Electron shielding&Z-eff and Rate Law.

[pfaction]

Didn't want to create 2 new topics.

So, for this one, I kind of figured it out. Shielding is protection of the valence electrons by previous electrons. Since calcium has 2, each feels a large pull. Arsenic has even more protons than Ca, leading to a decreased radius--due to increased Z-eff. The valence electrons are also more shielded, is it due to the 4S, 3D shielding? Or just the 4S? Is there a correlation between shielding and Z-eff?

This one I don't understand at all. What?!

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[MedPR]

Didn't want to create 2 new topics.

So, for this one, I kind of figured it out. Shielding is protection of the valence electrons by previous electrons. Since calcium has 2, each feels a large pull. Arsenic has even more protons than Ca, leading to a decreased radius--due to increased Z-eff. The valence electrons are also more shielded, is it due to the 4S, 3D shielding? Or just the 4S? Is there a correlation between shielding and Z-eff?

This one I don't understand at all. What?!

What are the answers? B and A?

 

[pfaction]

No, it's B and D. I got 33 right on a complete guess and I don't know how.

 

[MedPR]

32 is D? The rate is just rate=-reactant/time. According to the given equation, everytime you make 1 NO2, you consume 1/2O2 and 1NO. No idea why it would be D and not A.

 

[pfaction]

White text, even after reading I don't understand.

Explanation:
D. Reaction rate can be characterized by changes in reactants or products over time. In the generic reaction aA → bB, the reaction rate = –(1/a) Δ[A]/Δt= (1/b) Δ/Δt. Thus the rate expression for the given reaction would be = –(1/2) Δ[NO]/Δt = –Δ[O2]/Δt = Δ(1/2) [NO2]/Δt (choice D is correct).
​

 

[reburbia]

For reaction aA -> bB, I'm used to seeing reaction rate = k Δ ^b

apparently there's another reaction rate formula, –(1/a) Δ[A]/Δt, which you posted. If you knew this latter formula for reaction rate, isn't it just a matter of plugging in numbers?

 

[chiddler]

please include in title if it's a question from an official exam next time.

 

[MedPR]

For reaction aA -> bB, I'm used to seeing reaction rate = k Δ ^b

apparently there's another reaction rate formula, –(1/a) Δ[A]/Δt, which you posted. If you knew this latter formula for reaction rate, isn't it just a matter of plugging in numbers?

Yes, just plugging in numbers for knowing that equation. I guess none of the rates can be greater than 1.

 

[MedPR]

please include in title if it's a question from an official exam next time.

This is from an official exam? Great. Never opening any of pfaction's threads again. I noticed he did this in another thread today; mentioned AAMC 10 in the first post, but not in the title.

 

[pfaction]

No, this is from TPR. Sorry about that. I won't mention AAMC again unless i specifically state it.

 

[chiddler]

oh.

sorry. that format looked very official and classy lol

 

[pfaction]

No problem. I will clearly mark AAMC next time, if no AAMC marking it's TPR or from my own brain.

 

[SaintJude]

What's the explanation for 32? I kind of knew it was B, but only vaguely b/c I knew electrons in the 4s orbital in the outer orbital.

 

[pfaction]

32 is D. If you meant 23 I posted my theory, and I think it's right. The shielding of As is incr because of, at the least, 4S electrons absorbing some of the proton's attractivity.

It also really messes with my understanding of atomic radius. I understand that as we go L->R, protons increase, electrons increase, but Z-eff outpaces the electron increase so radius decreases...yet is somehow more shielded?!

I still don't understand #32...why is it what it is 🙁

 

[SaintJude]

What does Princeton say about 23? And I will agree with MedPr. It's a mistake. If anything it's A. For every 2 moles of NO that's consumed there are 2 moles of NO2 produced. So the rate of reactions should be 1:1.

 

[chiddler]

The reaction says that for every 2 NO that is reacted, one reaction occurs. Therefore, half of the change in NO = 1 reaction. And this tells us rate of reaction.

 

[pfaction]

White text ahoy:

Explanation:
B. The amount of shielding that the highest energy electrons of an atom feels is determined by the number of filled shells in that atom (inner core electrons), as well as subshells within an energy level (eliminate choice D). Because calcium and arsenic are in the same period (row), they have the same number of inner core electrons. However, the highest energy electrons in arsenic are in the psubshell and are shielded by the electrons in the ssubshell. Since the highest energy electrons in calcium are in the s subshell, there is more shielding for arsenic, eliminating choices A and C.

Also, I don't understand 32 so I'll hope someone can explain it real good?

​

 

[pfaction]

The reaction says that for every 2 NO that is reacted, one reaction occurs. Therefore, half of the change in NO = 1 reaction. And this tells us rate of reaction.

1. Why is it negative? Rates of reaction by definition is the rate at which reactants disappear, so is that why? I thought it was positive.
2. I understand what you say, so if B were -[O]/time, it would be right? (assuming A wasn't there, choosing between B C D)
3. What about (1/2) [NO2]/t?

 

[SaintJude]

The reaction says that for every 2 NO that is reacted, one reaction occurs. Therefore, half of the change in NO = 1 reaction. And this tells us rate of reaction.

Well, really it could be A& D. B/c D is just "choice A" divided by 2. It still has to remain a 1:1 ratio.

Edit: pfaction, Why all this white text?

And why it's negative: Because the reactants are being consumed in the process of formation of the products.

 

[chiddler]

You don't have to white out everything @_@

For #23, i'm a bit confused, especially after reading the table on the right. This indicates that shielding occurs between different n values. In arsenic and calcium, their valence electrons are both in the same n shell (the fourth) and therefore their shielding should be equivalent, no?

 

[pfaction]

Well, I think of shielding in this crude sense.

CA---|2|
Calcium has protons and just 2 electrons. There's no real shielding here. Both electrons feel the same effects.

AS-|2|----|3D shell??|----|4P#|
Much more shielded, by definition, the inner 2S (3D as well?) protect 4P from all the effects.

But my diagram also indicates the radius should be bigger, though I also understand that As has increased # of protons and this has a larger pull on electrons, drawing them closer.

This is really screwing with my head.

 

[chiddler]

Well, I think of shielding in this crude sense.

CA---|2|
Calcium has protons and just 2 electrons. There's no real shielding here. Both electrons feel the same effects.

AS-|2|----|3D shell??|----|4P#|
Much more shielded, by definition, the inner 2S (3D as well?) protect 4P from all the effects.

But my diagram also indicates the radius should be bigger, though I also understand that As has increased # of protons and this has a larger pull on electrons, drawing them closer.

This is really screwing with my head.

yeah! 3d shielding. i forgot.

 

[chiddler]

1. Why is it negative? Rates of reaction by definition is the rate at which reactants disappear, so is that why? I thought it was positive.
2. I understand what you say, so if B were -[O]/time, it would be right? (assuming A wasn't there, choosing between B C D)
3. What about (1/2) [NO2]/t?

1. You are measuring how fast reaction is going by measuring how fast reactants disappear. So it's -disappearance/time = appearance/time = rate of reaction

example:

i convert oranges into apples. how fast am I creating apples? by the negative of how fast i'm consuming oranges.

2. I think it would be correct. O2, anyway. Not O.

3. I think that would be correct.

I'm not very confident with my knowledge here. For example, I can't figure out why what medpr said is incorrect. I got the question right and I just wrote down my logic for answering it. I don't have much to back it up though.

I think the reason that he is wrong is because you can't have 1/2 O2 in reality. This isn't thermodynamics.

 

[pfaction]

That's really helpful, actually. I think MedPR was incorrect just because the limiting reagent here is Oxygen, so to make one molecule, it's 1NO+1/2O2=1NO2, none of the answer choices works except D?

 

[chiddler]

That's really helpful, actually. I think MedPR was incorrect just because the limiting reagent here is Oxygen, so to make one molecule, it's 1NO+1/2O2=1NO2, none of the answer choices works except D?

I think so. I wish i can help more.

 

[chiddler]

http://en.wikipedia.org/wiki/Reaction_rate#Formal_definition_of_reaction_rate

i don't know why i didn't notice this earlier...

look at the equations wiki has:

r = -1/a d[A]/dt

So for sure, you divide by the stoichiometric coefficient. I am more confident with the answers I gave. But still would like a more conceptual explanation if anybody has it.

(the question being: why must we divide by the coefficient?)

 

[pfaction]

Well, I apparently grasped the concept but I don't understand why/how. The RRate is the rate at which a reactant gets consumed.
2A+4B=C
Therefore, rate is C = -1/4B = -1/2A.
I don't understand why negatives in the reactants if that's what we're doing in the first place...rate = k[A]^2 * ^4
assuming this was an elemental step.

 

[sweet d]

http://en.wikipedia.org/wiki/Reaction_rate#Formal_definition_of_reaction_rate

i don't know why i didn't notice this earlier...

look at the equations wiki has:

r = -1/a d[A]/dt

So for sure, you divide by the stoichiometric coefficient. I am more confident with the answers I gave. But still would like a more conceptual explanation if anybody has it.

(the question being: why must we divide by the coefficient?)

The equation tells us that 2 mol of NO are required to react with 1 mol of O2 and they form 2 mol of NO2. Assuming a closed system, this means that:
- NO decomposes twice as fast as O2 decomposes (2 mol NO are used for every 1 mol O2)
- NO decomposes at the same rate that NO2 is formed (2 mol NO are used to create 2 mol NO2)
- NO2 is formed at twice the rate that O2 decomposes (2 mol NO2 are created with 1 mol O2)

This question is asking us for the rate, not the rate law, so think of it in simple terms: rate = (change in concentration)/(change in time). This is the speed at which the reaction occurs. In other words, the speed at which the reactants decompose or the speed at which the products form. Because we are in a closed system, those two rates should be equal to the rate of the reaction:

Rate of reaction = rate of decomposition of O2 = - ( [O2]f - [O2]i ) / (dt)
Rate of reaction = rate of formation of NO2 = ( [NO2]f – [NO2]i ) / (dt)

Obviously, those two rates are not equal (stated above, NO2 is formed at TWICE the rate that O2 decomposes). I assigned some arbitrary numbers to demonstrate:

At t = 0 seconds
[NO] = 4.0M
[O2] = 2.0M
[NO2] = 0.0M

At t = 1 second
[NO] = 2.0M
[O2] = 1.0M
[NO2] = 2.0M

Rate of decomposition of O2 = - ( 1.0M – 2.0M ) / ( 1s – 0s) = 1.0 M/s
Rate of formation of NO2 = ( 2.0M – 0.0M ) / ( 1s – 0s) = 2.0 M/s

It makes sense, since concentration is a measure of the number of particles and we don’t usually get the same number out as we put in. But the reaction cannot have multiple rates (rate law: rate of reaction = k[A]^n^m), so we have to normalize the rates of decomposition and formation. Dividing by the coefficient accomplishes this:

Rate of decomposition of O2 = - (1/1) * ( 1.0M – 2.0M ) / ( 1s – 0s) = 1.0 M/s
(1/2) Rate of formation of NO2 = (1/2) * ( 2.0M – 0.0M ) / ( 1s – 0s) = 1.0 M/s
(1/2) Rate of decomposition of NO = - (1/2) * ( 2.0M – 4.0M ) / ( 1s – 0s) = 1.0 M/s

Now you can describe the rate of the reaction:

rate = - (1/2) rate of decomposition of NO = - rate of decomposition of O2 = (1/2) rate of formation of NO2

Another way I've thought of this is by saying that the rate of the reaction describes how fast 2 mol NO disappear, how fast 1 mol O2 disappears and how fast 2 mol NO2 appears. When you measure the change in concentration / time, you lose those coefficients (you're measuring how fast 1 mol of each forms or decomposes, in a sense), so you have to add the coefficients back in to the final expression.

Well, I apparently grasped the concept but I don't understand why/how. The RRate is the rate at which a reactant gets consumed.
2A+4B=C
Therefore, rate is C = -1/4B = -1/2A.
I don't understand why negatives in the reactants if that's what we're doing in the first place...rate = k[A]^2 * ^4
assuming this was an elemental step.

If I drive a car backwards, am I traveling at negative speed? No. In physics we describe velocity with a speed number (+) and an arrow indicating the direction of travel (forward, backward, etc). In chemistry, we give the rate of change in concentration of a species and indicate whether it is disappearing at that rate or forming at that rate.

The negatives are there because reactants are disappearing and the rate of decomposition is going to be negative without them. The example from above:

Rate of decomposition of O2 = - ( 1.0M – 2.0M ) / ( 1s – 0s) = 1.0 M/s

O2 is decomposing, so we know the concentration is decreasing over time. However, the rate (speed) at which is disappears cannot be negative, so we compensate by adding a negative sign and then describing this as decomposition so that we know the reactant is disappearing at this rate. The rate of the overall reaction is just a positive number. You could also use the absolute value of the change in concentration, but I don't think that's the officially accepted form of the expression.

 

[SaintJude]

http://en.wikipedia.org/wiki/Reaction_rate#Formal_definition_of_reaction_rate

i don't know why i didn't notice this earlier...

look at the equations wiki has:

r = -1/a d[A]/dt

Oh, wow 😳. How did I miss this ?? I even underlined and put 2 stars next to this in my Kaplan book:

The rate of concentration change of each species should be divided by the species' stoichiometric coeffecient

Thanks chiddler!

 

[pfaction]

And thanks sweet d, I'm sure we'll pour over that in a few hours! When I'm up to gchem that is.

 

[chiddler]

thank you, sweet d