[justinh]

List the interactions that need to be broken when an ionic solid dissolves in water.



List the interactions that need to be formed when an ionic solid dissolves in water.



Can someone help me understand these questions?  In class my teacher did something like this and he wrote two columns:  bonds broken and bonds formed.  Then he just kind of new what to put down for each column. 

I don't care so much about the answer, I just really want to understand this kind of question and how to approach/do it.

[Corribus]

Think about the initial and final states. Can you characterize them in your mind - what do they look like? What interactions and bonds will they have holding them together?

I'll get you started.

Initial state: chunks of table salt in your hand and a glass of water
Final state: table salt fully dissolved in the glass of water

What does the salt look like before and after? What does the glass of water look like before and after? Zoom in and think of molecules/atoms involved and how they are held together and interact with other molecules/atoms that are around them.

[justinh]

Think about the initial and final states. Can you characterize them in your mind - what do they look like? What interactions and bonds will they have holding them together?

I'll get you started.

Initial state: chunks of table salt in your hand and a glass of water
Final state: table salt fully dissolved in the glass of water

What does the salt look like before and after? What does the glass of water look like before and after? Zoom in and think of molecules/atoms involved and how they are held together and interact with other molecules/atoms that are around them.

So before, it would be NaCl + H₂O
After would be Na⁺ + Cl^- + H₂O

So the molecules of NaCl are held toghther by hydrogen bonds and the molecules of water are held together by hydrogen bonds.  So first you would have to break those bonds, then break the bonds between the Hydrogen and Oxygen and the bonds between Na and Cl.  Am I on the right track?  This whole concept really confuses me and I havn't had chem for a while and this is suppose to be review from chem 1 (Im in chem 2) so forgive me if I'm a little confused.

[Corribus]

So the molecules of NaCl are held toghther by hydrogen bonds and the molecules of water are held together by hydrogen bonds.

Half right. Hydrogen bonds involve hydrogen atoms. Do you see any hydrogen atoms in NaCl? So, can NaCl be held together by hydrogen bonds? Here's a clue - is there such a thing as an NaCl molecule?

So first you would have to break those bonds, then break the bonds between the Hydrogen and Oxygen and the bonds between Na and Cl.  Am I on the right track?

Not really. Your first point of confusion seems to be regarding the definition of a molecule and what happens when one thing is dissolved in another.

Let's start with the first part: what is the definition of a molecule.  Water and table salt are very different substances. Water consists of discrete molecules of H₂O that are glued together by weak forces called hydrogen bonds. This is why water is a liquid - if there were not attractive forces between water molecules, they would just go everywhere. Each molecule of course consists of three atoms (two hydrogen atoms and one oxygen atom) and these atoms are held together by covalent bonds. So the atoms are glued together by covalent bonds (in which electrons are shared between atoms) to form molecules, and molecules are glued together by weaker bonds called hydrogen bonds to form a macroscopic liquid we call the substance water. Hydrogen bonds are attractions between a hydrogen on one water molecule and an oxygen on another nearby water molecule. They are intermolecular bonds (often called intermolecular interactions to avoid confusion. Contrast this to the covalent bonds between the atoms that make up each water molecule - these are intramolecular bonds.  We know the latter are stronger because heating up a glass of water causes boiling to a gas - in water vapor, the intermolecular hydrogen bond interactions are destroyed and the glue holding water molecules together disappears, but the water molecules are still water molecules. You have to use a lot more energy or expose the water to some other substances to destroy the intramolecular covalent bonds that glue together each atom of a single water molecule.

Table salt does not consist of NaCl molecules. Chlorine atoms like electrons so much more than sodium atoms do that they steal the electrons and hoard them to themselves. This is very different form the internal bonds of water molecules, where electrons are shared more evenly. Because all the chlorines end up negatively charged (extra electrons) and the sodium atoms end up positively charged (loss of electrons), they all tend to stick together in one large crystalline mass. The formula of sodium chloride is NaCl, but there's no discrete and distinguishable unit we could call a single NaCl molecule. This is what distinguishes any kind of salt. The bonds that hold all these ions together are called ionic bonds - every sodium atom is connected to nearby chloride atoms by forces of electric attraction, and vice-versa.

Having all that background, can you imagine now what happens when you dissolve salt in water? And remember, this process does not involve destroying any water molecules (to a good approximation), but it does result in disruption interactions between water molecules.

[justinh]

Wow!  That was super helpful!  Allow me to give this another shot.

So you have NaCl that you want to dissolve in H₂O.  Initially you would have to break the Hydrogen bonds between the water molecules so that NaCl can be dissolved.  Then you must break the ionic bonds that bond together the NaCl molecules which would separate it into Na⁺ and Cl^- ions.

So then you would have the ions of table salt dissolved within the water molecules.  Am I getting closer?

[Corribus]

Yes this is a fine answer for the first half of the question (what interactions need to be broken): ionic bonds in NaCl are broken and hydrogen bonds in water are broken (or, changed at least) to accommodate the influx of ions. Now what about the second half - are any new bonds/interactions resulting from the dissolution?

[justinh]

Yes this is a fine answer for the first half of the question (what interactions need to be broken): ionic bonds in NaCl are broken and hydrogen bonds in water are broken (or, changed at least) to accommodate the influx of ions. Now what about the second half - are any new bonds/interactions resulting from the dissolution?

Honestly, I have no idea.  I feel like it has something to do with polar stuff but I can hardly remember far enough back to Chem 1 to put together an answer.  How do I know what the answer is to the second part if I was given a question like this?

And just a clarification.  In H₂O, The hydrogen is bonded to the oxygen using covalent bonds, right?  Then all of the H₂O molecules are attracted to each other by ionic bonds, that way the water doesn't go everywhere.  Is this way of thinking correct?

Now the Na⁺ and Cl^- are bound together by ionic bonds right?  Well, what holds all the NaCl molecules together? 

By the way, thanks for all the help so far, it's been amazing!

[Burner]

Honestly, I have no idea.  I feel like it has something to do with polar stuff but I can hardly remember far enough back to Chem 1 to put together an answer.  How do I know what the answer is to the second part if I was given a question like this?

The first attached diagram may give you some clue. And yes, it has something to do with 'polar stuff'.

And just a clarification.  In H₂O, The hydrogen is bonded to the oxygen using covalent bonds, right? 

Yes.

Then all of the H₂O molecules are attracted to each other by ionic bonds, that way the water doesn't go everywhere.  Is this way of thinking correct?

No. As Corribus has told you, H₂O molecules are attracted to each other by hydrogen bonds. Hydrogen bonds are not ionic bonds - they are intermolecular forces.

Now the Na⁺ and Cl^- are bound together by ionic bonds right?  Well, what holds all the NaCl molecules together? 

By the way, thanks for all the help so far, it's been amazing!

In solid state, yes, Na⁺ and Cl^- are bound together by ionic bonds. NaCl does not exist as molecules - look at the second picture. All the Na⁺ and Cl^- are held together by ionic bonds as shown and no 'molecules' are involved.

[justinh]

Honestly, I have no idea.  I feel like it has something to do with polar stuff but I can hardly remember far enough back to Chem 1 to put together an answer.  How do I know what the answer is to the second part if I was given a question like this?

The first attached diagram may give you some clue. And yes, it has something to do with 'polar stuff'.

And just a clarification.  In H₂O, The hydrogen is bonded to the oxygen using covalent bonds, right? 

Yes.

Then all of the H₂O molecules are attracted to each other by ionic bonds, that way the water doesn't go everywhere.  Is this way of thinking correct?

No. As Corribus has told you, H₂O molecules are attracted to each other by hydrogen bonds. Hydrogen bonds are not ionic bonds - they are intermolecular forces.

Now the Na⁺ and Cl^- are bound together by ionic bonds right?  Well, what holds all the NaCl molecules together? 

By the way, thanks for all the help so far, it's been amazing!

In solid state, yes, Na⁺ and Cl^- are bound together by ionic bonds. NaCl does not exist as molecules - look at the second picture. All the Na⁺ and Cl^- are held together by ionic bonds as shown and no 'molecules' are involved.

It seems like I made a typo, when I said water was bonded by ionic bonds, I did mean hydrogen bonds.  So my thinking was correct, I just typed it wrong.  So I guess that means I am good with the bonds broken so far.

Now for bonds formed.  You said it does have something to do with polarity.  Honestly, I remember hearing about dipole moments and polarity stuff before, but I don't really member anything about it.  I'm looking at some videos now but I can't seem to boil it down to something I can understand.  Care to help?  What do I need to learn to answer part 2 of this question?

[Burner]

Now for bonds formed.  You said it does have something to do with polarity.  Honestly, I remember hearing about dipole moments and polarity stuff before, but I don't really member anything about it.  I'm looking at some videos now but I can't seem to boil it down to something I can understand.  Care to help?  What do I need to learn to answer part 2 of this question?

Polarity of molecules refers to the uneven distribution of bonding electrons in a molecule due to the different electronegativity of atoms in the molecules. Dipole moment is a measurement of polarity.

The uneven distribution of bonding electrons, and hence negative charges, lead to the formation of partial charges in a molecules, which are what δ+ and δ- show in the above diagrams.

Look at the first diagram(especially the 'dissolved ions' part) above again. What can you observe about the interactions between the polar water molecules and the ions? Can you name this interaction?

[justinh]

Now for bonds formed.  You said it does have something to do with polarity.  Honestly, I remember hearing about dipole moments and polarity stuff before, but I don't really member anything about it.  I'm looking at some videos now but I can't seem to boil it down to something I can understand.  Care to help?  What do I need to learn to answer part 2 of this question?

Polarity of molecules refers to the uneven distribution of bonding electrons in a molecule due to the different electronegativity of atoms in the molecules. Dipole moment is a measurement of polarity.

The uneven distribution of bonding electrons, and hence negative charges, lead to the formation of partial charges in a molecules, which are what δ+ and δ- show in the above diagrams.

Look at the first diagram(especially the 'dissolved ions' part) above again. What can you observe about the interactions between the polar water molecules and the ions? Can you name this interaction?

So due to the charge of the elements in the molecule, the positive H (in water) is attracted to negative Cl.  As well as the positive Na being attracted to the O (in water).  That is what holds everything together in the solution?  So is this the new interactions that are formed?

[Burner]

So due to the charge of the elements in the molecule, the positive H (in water) is attracted to negative Cl.  As well as the positive Na being attracted to the O (in water).  That is what holds everything together in the solution?  So is this the new interactions that are formed?

Yes. Can you name this type of interaction?

[justinh]

So due to the charge of the elements in the molecule, the positive H (in water) is attracted to negative Cl.  As well as the positive Na being attracted to the O (in water).  That is what holds everything together in the solution?  So is this the new interactions that are formed?

Yes. Can you name this type of interaction?

After looking through some stuff online would I read about ion-dipole interactions and dipole-dipole interactions.  Wouldn't there be both?

Also, I'm a little confused about how Ionic Bonds work.  After watching some videos online, it says that ONCE Na gives it's extra electron to Cl, that is when Na becomes negatively charged, and Cl becomes positively charged.  That charge is what holds them together in an ionic bond.  This condratadicts what I previously believed.  I thought that Cl and Na were already charged and they want to become neautral, that's why they give away/take electrons.  The way the video said it, the ions were neatural charge until they gave away/took electrons which gave them a +/- charge which brought them together with ionic bonds. 

[Burner]

After looking through some stuff online would I read about ion-dipole interactions and dipole-dipole interactions.  Wouldn't there be both?

They are ion-dipole interactions(attractions). The attraction happens between the partial charges of water molecules and the Na⁺ or Cl^- ions.

Also, I'm a little confused about how Ionic Bonds work.  After watching some videos online, it says that ONCE Na gives it's extra electron to Cl, that is when Na becomes negatively charged, and Cl becomes positively charged.  That charge is what holds them together in an ionic bond.  This condratadicts what I previously believed.  I thought that Cl and Na were already charged and they want to become neautral, that's why they give away/take electrons.  The way the video said it, the ions were neatural charge until they gave away/took electrons which gave them a +/- charge which brought them together with ionic bonds.

Na and Cl atoms are initially neutral. However, they want to become stable by having 8 electrons in their outermost electron shell(octet rule), thus they lose/gain electrons and become ions with positive/negative charges. These charged atoms, called ions, attracts each other with opposite poles.

Since Na has an electron configuration of 2,8,1 , it wants to lose 1 electron to become octet, while Cl has an electron configuration of 2,8,7 and thus want to gain 1 electron to become octet. Hence, when Na and Cl form a stable compound NaCl, we say that the electrons that Cl gained comes from the one that Na loses, so apparently Na gives it's extra electron to Cl.

[justinh]

After looking through some stuff online would I read about ion-dipole interactions and dipole-dipole interactions.  Wouldn't there be both?

They are ion-dipole interactions(attractions). The attraction happens between the partial charges of water molecules and the Na⁺ or Cl^- ions.

Also, I'm a little confused about how Ionic Bonds work.  After watching some videos online, it says that ONCE Na gives it's extra electron to Cl, that is when Na becomes negatively charged, and Cl becomes positively charged.  That charge is what holds them together in an ionic bond.  This condratadicts what I previously believed.  I thought that Cl and Na were already charged and they want to become neautral, that's why they give away/take electrons.  The way the video said it, the ions were neatural charge until they gave away/took electrons which gave them a +/- charge which brought them together with ionic bonds.

Na and Cl atoms are initially neutral. However, they want to become stable by having 8 electrons in their outermost electron shell(octet rule), thus they lose/gain electrons and become ions with positive/negative charges. These charged atoms, called ions, attracts each other with opposite poles.

Since Na has an electron configuration of 2,8,1 , it wants to lose 1 electron to become octet, while Cl has an electron configuration of 2,8,7 and thus want to gain 1 electron to become octet. Hence, when Na and Cl form a stable compound NaCl, we say that the electrons that Cl gained comes from the one that Na loses, so apparently Na gives it's extra electron to Cl.

Ok, the ionic bonding makes sense to me now, thanks!  So really all I have left to understand is the difference between ion-dipole and dipole-dipole.  After reading on the internet.  It would appear that this "bonds formed" would be ion-dipole interaction between the polar water molecules and the ions.  When would I need dipole-dipole interactions?