The Periodic Table

Haikus are awesome / Chemistry's even better / So pull up a chair
Jul 15 1:27 PM
@BuckThorn I was interested in the general formulas, but I was not able to find a derivation of them. An example of the formulas can be found at Chapter 12 of Atkins' Physical Chemistry (I think it is Chapter 12, it is the one that has the name of vibrational and rotational spectroscopy)
Jun 27 12:43 PM
Hi everyone... I am reading about rotational spectroscopy and I was interested in the derivation for the moments of inertia for symmetric rotors (which is a function of the bond length and angles). Anyone knows a source?
Mar 22 4:05 PM
haha
Mar 22 10:53 AM
@Mithoron Mithoron, parlez-vous français ?
Mar 1 11:43 AM
What type of bright future you mean in Argentina (what is ssplb)? The average income in Argentina is about 180 USD/month (this is real, lol). Moreover, there is an uncertainty whether the new president will support public education (research in Argentina comes 99.999% from the most important public university). There are current manifestations of PhD students because their salary will be cut off
Mar 1 11:38 AM
@BuckThorn I didn't consider Australia, don't know why though. I've studied all Europe, and all the master's in this field are for the most part in Norway/Denmark/Sweden. There are two in France, and an 'Erasmus type' shared between Spain, Belgium, and the Netherlands.
Mar 1 10:41 AM
Maybe everything will be alright, and I will find a place in IT or the field that you referred to in the last message. A similar situation would be the one that chemists tell me in Argentina. When things go wrong, they will always find a place to work in water quality, reporting Cd, Pb, Hg, etc and some statistics. That's not what they were hoping for, but they are safe
Mar 1 10:35 AM
@BuckThorn I considered it, but since it was related to chemistry I thought it would fit more in this place. Yeah, you are right. Undergraduate degrees are always taught in the mother tongue of the country, so I will have to learn it, but that's ok. Thank you very much for that information in ACS, time to read. The thing I fear the most is the worst possible scenario i.e., ending the degree with a paper in my hand, and if that paper is useful in the job market
Mar 1 10:27 AM
@Mithoron Yes.. it is pretty broad, but all the programmes are focused to give an overall view of the subject. Some of them have also have molecular dynamics, other ones put in the mix AI or machine learning, etc. The specialisation has that name, but any research I will do (if any) in the future will be the aligned with the department interests (and they are all different, of course).
Feb 19 11:39 AM
Thanks in advance
Feb 19 11:38 AM
I would like a simple life as it is in my homeland... I mainly teach and do research. However, imagine all goes wrong: I get no offers for PhDs, impossible to become a practical assistant, I suck at everything, or other things... What is the world today for someone that has in his/her hand a master which is oriented in theoretical chemistry? Does he/she have any job prospect outside academia/teaching? Maybe in pharmacy? Is the field plain dead outside universities?
Feb 19 11:34 AM
Hello!! I have a question for the chemists regarding careers.

I have a degree in chemical engineering. Last year I did an exchange in a country of Europe and was exposed to theoretical/computational chemistry, and I really want to go further in it. I am applying to several master's programmes which offer a specialisation on this subject. I don't stand a chance (due to the lack of credits) in organic, analytical, etc.; but since it is more related to PChem, I might have a chance (since it needs also phsyics and mathematics)...
 
Mar 16 9:51 AM
This has nothing to do with the post but I find amazing. Coming from a physics and a maths major, you are familiar with electrodynamics than chemists. The idea that hydrogen ions just escape from the aqueous solution to the gas phase is crazy due to electrostatic interactions , but maybe that needs some chemistry background. However, you just violated charge neutrality two times, the liquid has now become negative and the gas positive. Science is unforgiving and very hard... I will try to post something later to help you!
 
Mar 1 10:16 AM
Just to ensure, check that all your sources say what "mass" refers to.
Mar 1 10:15 AM
A problem when making comparisons is that no one specifies what they mean with "mass". If the battery maker has to be honest, it should be the TOTAL mass (with everything). But, some people choose something like "mass = mass of positive electrode", "mass = both electrodes + electrolyte", and so on. This cheating is helpful because if you are omitting a heavy part of your battery in the mass, the specific energy density will be great.
Mar 1 10:12 AM
And it is a surprise that you have found the opposite, i.e. that LiPo is more dangerous and presents a higher specific energy density.
Mar 1 10:11 AM
And I've read the sentence 'Polymer-based electrolytes pale in comparison with liquid-based electrolytes due to its mass, therefore reducing the specific enery density' like 1 million times.
Mar 1 10:10 AM
Hey, sorry for the reply I was very busy. It seems that you have done a lot of research, and then go for the Li-Pi for your drone. The reason why I told you that thing about energy density was because I've done my final thesis in LiS battery.
Feb 26 4:48 PM
Maybe you can find some data that makes a fair comparison, typically from a paper that suggest improvements on a new cell with a solid electrolyte.
Feb 26 4:47 PM
As an example, the figure you referred to doesn't mention the materials of the positive electrode or negative electrode, I imagine that it is just an "average". But the approximate energy densities are 200, 160, 220, and 150 when the material of the positive electrode is LCO, LMO, NMC, and LFP.
Feb 26 4:46 PM
A reliable assessment would be to grab a battery with the SAME positive electrode, SAME negative electrode, SAME separator, SAME tabs and housing, etc; but only change the electrolyte
Feb 26 4:45 PM
Although I understand what you mean, we need to make fair comparisons. There are many Li-Ion batteries, and they volumetric energy density varies across them
Feb 26 4:05 PM
We agree, we had a misunderstanding. In my experience, when someone writes 'energy density', it means energy per unit mass (also called specific/gravimetric energy density). However, you were referring to the volumetric energy density. You are right. Cells based in solid electrolytes do provide a lower volumetric energy density.
Feb 26 4:05 PM
I have no doubt that electrochemical cells which use solid electrolytes are safer. However, the fact that they also give a higher energy density is a surprise to me. We will wait for an answer...
Feb 26 4:05 PM
You say that you maxed out the energy density for a cell using a liquid electrolyte. If by because of that, you mean 'even though we maxed out the energy density, it is not enough', then that is the reason why I asked you. You want to have more energy density by employing a solid electrolyte?
Feb 26 4:05 PM
Hi! May I ask why you say that solid electrolytes max out the energy density? Frequently, they pale in energy density because they are heavier, even though sometimes they exhibit a better performance. For me, the energy density $E$ is defined as $E \equiv \int_0^t E(t')I(t') \, \mathrm{d}t' / \color{red}{m}$.
 
Feb 15 2:39 AM
It seems to me that now you moved the goalpost, or being just plain dishonest. Do you accept the need for the process to be mechanically reversible? If you do, then why didn't you agree with the book like 10 days ago?
Feb 15 2:38 AM
@ManRow Hello! You are asking a different question. This discussion initiated because you claimed that $\Delta H = Q$ regardless of reversibility. I link it here
Feb 15 12:13 AM
Ping at me when you have a book that says the contrary so we can discuss further
Feb 15 12:13 AM
Up to now, we have one source directly state that what you say is not correct, and this book is pretty much part of the syllabus of a lot of chemical engineering degrees across the world. Thus, it is a good source also.
Feb 15 12:09 AM
What we can do is the following. Bring me a book that states clearly that the equation is valid for irreversible process, and we can discuss further.
Feb 15 12:09 AM
It explicitly says only for the mechanically resversible
Feb 15 12:08 AM
Then it says:
However, only for the mechanically reversible, constant pressure process can heat and work can be calculated by the equations $Q = n \Delta H$, $Q = ...$.
Feb 15 12:08 AM
Of course this book refutes your claim. I will read it for you: postimg.cc/bSYg5C1t

For a mechanically reversible process, constant-p process, this result may be combined with Eq. (2.13): (shows the equation that $Q = \Delta H$).
Feb 14 10:12 PM
It seems you are very interested in this. We could also email some of the authors of the books and ask them this question, the email would be rather short, so there is some chance
Feb 14 10:09 PM
I searched many of them, but didn't find anything
Feb 14 10:07 PM
*processes
Feb 14 10:06 PM
Could you find a book where it says that the equation is valid for irreversible process? I think that would be helpful
Feb 14 10:06 PM
Hello, yes I saw it.. it seems that he agrees with me
Feb 13 1:51 AM
Yes, of course. We are only considering the "PV work" in $w$.
Feb 12 11:00 PM
To sum it up, is $Q = \Delta H$ valid for constant-pressure and irreversible processes?
Feb 12 10:58 PM
On the other hand, @ManRow thinks that the equation $Q = \Delta H$ is valid for processes that are isobaric, but reversibility doesn't matter. Thus, the equation is also valid for irreversible processes.
Feb 12 10:57 PM
@ChetMiller I summarise. I think that the equation $Q = \Delta H$ is valid for an isobaric, and mechanically reversible process. If the process is irreversible, that equation is NOT true. I leave you an image from Smith, Van Ness, and Abbott's book that state this in Chapter 2: postimg.cc/bSYg5C1t
Feb 12 8:15 PM
I am 100% sure that Chet Miller has read Smith, Van Ness, and Abbott's book a lot, because we are both chemical engineers. Thus, he might also disagree with what they say
Feb 12 8:13 PM
@ManRow May be you can find a book that has that proof and says that it is also true for irreversible processes, apart from your thoughts.
Feb 12 8:12 PM
@ChetMiller Can you read these messages?
Feb 12 8:12 PM
I tried searching The Atkins' book in insight and there is no comment about the reversibility of the process. I checked other books as well.
Feb 12 8:11 PM
I could find one book, which is the one I pointed in this photo postimg.cc/bSYg5C1t, that specifically says that only for a mechanically reversible process that equation is true.
Feb 12 8:09 PM
Yes, now it is much clearer to me. I understand your point. I pinged Chet Miller but he did not respond. May be you can try that yourself? In addition, did you find any book which explicity says that the proof holds for reversible and irreversible processes?
Feb 12 10:20 AM
How come you are able to cancel "-p{ext}dV + pdV" in the derivation? What does the "p" refer to, in your opinion?