The Periodic Table

1:53 AM

-1

Q: Describe energy (of a system) properties in a substance and give some examples?

It was my physical chemistry homework. I was tasked to answer it in qualitative perspective. I really can't understand what the question really means.. Am I supposed to: explain one form of energy, like heat, and the application of heat in a system? Say that the system is a tea in a glass, an...

What's this supposed to mean? I don't get this post..

user228700

2:11 AM

@Loong: Hello :-) Since I received an answer for my question even before new answers were posted, I'm wondering if I should accept an answer or not, since that would mean that you'd lose 50 points. I should also mention that none of the answers are as satisfactory as the answer I first received so that's the other reason that I'm hesitant to accept any of the answers posted.

getafix

2:32 AM

@KaumudiHarikumar I think this is between you and Loong, but I think the purpose of a bounty is draw more attention to an question by promising a reward, in this case 50 rep. People posted answer in the hopes of earning said reward, and removing it now would be unfair to them

user228700

2:53 AM

@getafix OK. I'll wait for @Loong to respond though.

user228700

Also, just one person has posted an answer so far. I'm gonna wait for at least 3-4 days before I go ahead and hastily accept an answer.

getafix

last I saw there were two?

orthocresol

3:08 AM

1. Bounties are non-refundable. 2. Bounties are non-cancellable. stackoverflow.com/help/bounty

I swear I dreamt of that stupid heat capacity question last night

getafix

3:19 AM

Hahaha, still haven't found it?

Btw, @orthocresol great job on O-O and S-S bond question..!! It was a delight..

@orthocresol I did try looking for the heat capacity question, you did offer a big bounty ;) no luck though

orthocresol

3:31 AM

@getafix You know, if two hours of my best searching efforts couldn't find it, I think it might just be worthwhile to write a new question...

UGH

getafix

3:48 AM

Umm perhaps

What was it about though? Like precisely..do you remember?

orthocresol

4:02 AM

It was just about Cp - Cv = nR

although it could be in many guises for example Cp,m - Cv,m = R

i believe there were at least two answers, one of which simply stated that for an ideal gas, U = 3/2 nRT

But I don't like that, because that's only half a proof, that is not the definition of an ideal gas and you don't need it to prove the equation anyway

So I wanted to write a more rigorous proof, but now I can't find the question.

You can see my chem.SE to-do list on my profile information, actually.

orthocresol

4:21 AM

Maybe it was the perfect gas/ideal gas question and I just remember wrongly.

sobs

I looked through all the results for "[physical-chemistry] answers:2" and couldn't find it.

user228700

4:40 AM

@orthocresol Oh, shucks :/

user228700

I've got a quick question. Textbooks keep referring to "Enthalpy" as the "heat content of a system". In my understanding, the quantity heat is only defined during processes and it is not a state function. On the one hand, people say " It is meaningless to talk about the heat content of a system" and on the other, people are defining enthalpy as the heat content of a system! What is the more useful definition of enthalpy that u have found..?

getafix

5:06 AM

@orthocresol is it possible that it was flagged/deleted?

I would like to write a proof along those lines too actually haha

orthocresol

@KaumudiHarikumar H = U + pV.

That is the definition of enthalpy

If you say it is the heat content of the system, that is an interpretation, not a definition

@getafix It is possible that I just remember wrongly. :(

@KaumudiHarikumar Also, you are right, heat is only defined for a process, not a state.

However, enthalpy changes are equal to heat changes, for a process at constant pressure.

Most chemical processes are constant pressure (think of doing a reaction in a test tube - everything is at atmospheric pressure), which is why some books may equate heat and enthalpy.

user228700

@orthocresol You mean constant volume..?

orthocresol

No, I mean constant pressure.

At constant volume, heat change = internal energy change

"heat change" is not really correct terminology, but I will let it slip...

user228700

@orthocresol Yes, OK.

orthocresol

Nah, it is my bad phrasing, not yours. What i called heat change should be more properly called "heat supplied to the system"

user228700

5:15 AM

I was wondering why we use this term(enthalpy)instead of plain old internal energy when speaking about bonds. Bonds are created so that's why we include the expansion work too?

orthocresol

Simple, because $q = \Delta H$ for const-p whereas $q = \Delta U$ for const-V

and const-p is more applicable than const-V

for most chemical applications

If you were to define a chemical process at const-V, then you would use internal energy

However, most of us define chemical processes at const-P, hence enthalpy is our friend.

CowperKettle

\o

orthocresol

@CowperKettle hi

CowperKettle

Is "molecular weight" used more often than "molecular mass"?

orthocresol

@CowperKettle Although it is not strictly correct, yes, MW is much more common.

CowperKettle

5:18 AM

@orthocresol Thank you! Because in Russian the standard phrase is "molecular mass".

orthocresol

@CowperKettle See, Russians know science better than we do.

shakes head at IUPAC

CowperKettle

My TRADOS dictionary suggests "molecular weight" all the time, and I follow its suggestion

user228700

@orthocresol OK. Thank you very much :-)

orthocresol

@KaumudiHarikumar No problem.

getafix

5:33 AM

Yeah in general, people confuse mass and weight in common usage all too often and it annoys the shit out of me

orthocresol

@getafix see, I'm primarily an organic chemist, (or I will be after I finish struggling through this year of inorganic and physical), so I don't really care that much :D

CowperKettle

> In order to determine the molecular weight of the non-glycosylated protein, we used PNGase F to separate N-glycans from the protein molecule.

I wonder if there's a better verb than 'separate'

orthocresol

5:49 AM

I don't know the context well enough to judge.

I think it is trying to cleave the groups?

or remove the groups?

CowperKettle

cleave might be better, thank you

orthocresol

Separate suggests that they are not covalently bound whereas I think this case it is trying to remove a covalently bound group.

If that's the case I'd go with cleave

CowperKettle

nods

getafix

@orthocresol I see haha. I really don't know what I will be sigh..

orthocresol

@getafix well, I changed my mind several times. For a really long time, I wanted to do medicine

So there's no guarantee that this time next year, I will still want to do organic :p

CowperKettle

5:59 AM

Do biotech, man

Gene modification, etc. (0:

orthocresol

@CowperKettle You actually need to know a lot of bio to do that

It's really cool, and probably most people in the scientific community have heard of crispr-cas9, but I'm not feeling the love for bio right now :(

CowperKettle

CRISPR

Clustered regularly interspaced short palindromic repeats (CRISPR, pronounced crisper) are segments of prokaryotic DNA containing short repetitions of base sequences. Each repetition is followed by short segments of "spacer DNA" from previous exposures to a bacteriophage virus or plasmid. The CRISPR/Cas system is a prokaryotic immune system that confers resistance to foreign genetic elements such as those present within plasmids and phages, and provides a form of acquired immunity. CRISPR associated proteins (Cas) use the CRISPR spacers to recognize and cut these exogenous genetic elements in a...

orthocresol

they're gonna get a nobel prize

getafix

I was into biochem but then my interests moved to supramolecular Chem

Think I'm gonna see how that works out, cause I like all of Chem..haha hard for me to narrow down

CowperKettle

I never even heard of supramolecular chemistry

Supramolecular chemistry

Supramolecular chemistry is the domain of chemistry beyond that of molecules and focuses on the chemical systems made up of a discrete number of assembled molecular subunits or components. The forces responsible for the spatial organization may vary from weak (intermolecular forces, electrostatic or hydrogen bonding) to strong (covalent bonding), provided that the degree of electronic coupling between the molecular component remains small with respect to relevant energy parameters of the component. While traditional chemistry focuses on the covalent bond, supramolecular chemistry examines the weaker...

getafix

6:07 AM

So yeah let's me do a bit o organic and inorganic in supramolecular Chem

CowperKettle

Supramolecular looks like a thing done more behind the monitor than with the beaker and tube?

getafix

Well it's broad. I have been spending my summer in a supramolecular lab, and although I have only been doing literature review..the lab has a heavy organic synthesis focus

Anyway might be moving to an organic lab after my summer vacation ends because right now, the lab I am in can only let me work over vacations

Stupid rules in my university..even right now I kinda forced them to make an exception

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