Submitted by Effective-Night-2646 t3_z4w7xm in askscience
I was reading about how the components in a mixture haven't reacted with each other so can be separated via boiling which got me wondering what does it mean for them to react with each other. I've always thought of it as bonds forming but this doesn't seem true here as bonds exist between the atoms in the mixture so how do you classify whether they've reacted or not?
I'd like to elaborate on that a bit regarding some important types of bonds and molecular interactions. I'll try to simplify as much as I can and proceed from weakest to strongest in terms of bonding strength.
Van-der-Waals-Force The weakest of the underlying forces are Van-der-Waals-forces. They occur between any kind of atom and/or molecule and are weakly attractive across small distances, until they become repulsive when atoms or molecules are pushed together too tightly.
Dipole interactions Molecules, which are basically arrangements of semi-permanently bound atoms, can have complex structures, depending on how many atoms are involved and how they are arranged. Based on that complexity, some areas of them will have a electromagnetic charge (positive or negative, just like a battery). Charged regions of a molecule will attract molecule regions of the opposite charge.
Hydrogen-bonds Hydrogen bonds are basically a special form of dipole interactions. In specific arrangements inside molecules, hydrogen (partially positively charged) and oxygen (partially negatively charged) will get into close contact and start attracting each other strongly because of their polarity. The term hydrogen bond, is a bit misleading, as this is not considered a bond that would form a new molecule, but one of the weaker interactions between molecules. However, it is such a common interaction, that giving it its own term is reasonable. These bonds, together with dipole interactions, are what make water (highly polar) dissolve other polar molecules, while oil (non polar) does not mix well with water.
Covalent bonds At this point, we are at what you would consider a bond formed by a chemical reaction. They appear when polar interactions like the ones before are so strong, that they start to rip on what composes atoms. An atom has a core, which contains neutral parts (neutrons) and positive parts (protons), as well as a shell of electrons, which are negatively charged. When a pull between two atoms is strong enough, one will start tugging on the electrons of the other. That leads to an arrangement, at which the electron basically belongs to both of them, forming a covalent bond. We call that a reaction, the result of which is a combination of two atoms that form a unison with new characteristics - a molecule (in contrast to everything up to this point, which would result in mixing but not an alteration of involved substances e.g. a reaction)
Ionic bonds As you can imagine, you can step up that scenario once more. If atom A is so strong, that it overpowers atom B in the tug of war, it can completely rip the electron from B. The result of this are two so-called ions. One, that lacks one piece of its internal negative charges, making it positively charged in total. The other one gained an internal negative charge in form of an electron and hence becomes overall negatively charged. What again happens is an attraction between negative and positive, pulling the two ions together and forming what is called an ionic bond, that in and on itself is even stronger than a covalent bond, as the difference in charges is much, much higher than in any bond I mentioned before.
TL;DR - a reaction requires an interaction force altering the structural composition of an atom or molecule. Mixing two compounds does not necessarily produce forces strong enough to do so.
Thanks for your answer. What sort of bonding would we expect in a metal alloy? I believe alloys are also classed as mixtures but we don't see intermolecular forces in metals if Im correct so does metallic bonding occur between the mixed atoms?
Metal bonds are a bit out of my area of expertise, as I am a biochemist and usually only encounter metals in my field sporadically.
Maybe someone in anorganic chemistry or material science can give us their view here? And please do correct me if I'm wrong.
As far as I understand, metal bonds are in a way similar to ionic bonds, in that bonding electrons are completely ripped from their atoms, but do not reside with the ion that took them. Rather, they are distributed and moveable throughout a crystal structure of the ions.
As to the question if metal alloys are classified mixtures - I do not know. I am inclined to believe they are not, but as I said, metal chemistry is beyond my area of expertise. Edit: u/passerculus elaborated on the nature of metal bonds. Refer to their comment for info about that, rather than my half knowledge!
Metallurgy and inorganic chemistry quickly blends into condensed matter physics.
Metals and semiconductors will arrange themselves in specific crystal structures that depend on their electron valence. In the crystal the outermost electrons tend to get shared collectively. What that looks like is the energy levels that once belonged to the discrete atoms blend together to form bands. The band that corresponds to the original valence orbitals is called the valence band, and the next higher the conduction band.
The difference between metals and semiconductors is in the latter there is an energy gap between the bands, where in a metal the band energies meet at specific points in the crystal. Think of them as two sets of roads, local streets and a highway system. In a metal there are plenty of on-ramps and the electrons can move all about the crystal.
In a semiconductor there are no on-ramps, so the local streets are totally log-jammed and electrons stay stuck in place. If you manage get rid of a valence bad electron (e.g. thermally kick it up to the conduction band) there is now a little wiggle room and electrons can hop from atom to atom in the crystal.
TL,DR its sort of like covalent bonds, but instead of molecular orbitals they are really big
I always explain the difference between bonds and intermolecular forces this way: bonds are like gluing two pieces of paper together and IMFs are like magnets. You can tear the papers apart, but they will not be the same as when they were together. Things held together by IMFs can be separated and they’ll be exactly the same as when they were together. Of course, I am explaining things to high school or gen Chem students, so the analogy is enough.
Let's take a different mixture. Sodium Chloride (NaCl) - salt - and water.
Fun thing about NaCl is that they're like a nuclear couple, but they like to flirt with other NaCl. They create intramolecular attraction, but don't really BOND to other NaCl molecules. So a salt crystal doesn't become Na2000Cl2000 - it's just NaCl - just stacked like legos, or magnets.
In swoops in water, and it readily dissolves those intramolecular bonds. But may or may not actually break down that Na Cl.
So we can boil back out the water and we just end up with crystalline bar magnets stuck together again.
Except... water CAN break down the Na and Cl with sufficient energy levels, so you end up with an ocean of H2O, Na, Cl, and NaCl, but the Na and Cl don't break down the H2O.
Boil H2O out, though, and it's reversed back to NaCl, and you wouldn't necessarily be any wiser.
Except you can send in more chemicals that will fetch either Na, or Cl, but not H2O and not NaCl. Usually when that happens, it creates a film that floats to the surface, turns to a gas, or settles to the bottom.
But there are any other number of properties you might test. E.G. how does it react to UV light? What color is it? Lasers? What's the density? Etc.
[deleted]
[removed]
Objective_Regret4763 t1_ixtewwt wrote
Bonds forming is an indication of a chemical reaction. A mixture does not create new bonds or break bonds. Sugar, for example, just mixes in with water. Each sugar molecule is surrounded by water molecules and becomes an aqueous solution. These are not “bonds” but rather intermolecular forces that hold the mixture together the way it is.