thanks for that least. helpful. email. ever.

The BBQ Pit
1

i have a chemistry exam tomorrow.

for the first time in my life, i do not have to resort to cramming the night before, as i spent all weekend studying chemistry, reading, doing problems, writing notes, etc. tonite is just review.

that being said, there are a few things i am unsure of - i understand the concepts, but while working through the practice exams and extra homework sets we were given, i came across a few problems which i was very surprised to get wrong - i had thought i understood the concepts behind them, and it is unsettling to realize that maybe actually i dont. the problem had to do with the different potential energies between various electron-nucleus combinations.

so i emailed the professor, just like he told us to. his exact words were probably something like “please feel free to email me if you have any questions about the material, i’ll get back to you as soon as i can.” i had kind of figured out why i had gotten the problem wrong (math confusion) and had outlined my interpretation of where my train of thought had gotten derailed, and how i thought to get it back on track.

i wanted either clarification or explanation. possible answers would have included “yes” (as in yes, you understand) or “no, it’s like this - blah blah blah.”

this is what i got: “the gold nucleus has an enormous number of protons…”

no. really? seriously? it does? dont you think that any moron with a number line and a periodic table could figure that out in 30 seconds? do you really think that’s what i was confused about, and i just tossed in all that stuff about math to try and throw you off the trail? that had absolutely nothing at all to do with what i was confused about. if i dont know by now that gold atoms are ripe with protons, i might as well break into the chem lab and drown my ignorance with beakers of isoproyl alcohol and mercury then stick my head on a bunsen burner to finish the job. it’s not like you and the book dont dote on gold like it was designed by god specifically for use in intro-chem. it’s like you are in the pocket of the gold atom lobby, and you get a free gold bar every time you use that element in an example.

yeah. i know gold has a lot of protons - 79 to be exact. i knew that without having to look it up. because i actually understand most of what is going on. what i dont understand is the math concept which i carefully laid out for you in the email, and which you completely ignored and glossed over in favor of reminding me of the protonous nature of your favorite element.

THAT WAS THE MOST FUCKING USELESS REPLY I HAVE EVER GOTTEN IN MY LIFE, and based on some of the things you’ve said in the class, you’re pretty close to attaining a monopoly on my top five list of useless replies. sweet spreadable christ on a nutri-grain biscuit, it only took me three seconds to read your email, but those might be the most sorely wasted three seconds of my entire multi-seconded existence. why not respond “a duck just broke into my refrigerator and ate all of my pickles?” if you are going to ignore my question, fine, but you dont have to insult my intelligience in the process.
now that you’ve so nicely windexed the window of my misunderstanding with your ground-shattering proclamation that “the gold nucleus has an enormous amount of protons…” i can get back to my studying, secure in the knowledge that my professor is as eager for me to grasp the material as i am eager to grasp it, and only confused as to whether or not he just didnt understand my question, or if he is a big fucking jerk.

love
yams!!

2

You know, now you HAVE to tell us what the problem was, and what you emailed to the prof.

Tomorrow after the exam will be soon enough. No pressure.

ETA: Is there any way at all he could have mixed up his response to you with one intended for someone else?

3

Maybe you should ask your prof to email you a shift key.

4

Anyone else reading this dirty?

Oh, and “YAMS!!!”
-bbs2k

Oh, and he had to save all his capital letters for that one sentence, the whole thread doesn’t work without ALL THOSE CAPITALS SO YOU KNOW HE’S YELLING!

5

What a fucking crybaby! Don’t you realize how radically it would change the problem if a gold atom had a GINORMOUS number of protons instead of merely an enormous number.

6

friedo, this post is for you.

What follows is the second least useful bit of information I have ever received, courtesy of the same professor.

Last week in class, we were discussing potential energy diagrams, which is essentially an x and y axis, with a parabolic-sort of line representing the potential energy (V) of a particle in relation to it’s position from another particle in fixed position at the origin, and a horizontal line drawn to represent total energy E.

The professor had drawn the line representing total energy E, but he either didnt explain what it was or I missed the explanation, so I raised my hand and asked “What does that second horizontal line represent?”

Professor points to the x-axis. “This line?”

Now. I am what is known as a non-traditional student. I received a BA some years ago, went off into the real world, decided I wanted to study medicine, and so am now back at school, taking pre-med classes. So I can understand, maybe nontraditional students are traditionally retarded in the ways of the Cartesian Plane, but Holy Hydrogenated Christ in a Dirty Deep Fryer, if I dont know at this point what the x-axis is, I wouldnt have matriculated at this particular, Ivy-League university; I would have enrolled at the nearest junior high school so I could remaster long division and basic geometry.

“No, not that line,” says I. “The one above it.”

Professor points to the line in question. “This one?”

“Yes.”

“Oh,” says Proff. “That’s the line on the board I drew connecting this line to the y-axis.”

Again - really? Seriously? So you picked up a piece of chalk and used it to connect two points on the board that you felt might like to get better acquainted? Do you have OCD and are compelled to draw horizontal lines at random? Or maybe, did you draw that particular line, at that particular point, for a particular reason?

Thus concludes my story of the second most useless piece of non-information I have ever received. I used to live in a country where, if a person didnt know the answer to your question, they would make something up on spot, just so they had something to tell you. This kind of non-info, in my opinion, is worse.

love
yams!!

7

(I am continuing with the usage of shift key, again in honor of friedo.)

Maybe if I post it now, somebody could come along and confirm that my mathematical reasoning is, in fact, correct. Here is the pertinent part of the email:

in the extra problem set from chapter three, i am confused over
question number 3. in case you dont have it in front of you, here
it is:

which of the following has the lowest potential energy?
a) an electron and a hydrogen nucleus separated by 4A
b) an electron and a helium nucleus separated by 4A
c) an electron and a hydrogen nucleus separated by 2A
d) an electron and a lithium nucleus separated by 4A
e) an electron and a gold nucleus separated by 10 A
so using the equation V(r) = - Ze^2/4pi(8.854x10^-12)r these are
the potential energies i got:

a)-5.766 x 10^-19 J
b)-1.533 x 10^-18 J
c)-1.153 x 10^-18 J
d)-1.736 x 10^-18 J
e)-1.822 x 10^-17 J

so i picked (a) as having the lowest potential energy, but the
correct answer is (e), which leads me to two possible conclusions:

either i am using the wrong equation, or somehow using it
incorrectly, or i am getting all turned around with negative
numbers with 10 raised to negative numbers. i am pretty sure that
10^-19 is actually smaller than 10^-17, but if it is smaller, that
means it is closer to zero, which, if it’s negative, would actually
mean it’s bigger? so since (e) is the biggest actual number, and
since it is negative, that means it is farther from zero, and
therefore smallest?

For those in the know, I couldnt figure out how to type the proper symbol for ‘angstrom’ (capital A with a little circle on top), so I just typed A, figuring it was close enough.
And I realize my line of thought in that last paragraph might not be clear (it was as clear as I could make it at the time), so to rephrase:

10^-10 is bigger than 10^-20 - on a number line, 10^-10 is farther from zero

-10^-10 is bigger than -10^-20 - on a number line, -10^-10 is farther from zero.

So even though 20 is bigger than 10, when you are dealing with negative numbers and negative exponents, the bigger the negative exponent, the smaller the actual number?

And just to head this inevitability off at the pass - ok, it is not a smart question. I thought about it, and thought about it, and I think I figured it out, but I dont think it is a ridiculous question, as it is kind of counterintuitive. And even if it is the Dumbest Question Ever to Be Converted into Binary and Sent Across the Internetic Ether, my professor’s reply still doesnt have a damn thing to do with what I was asking. I had clearly already calculated the various potential energies. A little comparison would show that I had, indeed, taken note of the enormous number of protons in the gold nucleus (again, 79) and made my calculations accordingly.

love
yams!!

8

You are right, -5.766 x 10^-19 is less than -1.822 x 10^-17.

I don’t know how to read the formula you typed out, so as far as I can tell, you’ve either used the wrong formula, or applied it incorrectly.

I have a question which might be relevant to the professor’s reply. What does “separated by X angstroms” mean in the context of this problem? Does it mean the centers of the two objects in question are 10 angstroms apart?

How big is an angstrom, and how big is a Gold nucleus in angstroms?

I’m just shootin’ in the dark here…

-FrL-

-FrL-

9

Å <-- Happy to help.

10

But is it? If you take the absolute value of both numbers, -5.766 x 10^-19 is smaller than -1.822 x 10^-17. But if you plotted both numbers on the number line, -5.766 x 10^-19 is smaller, so it would be closer to 0, and since it is negative, wouldnt that mean that is actually a bigger (ie, less negative) number than -1.822 x 10^-17, which would make -1.822 x 10^-17 smaller?

“separated by x angstroms” means that, for example as in option (e), you have a gold nucleus centered at the origin, and you have an electron 10 angstroms away from the origin, and at that distance, you want to figure out the potential energy of the electron, as that potential energy changes with distance. The whole problem is just comparing how potential energy varies with distance and with the amount of charge.

An angstrom is 10^-10 meters.

I dont think the size of a gold nucleus has anything to do with anything, though. We havent done any problems so far that take into account the size of the particles in question, just their respective charges and the distances between them.

Let me try again with the formula:

V(r) = - [Z(e^2)] / [4pi (8.854 x 10^-12)r]

where Z is the number of protons, e is the charge on an electron (1.602 x 10^-19), 4pi is 4x3.14… (cant find symbol for pi), the big number in paranthesis is something called permittivity of the vaccuum (it is a universal constant, I dont know where on the keyboard to find its symbol) and r is the distance between the particles in angstroms.

love
yams!!

11

Hey! How do you do that??
love
yams!!

12

I smell a catch phrase.

The SDMB: “It’s more helpful than your email!”

13

-2x10[sup]-17[/sup] < -5x10[sup]-19[/sup]

|-2x10[sup]-17[/sup]| > |-5x10[sup]-19[/sup]|

I don’t know how something has a negative energy, but I’m not a chemist.

My favorite unhelpful professor answer:

Prof: Let R be the curvature tensor.
Student: What’s the curvature tensor?
Prof: R is.

14

“smaller” does not mean “closer to minus infinity”.
Tenebras, “negative energy” can mean either “it loses energy in the process” or “it has less energy than our arbitrary reference point.” Since the reference point is arbitrary, we could choose a different (lower) one and have the value come positive. Except for internal energy U, which is always positive (its reference point is not arbitrary but an actual zero), any other of the kinds or energy used in chemistry can be negative just fine.

15

In the case of potential energy between charges, the reference point is the point where the charges are infinitely separated. Since the two charges in the problem are of opposite signs (they attract, the point of lowest energy is when distance is zero), bringing them close from infinity actually releases energy, hence the negative sign.

It’s not even chemistry, it’s physics.

16

So the equation you’re using is V = -kQq / r, where k = 1/4πε0, hence V = Qq/4πε0r, which seems right to me. So, you’ve substituted Z for Q, as the charge on the nucleus is Z, and e as the electron charge, also with ε0 = 8.854x10^-12.

Hence, your equation seems fine to me. Negative signs in those equations are a bit funny, and fairly arbitrary, as Nava says.

17

One thing I would add is surely r should be in m? ε0 has the units of F m^-1, A = 10^-10 m

18

Yes! I dont know what that business with the k is, but once you convert it around, that’s it (but you have to square the electron charge).

That little robot-E thing - how do you call it? And where do you find it on a keyboard?

I think my equations are fine, and my calculations thereof as well, and my question at this point isnt really a chemistry question, or a physics question, but a math question.

In these problems the potential energy V is plotted on the y-axis. So if I have two numbers, -10^-17, and -10^-19, and I am plotting them on my y-axis, -10^-17 is going to be lower on the graph, and would therefore represent a lower potential energy. This is because |-10^-17| is bigger than |-10^-19|, it’s magnitude is bigger, it would be farther away from zero, lower down on the y-axis, and thus less - right?

love
yams!!

19

Have you tried taking into account the enormous number of protons in the nucleus of a gold atom?

20

The “little robot e” is called “epsilon”, for the permittivity of free space (a vacuum), it’s called “epsilon-nought”. I just got it from a Wikipedia article when I went to check you had the right value. This isn’t a chemistry question at all, and it is just interpretation of numbers and plugging numbers into an equation.

When I calculated the values out for myself, I got different answers to you which agreed with your professor. I think your orders of magnitude are skewed on it, and you need to recalculate carefully.

ETR stupid blathering from last nights 7th pint