The Periodic Table

hBy2Py

2:13 AM

@DHMO There is no electronic SE for the dihydrogen dication -- it has no electrons. Did you mean something else?

user228700

2:39 AM

Hi everyone :-) Does anybody know about the "levelling" and "differentiating" effect of solvents..?

user228700

3:20 AM

@JohnRennie: Are u familiar with this concept?

John Rennie

4:58 AM

@KaumudiHarikumar I hadn't come across the idea before, but it seems straightforward enough. What in particular did you want to know?

user228700

@JohnRennie As usual, I came across several different definitions and they are confusing me.

user228700

The first, oversimplified version given by my textbook goes like:

John Rennie

I think the point is that if all acids are 100% ionised then they produce the same $[H+]$.

user228700

John Rennie

That seems pretty clear ...

user228700

5:07 AM

@JohnRennie Yes, but I wanted to have an idea about why this effect is observed and when I googled it, I found some other definitions that confused me a bit...

John Rennie

OK, what other definitions are causing you grief?

user228700

"No acid stronger than $H_3O^+$ can exist in water"

user228700

How come?

user228700

It is my understanding that acid-base reactions tend to proceed in that direction of the stronger acid(/base) to the weaker acid(/base)

user116211

5:13 AM

@KaumudiHarikumar stronger acid turns to weaker base...

user228700

@MAFIA36790 I'm sorry, I've edited that. I didn't mean acid converts to acid. I meant that it goes to the side which has the weaker acid among the two.

user228700

So, in case a stronger acid than $H_3O^+$ is mixed with water, then the reaction will proceed in the direction leading to the formation of the weaker acid, $H_3O^+$, correct? I dunno, I'm not entirely confident about that statement. Is this correct?

user116211

Hey Hey @shadow ;)

user116211

@KaumudiHarikumar Letting off, for a moment, the truthfulness/falsity of the statement, tell me what makes you dubious of the fact.

user228700

@MAFIA36790 Which fact?

user116211

5:21 AM

@KaumudiHarikumar You mentioned above.

user228700

@MAFIA36790 Well, I kind of feel like I haven't fully understood how these reactions work so I dunno if saying "leading to the formation of the weaker acid $H_3O^+$" and all makes any sense...

John Rennie

> No acid stronger than $\ce{H3O^+}$ can exist in water

That's another way of saying the acid is completely dissociated in water, though it's a rather less obvious way of saying it.

user228700

@JohnRennie How are those two statements equivalent..?

user116211

@KaumudiHarikumar Tell me first why acid-base reactions favour weaker acid-base products.

John Rennie

Suppose you have an acid HA. If it is a stronger acid than water it completely dissociates to give H+ and A-. OK so far?

user228700

5:28 AM

@MAFIA36790 Oh, I just read that in my textbook. I don't really understand why...

user228700

@JohnRennie Yep.

user116211

@KaumudiHarikumar Hint: Acid-base reactions are under equilibrium-control.

user116211

Now, think of the position of the equilibrium...

John Rennie

And when we write H+ we really mean $\ce{H3O^+}$ since we don't have individual proton floating around.

user116211

Think of the concerned thermodynamic potential...

user228700

5:30 AM

@JohnRennie Yes...

John Rennie

So when we put our strong acid in water we get a mixture of water, $\ce{A-}$ and $\ce{H3O^+}$. Yes?

user228700

@JohnRennie Yeah...

John Rennie

So what is the strongest acid present?

user228700

Uh, $H_3O^+$?

John Rennie

Correct! So regardless of what A is the strongest acid is going to be hydronium. That's why the statement is equivalent to saying the acid completely dissociates.

user228700

5:39 AM

@JohnRennie Ah...

user228700

Okay, I see.

user228700

Lastly, can you help me to understand this:

user228700

"Because of the leveling effect, you don't really have hydrochloric acid or sodium hydroxide floating around in water, as would be predicted by Arrhenius Theory. Instead, you have lots and lots of the threshold acid or base and extra ions capable of forming hydrochloric acid or sodium hydroxide once the water (or any solvent) is removed, as predicted by Brønsted-Lowry Theory."

user228700

?

user228700

Oh, BTW sir! My friend who is doing his undergraduate degree in IIT right now has just bought "Atkins" and he's offered to lend it to me for a whole week so do u think that will help me/will be enough time to understand the concepts of chemical+ionic equilibrium, as well as Thermodynamics? (Given that I've already done thermo. in physics...)

user228700

5:46 AM

With regard to the paragraph that I've quoted, what do they mean by "...according to the Arrhenius theory"?

user116211

@KaumudiHarikumar There is free pdf available online; try not to get the edition of Atkins-Paula; they dropped out many important discussions.

user228700

With regard to the paragraph that I've quoted above, what do they mean by "...according to the Arrhenius theory"?

user116211

@KaumudiHarikumar A very good book; it really would give you a very good insight on the concerned topics.

user228700

@MAFIA36790 Online?! Okay, I will check it out. Which edition are u talking about? (That has cut out some parts, I mean)

user116211

@KaumudiHarikumar Prior to the 2005/2006 edition, if I remember that was the year when Paula became the co-author.

user228700

5:49 AM

@MAFIA36790 So the editions published before 2005/2006 are good, you mean?

user116211

@KaumudiHarikumar Both are good; but the earlier versions had more relevant discussions in them.

user228700

@MAFIA36790 Oh, OK. Do u think I'll be able to finish all those chapters in a week?

user116211

@KaumudiHarikumar That depends; it took me a month or more to complete the concerned chapters as then it was my first serious study of physical chemistry; also I had other books to follow....

user228700

I mean, I realize that that's very subjective, but still, now that you know about my pace a little bit...

user228700

@MAFIA36790 Well, I can tell you that this is probably the 5th/6th time I'm learning these chapters as I have been constantly revising these chapters over the past two years but still, I don't have the adequate in-depth knowledge required for JEE. I've already spent a lot of time and I hope I'll be able to finish within the next week :/

user228700

5:55 AM

Anyway, @JohnRennie: What about that paragraph..?

user116211

@KaumudiHarikumar The chapters are small; you can skip some part and a chapter not relevant to JEE (can't remember the name; but contained phase rules and Clapeyron equation derivation etcetera). There must be six chapters that cover the concerned topics. Since, you more or less know what you are reading; you should take not more than two weeks or so. But read slowly without considering the time - this is my advice for sure.

John Rennie

Busy at work, back soon ...

user228700

@MAFIA36790 Actually, Clapeyron equation is given in my coaching textbook so it must be relevant. I'll skip those topics that haven't been given in my textbook; my intention is to get a broader picture. I certainly cannot spend two more weeks. I'll try my very best. Thank you :-)

user228700

@JohnRennie OK sir.

John Rennie

I'm back!

I assume the threshold acid or base means the acid/base formed from water i.e. hydronium or hydroxide. If so the paragraphs just says (again!) that the acid or base is completely dissociated.

user116211

6:08 AM

@KaumudiHarikumar Seen that many coaching materials do try to cover many more things that aren't even important; however the chapters are quite intuitive and lucid; so don't think you should face any problem.

user228700

@MAFIA36790 Well, yeah, but I'm trying to know as much as possible...

user228700

@JohnRennie Yeah, I too don't understand what they mean by "threshold acids/bases" :/

user228700

One second. Brb.

user228700

6:25 AM

I'm back. Sorry about that; something important came up...

user228700

Let me look up "threshold acid/base"...

user228700

Oh, it's all about something called the "lactic acid threshold" :/

user228700

OK, never mind that. @JohnRennie: What do they mean by "...according to Arrhenius concept"?

John Rennie

@KaumudiHarikumar no idea. The only thing associated with Arrhenius I know of is the Arrhenius equation and this doesn't seem relevant.

user228700

@JohnRennie Well, there is the Arrhenius concept of acids and bases. U are not familiar with that..?

John Rennie

6:32 AM

@KaumudiHarikumar Oh, OK, I guess I just consider that part of the furniture rather than a theory.

I still don't understand what the paragraph means.

This could mean it's too deep and meaningful for me to understand, or it could mean that it's badly written :-)

user228700

@JohnRennie OK :P But how does the Arrhenius concept predict that the acid(/base) introduced to water floats around as such..?

user228700

The Arrhenius concept simply states that an acid dissociates to give $H^+$ and than a base gives $OH^-$, correct?

user228700

What's that to do with this "independent floating around"..?

John Rennie

I've no idea. I can't see why the (almost) complete dissociation of a strong acid in water is in contradiction to Arrhenius' theory.

Back to work for the next few minutes ...

user228700

Hm, OK...

user228700

6:37 AM

OK sir, never mind that paragraph then. BTW, MIT also teaches Le Chatelier's principle like the way my textbook does: "The system moves in that direction in which the applied stress is relieved".

user228700