]

Hello:
Wondering about water autoionization...

Autoionization is an endothermic process. When we lower the rxn temperature, we push autoionization in the reverse direction, favoring formation of H2O. So [H] and [OH] decrease.

If both decrease (in equal proportion), the product, Kw = [H][OH] will be smaller than 10^-14, right? So Kw= 10^-14 only applies at 25d? And at lower temp, Kw becomes smaller as water molecules autoionize less and less?

At the temp for which Kw = 10^-18, would ph = 9 refer to a neutral solution, ie the point at which [H] = [OH]?

As autoionization is reversed, what do we say about acidic nature of water-- it's become less acidic and less basic?

I don't think Gen Chem cover therodynamics of acid-base: is acidification endothermic as a rule, or rxn-dependent?

Thanks a lot.

 

[BrokenGlass]

Hello:
Wondering about water autoionization...

Autoionization is an endothermic process. When we lower the rxn temperature, we push autoionization in the reverse direction, favoring formation of H2O. So [H] and [OH] decrease.

If both decrease (in equal proportion), the product, Kw = [H][OH] will be smaller than 10^-14, right? So Kw= 10^-14 only applies at 25d? And at lower temp, Kw becomes smaller as water molecules autoionize less and less?

At the temp for which Kw = 10^-18, would ph = 9 refer to a neutral solution, ie the point at which [H] = [OH]?

As autoionization is reversed, what do we say about acidic nature of water-- it's become less acidic and less basic?

I don't think Gen Chem cover therodynamics of acid-base: is acidification endothermic as a rule, or rxn-dependent?

Thanks a lot.

All of your questions here seem to be interrelated. Basically,

H2O <-> H+ + OH-

Regardless of temperature, [H+] = [OH-] when water self-inoizes.

By definition, [H+] = [OH-] for a neutral solution. Depending on temperature, this may happen at various pH values.

 

[axp107]

Are we supposed to know how to do a problem like this?

pKa of H2SO3 = 1.81
pKA of HSO3- = 6.91

What is the pKb for HSO3-

A) 7.09
B) 121.19

I was like.. ok you just do: 14 - 6.91... = 7.09

Apparently thats wrong! Am I missing something here?

The answer is B) 12.19 (typo earlier)

So if they asked whats the pkb of H2SO4... what then.. it doesn't exist right? b/c H2SO4 isn't the conjugate base of anything

 

[BrokenGlass]

The answer is B) 12.19 (typo earlier)

So if they asked whats the pkb of H2SO4... what then.. it doesn't exist right? b/c H2SO4 isn't the conjugate base of anything

Sulfuric acid has 2 oxygens double-bonded to the central sulfur atom and 2 hydroxyl groups single-bonded to the central sulfur atom.
The oxygens of sulfuric acid have lone pairs of electrons, so they can accept a proton, thus resulting in a conjugate acid of sulfuric acid. This means that sulfuric acid is a conjugate base of something.

Someone correct me if I am wrong.

 

[bigkdawg161]

I was doing a Kaplan Kinetics/Equilibrium topical and I was stumped by a pretty easy passage made difficult by the arrehnius equation. This question involved finding the value of Ea. Can someone show the manipulation of the formula to get the Ea? Thanks alot.

 

[BrokenGlass]

I was doing a Kaplan Kinetics/Equilibrium topical and I was stumped by a pretty easy passage made difficult by the arrehnius equation. This question involved finding the value of Ea. Can someone show the manipulation of the formula to get the Ea? Thanks alot.

Is this what you want?

k = Z * e ^ (-Ea/RT)

ln k = ln Z - Ea/RT

Ea/RT = ln Z - ln k

Ea = (ln Z - ln K) * RT

Ea = (ln (Z/k)) * RT



ln k = ln Z - Ea/RT has the form y = mx + b, so if you plot ln k vs 1/T, slope is -Ea/R, and Ea = -(slope * R)

 

[scotties123]

CBT 9 gives this reaction:
Al(OH)3 + NaOH --> Na[Al(OH)4]
and asks: Al(OH)3 behaves as a:
a) lewis acid
b) lewis base
c) bronsted acid
d) bronsted base

i know that c and d are incorrect because no protons are being exchanged. I chose answer B and got the question wrong. The solution says the correct answer is A because it accepts a pair of electrons from OH-. However, I chose B because i figured it was donating a pair of electrons to Na+. Clarification please? thanks.

 

[corbis11]

Al(OH)3 is accepting a pair of e- and forming another covalent bond=definition of a lewis acid

 

[scotties123]

Al(OH)3 is accepting a pair of e- and forming another covalent bond=definition of a lewis acid

i see how its accepting the electrons from OH- but isnt it also donating electrons to Na+ to make a bond with it?

 

[Foghorn]

i see how its accepting the electrons from OH- but isnt it also donating electrons to Na+ to make a bond with it?

Bronsted base = an H+/proton acceptor; Lewis base = electron pair donor; Ionic compound = Metal cation + counter ion. The reactants and product are ionic compounds. In order to form Na[Al(OH)4], the ionic bond between NaOH dissociates. Therefore you know the electron pair originates from OH- & forms an ionic complex with Al(OH)3. Finally, Na+ associates with [Al(OH)4]- to get the electrically neutral product.

 

[scotties123]

Bronsted base = an H+/proton acceptor; Lewis base = electron pair donor; Ionic compound = Metal cation + counter ion. The reactants and product are ionic compounds. In order to form Na[Al(OH)4], the ionic bond between NaOH dissociates. Therefore you know the electron pair originates from OH- & forms an ionic complex with Al(OH)3. Finally, Na+ associates with [Al(OH)4]- to get the electrically neutral product.

gotcha. thanks. just didnt see how the Na wasnt affecting it.

 

[4s4]

Why does the temperature decrease when a gas expands adiabatically? I thought PV = nRT so increasing V at constant P means increasing T? Thanks.

 

[4s4]

Why does the temperature of a gas decrease if it expands adiabatically? Thanks.

 

[gridiron]

Why does the temperature of a gas decrease if it expands adiabatically? Thanks.

For the MCAT, when a gas expands it will cool. You can use the first law of thermodynamics to explain why the temperature decreases during adiabatic expansion. The first law states (for MCAT purposes):

change in internal energy = q + w

where q is heat and w is work. You may also see q - w. It is the same thing but from a different convention. For a adiabatic process, q=0 because there is no heat loss or gain. That leaves the w term. The work done by the gas is equal to: p*(change in V) . Since the gas expands, this will be a negative term: -P(change V). This means, upon expansion, the internal energy of the gas decreases. The total energy remains constant, so what happens to the temperature. Since work is done during gas expansion, the temperature will drop. (From thermodynamics, change in internal energy is equal to Cv(change in temperature; actually dU = CvdT--a decrease in internal energy means a decrease in temperature).

You can also think in terms of kinetic energy. For MCAT purposes, kinetic energy is directly proportional to temperature for an ideal gas. When a gas expands, during a adiabatic process, the molecules are more spread apart. This means, from a kinetic energy and speed perspective, compared to when the molecules are compressed, during expansion the molecules "slow" down. This means the temperature decreases because kinetic energy is directly proportional to temperature.

I hope this helps and good .

 

[axp107]

I just encountered a passage about the solubility of certain compounds

Assume a compound is soluble in dilute acid and base...

This compound probably contains:

A) carboxylic acid and amide
B) amine and amide
C) amide and nitro
D) amine and carboxylic acid

What does being soluble in dilute acid and base mean??? That it reacts with acids and bases..?

The answer is D) .. how can we deduce this? are we supposed to know solubility rules for all these functional groups? All I know is like dissolves like

 

[heymanooh1]

I just encountered a passage about the solubility of certain compounds

Assume a compound is soluble in dilute acid and base...

This compound probably contains:

A) carboxylic acid and amide
B) amine and amide
C) amide and nitro
D) amine and carboxylic acid

What does being soluble in dilute acid and base mean??? That it reacts with acids and bases..?

The answer is D) .. how can we deduce this? are we supposed to know solubility rules for all these functional groups? All I know is like dissolves like

Look up what amphoteric means.

 

[axp107]

So does being soluble in acid and base just mean it reacts with both..

I know what amphoteric means..

but are we supposed to know amine groups are basic .. what about amide groups then?

 

[scotties123]

So does being soluble in acid and base just mean it reacts with both..

I know what amphoteric means..

but are we supposed to know amine groups are basic .. what about amide groups then?

go2 the separation methods chapter in ur orgo book and read about Extraction. it should explain a lot.

 

[axp107]

go2 the separation methods chapter in ur orgo book and read about Extraction. it should explain a lot.

Can anyone explain or give me a link.. I don't really know what to search for. My ORGO book doesn't have any "techniques" sections in it. Plus, I haven't taken orgo lab yet.. the Kaplan book briefly talks about it .. and I don't think EK even touches on it.

 

[gridiron]

Can anyone explain or give me a link.. I don't really know what to search for. My ORGO book doesn't have any "techniques" sections in it. Plus, I haven't taken orgo lab yet.. the Kaplan book briefly talks about it .. and I don't think EK even touches on it.

You can try this link:

http://orgchem.colorado.edu/hndbksupport/ext/ext.html

 

[axp107]

Ok so in extraction.. you do multiple extractions to get the purest form of whatever you're trying to extract, in 1 layer.

That doesn't really help me answer the question :/ whether amides/amines are basic or not

 

[pezzang]

IS it okay to mix partial pressure and concentration when calculating the equilibrium constant? I didnt think that pp of some gaseous reactant/products can be mixed with concentrations of aq reactants/product. For ex in EK 1001 Gen chem #285,
3NO2(g) + H2O(l) -> 2HNO3 (aq) + NO (g)

K = P(NO)[HNO3]^2 / [NO2]^3

EK answer says that the statement for K(eq) above is correct... Is it correct?

 

[tennisboy85]

That doesn't really help me answer the question :/ whether amides/amines are basic or not

Axp, amides are actually neutral. To see this, you'd have to draw the resonance structure of amides. So amides are R - C=O - N - R2, right? Now, move the pi bond from C=O to the C - N bond, this would create: R - C - O (negative charged) = N(+) - R2

The second form is actually the preferred form as negative charge is on the more En atom (oxygen), and here you can see the -ve and +ve charge cancels, leaving it a neutral molecule. Majority of amides exist in this form, I believe.

I hope that made sense =S I wish there was a draw function.

Amines, on the other hand, are definitely basic. The lone pair on the nitrogen can act as nucleophile (think Lewis Base = e- donators) and can abstract/bond to things (such as H+ and others)

Hope this helps.

 

[scotties123]

TPR has a couple diagrams under their acid-base chemistry chapter that are confusing me. They show a curve of weak acid HF titrated with strong base NaOH. The thing is though, the pH of HF starts at about 2.8 before titration. My question is, how low of a before titration pH does an acid have to have to be considered a weak acid. I know HF is a weak acid because of the rules of acidity, but if just a curve is given and it's asked if it's a weak acid or strong acid that you start with, what kind of pH should weak acids start at compared to strong? thanks.

EDIT: I also know that the pH of a weak acid at equivalence will be greater than 7 when titrated with a strong base, but what if a weak acid is titrated with a weak base? what pH is the equivalence point usually around?

 

[gridiron]

TPR has a couple diagrams under their acid-base chemistry chapter that are confusing me. They show a curve of weak acid HF titrated with strong base NaOH. The thing is though, the pH of HF starts at about 2.8 before titration. My question is, how low of a before titration pH does an acid have to have to be considered a weak acid. I know HF is a weak acid because of the rules of acidity, but if just a curve is given and it's asked if it's a weak acid or strong acid that you start with, what kind of pH should weak acids start at compared to strong? thanks.

EDIT: I also know that the pH of a weak acid at equivalence will be greater than 7 when titrated with a strong base, but what if a weak acid is titrated with a weak base? what pH is the equivalence point usually around?

Hey! for the MCAT, there is not cut-off pH for an acid to be considered strong or weak. You have to know the strong acids for the MCAT--perchloric, hydroidoic, hydrochloric, sulfuric, hydrobromic, and nitric acid. The pH of the acids are dependent on the concentration of H ion. So, a strong acid could have a pH of 3 due to a low concentration of H ion. There is not particular pH to define a weak acid and you won't need that for the MCAT. If there is a question whether a acid is weak or strong, you already know the common strong acids for the MCAT, a titration curve should be provided or information in the passage.

To find the equivalence point for a weak acid/base titration, you need to know the pKa of the weak acid/base in order to determine which is a weaker acid or base. From this, you can use Henderson-Hasselbach or approximate the equivalence point. For example, if both the acid and base are "equally" weak, then the equivalence point will be around 7.

I hope this helps and good .

 

[BrokenGlass]

TPR has a couple diagrams under their acid-base chemistry chapter that are confusing me. They show a curve of weak acid HF titrated with strong base NaOH. The thing is though, the pH of HF starts at about 2.8 before titration. My question is, how low of a before titration pH does an acid have to have to be considered a weak acid. I know HF is a weak acid because of the rules of acidity, but if just a curve is given and it's asked if it's a weak acid or strong acid that you start with, what kind of pH should weak acids start at compared to strong? thanks.

EDIT: I also know that the pH of a weak acid at equivalence will be greater than 7 when titrated with a strong base, but what if a weak acid is titrated with a weak base? what pH is the equivalence point usually around?

http://forums.studentdoctor.net/showpost.php?p=5294076&postcount=927

http://forums.studentdoctor.net/showpost.php?p=5331316&postcount=942

You never TITRATE a weak reagent with a weak reagent. If you want to know whether you are starting with a weak acid or a strong acid (and titrating it with a strong base), just check for lipo-weakness. A weak acid curve will have a small vertical region (called a lip) preceding the first horizontal region.

 

[BrokenGlass]

IS it okay to mix partial pressure and concentration when calculating the equilibrium constant? I didnt think that pp of some gaseous reactant/products can be mixed with concentrations of aq reactants/product. For ex in EK 1001 Gen chem #285,
3NO2(g) + H2O(l) -> 2HNO3 (aq) + NO (g)

K = P(NO)[HNO3]^2 / [NO2]^3

EK answer says that the statement for K(eq) above is correct... Is it correct?

Post the problem and the answer key in their entirety.

 

[pr0foundbsguy]

Hey guys, I'm having a conceptual problem with one of the topics in the EK books (CH3-3). They state that "if a series of slabs were lined up end to end between hot and cold reservoirs, the rate of heat flow, Q/t, would be the same in all the slabs even if they each had different lengths thicknesses, and different thermal conductivities."

How is this possible? Especially if you compare heat to the fluid model where the velocity of the fluid will depend on the the thickness and length of the conduit? I'm sure this is a really simple topic but I'm just having a hard time visualizing it.

Thanks for your help in advance.

 

[BrokenGlass]

Hey guys, I'm having a conceptual problem with one of the topics in the EK books (CH3-3). They state that "if a series of slabs were lined up end to end between hot and cold reservoirs, the rate of heat flow, Q/t, would be the same in all the slabs even if they each had different lengths thicknesses, and different thermal conductivities."

How is this possible? Especially if you compare heat to the fluid model where the velocity of the fluid will depend on the the thickness and length of the conduit? I'm sure this is a really simple topic but I'm just having a hard time visualizing it.

Thanks for your help in advance.

The flow rate (Q) is the same, but velocities will differ.

 

[tennisboy85]

"if a series of slabs were lined up end to end between hot and cold reservoirs...

What does that even mean

 

[free_radical]

i think it means like this:

if you have five blocks of stone lined up one against the other (like 5 really thick dominos, touching each other), and the one all the way at the right was hot, and the one all the way at the left was cold, so intuition will tell us that heat (Q) will flow from the hot block to the cold block, via the middle 3 blocks.

then there are 3 factors:

-the amount of time it takes for a given amount of heat to flow (Q/t, or "I")
-the "resistance" each middle block has to heat flow (R)
-the difference in temperature between the hot block and the cold block (deltaT).

simplifying things, you can relate the 3 factors by an equation

"deltaT= (I)(R)", analogous to ohm's law.

the intuitive assumption is that I is constant (otherwise heat wouldn't flow right to left properly, because one of the middle blocks would be colder than a block on its left, making further heat flow from right to left impossible).

based on that assumption, the lower the resistance to heat flow, the lower the temperatire difference between the two blocks (that's per the equation- deltaT=IR, so if I is constant, and R is decreased, deltaT is decreased as well)

hope that helps- if i misstated something or left something out, Q/grid/broken or one of the others will know it better. g'luk.

 

[tennisboy85]

Why is delta T = IR? Isn't that the equation for electric potential (V=IR)?

Also, I kind of got lost in your confusion I'm not sure if it's because I don't really understand this whole thermal transfer stuff. Is there somewhere that I can go to read up on this?

I have EK (an older version, I think 5?), Kaplan and TPR books - but I haven't used them all. If it's mentioned there, could you point me to the appropriate place? If not, maybe somewhere online?

Thanks a bunch.

 

[free_radical]

yes- the equation does look very similar to ohm's law. it is purposely manipulated from the original more complicated equation in order to give you a feel for the relationships therein. (another very similar form would be relating pressure difference to fluid flow in real gases, giving you deltaP=IR, where the resistance is viscosity etc. i'm not really sure about this one, actually)

i believe you can find a very good synopsis of this in examkrackers chemistry manual, in the thermodynamics chapter where "heat" is discussed, under the subsection of conduction.

in a very general, abstract form:

things flow, if there's a difference in something form one end to the other. but some things inhibit that flow.

the things that flow, the difference at either end of the flow which causes the flow, and the inhibition are particular to specific contexts.

if the flow is referring to flow of electrons, then the difference at either end would be the voltage (the potential difference), and the resistance to that flow would be the fact that the electrons keep bumping into the atoms of the medium through which they flow. ditto for fluids, ditto for heat.

again- dont take everything i say as truth- there are many here who know better than i; i havent even taken the mcat yet. gluk.

 

[gridiron]

yes- the equation does look very similar to ohm's law. it is purposely manipulated from the original more complicated equation in order to give you a feel for the relationships therein. (another very similar form would be relating pressure difference to fluid flow in real gases, giving you deltaP=IR, where the resistance is viscosity etc. i'm not really sure about this one, actually)

i believe you can find a very good synopsis of this in examkrackers chemistry manual, in the thermodynamics chapter where "heat" is discussed, under the subsection of conduction.

in a very general, abstract form:

things flow, if there's a difference in something form one end to the other.

That is the result of the law of entropy.

but some things inhibit that flow.

the things that flow, the difference at either end of the flow which causes the flow, and the inhibition are particular to specific contexts.

if the flow is referring to flow of electrons, then the difference at either end would be the voltage (the potential difference), and the resistance to that flow would be the fact that the electrons keep bumping into the atoms of the medium through which they flow. ditto for fluids, ditto for heat.

again- dont take everything i say as truth- there are many here who know better than i; i havent even taken the mcat yet. gluk.

 

[tennisboy85]

Thanks!

 

[free_radical]

Question-

Filling a transition-metals' orbitals goes from .......3p->4s->3d->.......

So, it's a safe assumption that the highest energy electron in a transition metal of the 4th period, such as Iron, is 3d; namely, that since 4s is filled before 3d, that 4s is of lower energy.

Nevertheless, in Examkrackers, when discussing the first electron to be pulled off, it says you always pull off from the outermost shell first, and therefore when pulling off an electron, 4s would be pulled off before 3d. But isn't 3d a higher energy; i.e. it's less strongly bound to the nucleus?

Thank you!

 

[Foghorn]

Question-

Filling a transition-metals' orbitals goes from .......3p->4s->3d->.......

So, it's a safe assumption that the highest energy electron in a transition metal of the 4th period, such as Iron, is 3d; namely, that since 4s is filled before 3d, that 4s is of lower energy.

Nevertheless, in Examkrackers, when discussing the first electron to be pulled off, it says you always pull off from the outermost shell first, and therefore when pulling off an electron, 4s would be pulled off before 3d. But isn't 3d a higher energy; i.e. it's less strongly bound to the nucleus?

Thank you!

3d is lower in energy because it's in prinicple quantum # n = 3. The reason why 4s fills first is based on experimental data for radial distribution of electrons. 4s electrons are closer to the nucleus due to penetration effects compared to 3d electrons, meaning 4s electrons on average spend more time closer to the nucleus relative to their 3d counterparts. 4s electrons are also farther away from the nucleus based on radial distribution data.

You don't need to know this level of detail for the MCAT

 

[Creightonite]

I got a question about LeShatellie principle (sp?)

Why addition of innert gas does not change the rate of the reaction? It would increase the pressure, correct. I also think that adding some of the moles of the innert gas will decrease the partial pressures of other gases. How is this situation different from just increasing the pressure?

for example:

X(g) + Y(g) --> Z(g)

 

[jochi1543]

I got a question about LeShatellie principle (sp?)

Why addition of innert gas does not change the rate of the reaction? It would increase the pressure, correct. I also think that adding some of the moles of the innert gas will decrease the partial pressures of other gases. How is this situation different from just increasing the pressure?

for example:

X(g) + Y(g) --> Z(g)

I may be wrong, so take this with a grain of salt, but if I saw this question on the exam, I would respond that 1) the inert gas doesn't take part in the reaction, and thus will not affect the relative quantities of reactants vs. products 2) it is added to the entire reaction chamber as opposed to 1 side of the reaction, and so it won't have an effect as per Le Chatelier's principle the way it would if it were only added, say, to the right side (making the entropy on the right side same as on the left and thus affecting the equilibrium).
Again, that's just a guess.

 

[RoadRunner17]

I would like to note that only volatile gases really contribute to the partial pressures of a system. Not sure why inert gases don't affect the equilibrium concentrations though.. perhaps they have to actually be part of the reaction in order for Le-Chatelier's to apply. Need a chem expert here to answer this.. I'm more of a physics guy =P

 

[cloosh]

two questions:

1) how do i know, if asked, which metals (if given a choice) would react most violently with water or anything else for that matter?

2) okay i'm confused about solubility constants. a Ksp measures the solubility of a solid, and the normal number that they give you is the saturated version, right? also, if you compare two solubility numbers, would the lower number be more or less soluble? thanks.

 

[jochi1543]

two questions:

1) how do i know, if asked, which metals (if given a choice) would react most violently with water or anything else for that matter?

The ones in the first column (I believe "alkali" is the name of the first column). They only have 1 electron to lose in order to obtain a noble gas configuration, so they tend to react very fast. If you need to pick from several alkali metals, you need to consider the size of the element (which increases as you go down a column). Reactivity increases with size, because the bigger the orbital, the farther the electron from the nucleus, and the easier it is to remove.

 

[cloosh]

ah okay that makes sense, thanks jochi.

for the solubility thing, what i meant was the Ksp, not the solubility. in relation to that, the Ksp is a measured value when the solution is saturated right? So if you get a number lower than the Ksp, then it's unsaturated, and a number higher than the Ksp, the solution is supersaturated right? thanks.