Submitted by samuelma t3_z8r5za in explainlikeimfive
Fluffy-Jackfruit-930 t1_iycvh9f wrote
When a solid dissolves in a liquid, it basically becomes liquid. This is kind of like melting - the solid becomes a liquid. For this to happen, the chemical bonds holding the solid together have to break. Breaking the bonds takes energy - this can come from heat, so when a substance melts it cools the area around it. The same happens when a solid dissolves - it cools the liquid around it.
However, there is a big difference between melting and dissolving. When a solid dissolves in a liquid, it becomes part of the liquid by bonding with the liquid molecules. This creates new bonds and making these new bonds releases energy as heat. This refunds some of the heat used to break up the solid in the first place.
For most solids, the heat refund is less than was paid to break the solid up. This means that dissolving costs energy and eventually you reach a balance where no more will dissolve. If you add more heat, there is more energy available and the balance points moves so that more ends up dissolved.
For some solids, the refund is actually more than was paid. In this case, adding heat reduces solubility bexause the energy balance point moves the opposite way.
The same thing happens with gases. They have super weak bonds when gases, and the bonds when dissolved are stronger, so you get an energy refund when they dissolve. Putting more energy in moves the balance point to reduce solubility.
ialsoagree t1_iycz5rd wrote
>However, there is a big difference between melting and dissolving. When a solid dissolves in a liquid, it becomes part of the liquid by bonding with the liquid molecules. This creates new bonds and making these new bonds releases energy as heat. This refunds some of the heat used to break up the solid in the first place.
This is just not correct at all.
Let's take a simple example of salt being dissolved in water.
It's kind of true that the bonds that make up the crystal lattice for salt are broken as part of the dissolving process. This occurs mostly because the sodium and chlorine are already highly ionized (the electron being shared between them is spending most of it's time around chlorine, and we tend to short hand this by calling it an Na+ and Cl- ionic bond), so separating them is relatively easy if you have something with sufficient ionic attraction (like the partial charge that water carries due to the strong electron attraction of the oxygen atom in H2O).
But new bonds are NOT created. There is hydrogen bonding, but there's no chemist in the world who would say that hydrogen bonds are a form of chemical bonding. It's a dipole-dipole attraction, similar to a Van Der Waals force but much much stronger.
Secondly, this process doesn't release energy. The process of dissolving salt in water is endothermic - that is, energy is absorbed in the process of dissolving salt into water, not released, so both the salt and the water will get slightly cooler as a result of the salt dissolving.
Chromotron t1_iyd42qz wrote
> But new bonds are NOT created. There is hydrogen bonding, but there's no chemist in the world who would say that hydrogen bonds are a form of chemical bonding.
They didn't say "chemical bond", but just "bond". Just read what you wrote yourself again, even you used "hydrogen bonding", and indeed, there are many types of bonds and all of them involve some energetic changes. It doesn't matter if hydrogen bonds, ionic bonds, chemical bonds, or magical bonds.
> Secondly, this process doesn't release energy.
They didn't claim it does. Even in what you quoted they say "This refunds some of the heat used to break up the solid in the first place", implying that in total, energy is still spent. And it is correct that the break-up requires energy to overcome the ionic bonds, yet some other salts release quite a bit of energy when dissolved in water, exactly due to the reason they gave.
ialsoagree t1_iyd4nav wrote
>They didn't say "chemical bond"
Yes they did:
...it becomes part of the liquid by bonding with the liquid molecules.
Nothing is "bonding with the liquid molecules" - that's not happening at all.
>even you used "hydrogen bonding"
Which is decidedly NOT "bonding with the liquid molecules" exactly like I stated.
>It doesn't matter if hydrogen bonds, ionic bonds, chemical bonds, or magical bonds.
Yes it does, these are all WILDELY different things.
If you don't understand chemistry don't pretend to.
>They didn't claim it does.
Did you not read their post?
making these new bonds releases energy as heat
When you can actually read what they posted and my reply, get back to me.
Chromotron t1_iyd79iq wrote
Are you seriously saying that "hydrogen bonding" is not "bonding""? Because that is clearly silly.
And yes, they said the new bonds (the hydrogen ones, duh!) release energy; after more(!) energy was used to break ionic bonds.
You are just trying to be pedantic and fail at it.
ialsoagree t1_iyd7zq3 wrote
I'm saying it's not "bonding to a molecule" if you disagree you need to take introductory chemistry.
Further, I just want to point out, hydrogen bonding only applies to ionic substances being dissolved in specific solvents. So many things dissolve without hydrogen bonding at all.
You clearly don't understand the basics of chemistry.
Aurigae54 t1_iydggw9 wrote
Thats not true. Hydrogen bonding is an intermolecular force of attraction, as in between two molecules, and it certainly doesn't only apply to ionic substances being dissolved. The reason why water is a liquid at room temperature is because of hydrogen bonding between the positive hydrogen ends of one water molecule being attracted to the negative end of a second water molecule.
I think the point you were getting at was intramolecular vs intermolecular forces of attraction, and that in general intramolecular forces are considered to be true bonds, as they hold the atoms in different molecules and compounds together whereas intermolecular forces, not being nearly as strong as intra, are generally demoted to just being weak attractive forces, with hydrogen bonding being confusingly named as its not even a true bond by these metrics. This whole thread seems pretty semantic, 'bond' is just a strong word to describe an attractive force, at the end of the day and at its core a triple covalent bond is not that different from dispersion forces, they are just two ends of a spectrum representing how little/how much energy you need to break an attraction.
ialsoagree t1_iydj3qr wrote
You said "that's not true" but then discussed a bunch of stuff I never mentioned. I never mentioned whether or not hydrogen bonding only applies to dissolving substances. I never mentioned water being a liquid at room temperature.
But I will address this:
>they are just two ends of a spectrum representing how little/how much energy you need to break an attraction.
At a physical level I agree with you. But not at a categorical level. These things are categorically distinct when we talk about them because of the size of disparity in energy required.
Let a cup of salt water sit and salt will spontaneously crystallize out of the water within hours. Just through Brownian motion.
Stable molecules could take billions of years to change their structure, or longer. This is why we categorize "water" as it's own molecule, and "salt" as it's own molecule, but we don't categorize "salt water" as a molecule - we categorize it as a solution of 2 molecules.
Chromotron t1_iyd8gzo wrote
Well, I will leave my expertise of chemistry to the other readers (and my post history), but you clearly can only do insults (and down-votes, it seems; surely a sign of a healthy adult scientific discussion!).
ialsoagree t1_iydb6tk wrote
>you clearly can only do insults
Says person who referred to my posts as "pedantic" because they corrected incorrect statements.
The only difference is - all of my posts have contained content that directly supports my position by describing how chemistry actually works. This post of yours doesn't, it JUST serves to insult me.
>and down-votes
Yes, because you haven't down voted any of my posts, right? *Eye roll*
If you want to have a "healthy adult scientific discussion" I'm happy to do that.
But saying "hydrogen bonding is a form of bonding with molecules" isn't an adult scientific discussion. It's a blatant misrepresentation of actual chemistry. A hydrogen bond - as I stated in my very first post - is a dipole-dipole interaction. It's an inter-molecular force (a force between two separate molecules, not a bond) similar to Van Der Waals forces but many magnitudes greater in strength.
Further, hydrogen bonding doesn't even apply to the way many things dissolve. So even if we could ignore chemistry and say that hydrogen bonding is a form of bonding to a molecule - which it's decidedly not - that STILL wouldn't make the post I replied to correct, because it only applies to a subset of things that dissolve.
Organic molecules, for example, can dissolve through Van Der Waals forces or/specifically London dispersion forces.
Something being dissolved can be endo or exothermic, there's no hard and fast rule about whether dissolving something will heat or cool a liquid. Saying it's one way or another just isn't accurate, it depends entirely on what you're dissolving.
Chromotron t1_iydcjkw wrote
> Something being dissolved can be endo or exothermic, there's no hard and fast rule about whether dissolving something will heat or cool a liquid. Saying it's one way or another just isn't accurate, it depends entirely on what you're dissolving.
Look, you get hinged on this again, yet the original post never said it is just one or another. It differentiated between the exothermic "reaction" of the formation of hydrogen bonds (or van-der-Waals forces or others, unmentioned ones) and the endothermic dissolution of the (often ionic, but also not explicitely mentioned) bonds in the solid. Those are two things, their sign is (almost always) pretty clear and opposite, and the heat/cold of dissolution comes from their (signed!) sum. Hence it can be either way, and this was mentioned in that post.
Yes, they left out some detail and might( have wrongly implied that the total is always negative, i.e. dissolution is endothermic, but this was not what you called out and this is ELI5, not a journal paper.
ialsoagree t1_iydecbl wrote
>Yes, they left out some detail and might( have wrongly implied that the total is always negative, i.e. dissolution is endothermic, but this was not what you called out and this is ELI5, not a journal paper.
Sure, you could argue that his statement was specifically saying that hydrogen bonding - or any other force being used to maintain the solvation of the solute - is exothermic.
But my point remains that this isn't a bond between molecules, and that is an ELI5 level topic.
There is no point in chemistry where anyone would ever suggest that solvation involves molecules bonding. No chemistry teacher would ever say that hydrogen bonds, Van Der Waals forces, or any other intermolecular force is "bonding with the liquid molecules" or that "this creates new bonds."
Chromotron t1_iydftjk wrote
Let me quote Wikipedia, but this is also in agreement with what I learned and sources are given there as well: '''The strength of chemical bonds varies considerably; there are "strong bonds" or "primary bonds" such as covalent, ionic and metallic bonds, and "weak bonds" or "secondary bonds" such as dipole–dipole interactions, the London dispersion force and hydrogen bonding.'''
So bonds are just that, bonds. Some are just stronger or more commonly encountered.
Also, dissolving salts involves their ionic bonds, so if we are pedantic, this part involves even more bonds. But I get some feeling that even ionic bonds are not "bonds" in your mind, only covalent ones?
ialsoagree t1_iydhqgw wrote
I appreciate that wikipedia may describe it like this, but that doesn't make it an accurate description used in chemistry.
This paper, for example, specifically looks at the differences between chemical and hydrogen bonding (specifically by look at bond strength).
>But I get some feeling that even ionic bonds are not "bonds" in your mind, only covalent ones?
I'd argue that the distinction is more categorical than physical.
All chemical bonds are covalent bonds, there's just a disparity in how much the electrons are actually shared.
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