Problem Solving Strategies

3:31 AM

@JohnRennie I have a doubt in understanding sorption and desorption (surface chem) can you clear it?

3 hours later…

6:35 AM

@napstablook hi :-)

What's the problem?

@JohnRennie In this chat which @RajorshiKoyal asked ..Why did you say that negative deviation has more to do with delta H mix <0 or >0

I mean what has hydrogen bonding to do with this

Why will the enthalpy change because of association?

Secondly for upper and lower limit in Elastic Modulus vs volume of dispersed phase can u explain how the rule of mixtures explain two bounds?

@user586228 Suppose we start with an ideal gas. In an ideal gas there are no interactions between the gas particles. If we mix ideal gases the mixing is driven only by entropy because no interactions means there cannot be any change in internal energy. OK so far?

@JohnRennie Are you familiar with rule of mixtures

In a liquid there must be interactions because liquids form due to interactions between the particles. So even an ideal liquid this is fundamentally different from an ideal gas because we cannot assume there are no interactions.

6:49 AM

But suppose we start with a pure liquid A. If you look at a molecule of A then on average it will be surrounded by N other molecules of A and it will interact with them. The total interaction energy per molecule will be related to the strength of the A-A interaction and the number of surrounding molecules N.

Now suppose we add some other liquid B so we now have an AB mixture. That means each molecule of A is surrounded by some A molecules and some B molecules.

then

If there are on average Na molecules of A and Nb molecules of B around an A molecule, and the A-A interaction energy is Ea and the A-B interaction energy is Eb, then the total interaction energy will be something like NaEa + NbEb.

Now, in an ideal solution the energy doesn't change on mixing. Before mixing the interaction energy was NEa, and after mixing it is NaEa + NbEb, and in an ideal solution these would be the same.

6:53 AM

i.e. N Ea = Na Ea + Nb Eb

Let's assume that the molecules of A and B are roughly the same size, so the total number of molecules around an A molecules doesn't change - all that happens is some of the neighbouring A molecules get replaced by B moecules. So Na + Nb = N.

Then the interaction energy can only be constant if Ea = Eb i.e. if the strength of the A-A interaction is equal to the strength of the A-B interaction.

But molecules aren't just featureless spheres. The have a shape, and they have electric dipoles.

It's possible that the shapes of the molecules of A an B could mean that an A molecule and a B molecule can fit together very closely, more closely than two A molecules or two B molecules can fit.

6:59 AM

In that case the strength of the A-B interaction would be different from the A-A and B-B interaction strengths.

And this would be a non-ideal solution. Depending on exactly how A and B interact we could get energy released or absorbed on mixing i.e. ΔH > 0 or ΔH < 0.

When this happens we expect to see a deviation from Raoult's law and Henry's law.

But why negative or positive deviation based on this assumption?

How is Henry's law different from Raoult's law

If we take Raoult's law, this works because the molecules in the solution are in equilibrium with the vapour. But most gases are close to ideal to in the vapour phase we expect ideal behaviour.

@JohnRennie I could not understand anything..can u be slightly more descriptive about this...?

7:03 AM

So if the vapour phase behaves like an ideal gas, but the liquid phase is non-ideal, we are going to get deviations from laws like Raoult's law that assume all phases behave ideally.

How are Henry's law and Raoult's law different from each other

@user586228 I thin this will turn into a long discussion, and I don't have time for this today.

Just answer this

You need to already know and understand Raoult's law and Henry's law for any of this to make sense.

Rule of mixtures

In materials science, a general rule of mixtures is a weighted mean used to predict various properties of a composite material made up of continuous and unidirectional fibers. It provides a theoretical upper- and lower-bound on properties such as the elastic modulus, mass density, ultimate tensile strength, thermal conductivity, and electrical conductivity. In general there are two models, one for axial loading (Voigt model), and one for transverse loading (Reuss model).In general, for some material property E {\displaystyle E} (often the elastic modulus...

Check this out Rule of Mixtures..

7:05 AM

OK ... ?

I do not understand how Isostress condition and Isostrain condition is used to decided longitudinal loading and transverse loading with the upper and lower bound for young's modulus

This discussion is more impotant for the time being.

It's not an area I know anything about. We are straying away from physics and into engineering again.

5 hours later…

12:22 PM

Guys

I am having a lot of confusion while making notes

Should I use a register

or making notes on separate pages

The disadvantage of register is that if I have to cut or ignore a page.I will have to cut it.Then , let us say if there are some important points that should be written after page 3 (on page 4 you had to write )but you cannot write it there because you have already written sth there now . Then writing that information on further pages makes it difficult to revise.

In pages , you can just attach that page which you needed exactly at the page number where you wanted to . But it is difficult to keep these pages safely

1:01 PM

@Physicsismylife you still need the register where you will write classnotes. It is important because if the memory of how you were taught in class is renewed it will help you remember it much better.

seperate notes shouldn't be made for all chapters, only specific topics where you have a problem in grasping or memorising because now you need another memory of how you learned it since the previous memory doesn't help. Also make folders for all subjects where you will store these seperate notes. That usually works with me.

disadvantage of separate notes is when you set out to make them you mostly try to include as much info as possible which takes a lot of energy and time(I wasted quite some time in this.) Unfortunately at some point this disadvantage can't be ignored against the advantages

@JohnRennie I don't understand the following

A) sorption means either adsorption or absorption is going on or both need to happen simultaneously to be called sorption?

separate pages

A4 sheets

@napstablook

B) desorption also means emitting through the surface or from the surface or both need to happen simultaeously?

I made a file

what was that used for?

1sec I will post an image to give you more clear view of mint nites

This page is most likely to tear off

I hope you can get that

1:09 PM

oh so you are adding notes using binding files?

If you want to remove some information in register , then it is better to use these sheets I think

Yes @napstablook

why would you want to remove notes?I thought you wanted to add notes?

Let us say slowly you understand a chapter more , you won’t write that notes after 20 or 30pages right

Sometimes you may want to write some information not on that pag or position

then you remove it and add it somewhere else

What do you think

It is better than register I think but I don’t like it when pages tear off

1 hour later…

Q: Is this correct for pressure exerted by a liquid?

2:16 PM

$P_0$ is atmospheric pressure and P1 is height * density * gravity. So , is the diagram correct for saying that Total pressure in the liquid = P0 +P1. Since the direction of P0 vector in downwards?

2 hours later…

4:36 PM

Why are they assuming forces to be same

c stands for composite that is mixture of both fibre and matrix.