Thread: Formation of Ions in Solution.

We are told that when chemical solids like NaCl dissolve in water and go into solution they separate into ions:

NaCl(aq) → Na+(aq) + Cl−(aq).

But if a compound like NaCl, for example, dissociated into ions in a solution of water then why would the chlorine gas not simply bubble out of solution or the sodium not react with the water, which it ordinarily would do, producing hydrogen gas?

2 Na + 2 H2O - > 2 NaOH + H2

As far as I am concerned I have a difficulty in seeing how a liberated sodium ion WOULD NOT react with the water solution it was contained in.

Can anyone help?

The ions and the atoms have different properties. When natrium and chlorine dissociates, those ions are surrounded with molecules of water, but because of electrical repulsion have no contact with those same molecules, which inhibits the reaction.

Originally Posted by Sindrato

The ions and the atoms have different properties. When natrium and chlorine dissociates, those ions are surrounded with molecules of water, but because of electrical repulsion have no contact with those same molecules, which inhibits the reaction.

Okay but wouldn't a sodium ion be even more reactive than in its ordinary state?

Further as water is dipolar it contains both a positive and a negative charge. How would this interact with the sodium ion?

Why would a sodium ion be reactive, or any ion for that matter? Since it's Na⁺ it has a stable octet of electrons in its outer shell, and will not under normal circumstances react with anything.

The negative charge on the oxygen in water will attract the positive sodium ion, but it will not actually react with it in any way.

To add, water contains both positive and negative charges, but the negative charge "turns to face" the ion.

Originally Posted by AlexP

Why would a sodium ion be reactive, or any ion for that matter? Since it's Na⁺ it has a stable octet of electrons in its outer shell, and will not under normal circumstances react with anything.

The negative charge on the oxygen in water will attract the positive sodium ion, but it will not actually react with it in any way.

Fine AlexP, but the sodium ion now has an imbalance of protons to electrons and further has an affinity to electrons which is equal to its first ionisation energy 495.8 kJ mol-1.

Quoting Wiki on Bond Dissociation Energy:

For example, an O-H bond of a water molecule (H-O-H) has 493.4 kJ/mol of bond dissociation energy, and 424.4 kJ/mol is needed to cleave the remaining O-H bond. The bond energy of the covalent O-H bonds in water is 458.9 kJ/mol, which is the average of the values.

And then we can't ignore the fact that Sodium would have reacted with water anyway

Let's point it this way. When water and sodium react, in the form of oxygen receives one electron from natrium, which substitutes for the electron it previously received from hydrogen. So they both have stable electronic configurations. However, when you have a sodium ion , it has no free electrons to give to the oxygen, as if one more electron was taken from the sodium, it's electronic configuration would be which is obviously unstable.

Originally Posted by Sindrato

Let's point it this way. When water and sodium react, in the form of oxygen receives one electron from natrium, which substitutes for the electron it previously received from hydrogen. So they both have stable electronic configurations. However, when you have a sodium ion , it has no free electrons to give to the oxygen, as if one more electron was taken from the sodium, it's electronic configuration would be which is obviously unstable.

I think you are looking at things a little too simplistically Sindrato. I mean how does Ozone form, O3? Its formed from ions which in turn were created by the Sun's ultraviolet light.

What Sindrato just said is perfectly true. The sodium atom has already been ionized at it is in the form of a salt (Na⁺Cl^-), and will not react with the water for as long as it remains an ion (which it will given anything short of electrolysis). You simply need to accept that ions are stable and not reactive. And what you mentioned about the proton-electron imbalance doesn't matter; the imbalance simply causes the charge of the ion.

Originally Posted by AlexP

What Sindrato just said is perfectly true. The sodium atom has already been ionized at it is in the form of a salt (Na⁺Cl^-), and will not react with the water for as long as it remains an ion (which it will given anything short of electrolysis). You simply need to accept that ions are stable and not reactive. And what you mentioned about the proton-electron imbalance doesn't matter; the imbalance simply causes the charge of the ion.

Putting it very simplistically now, to a physicist Na as a neutral atom is not necessarily that reactive because it is neutral, it has a perfect balanced of protons to electrons. An Na+ ion however has a high affinity for electrons and is not neutral and should therefore be MORE reactive that the neutral Na atom.

Make any sense?

Nobody said that it isn't reactive, just that it won't react with water. It has an affinity for electrons, but water has no free electrons to give to natrium.

Originally Posted by Sindrato

Nobody said that it isn't reactive, just that it won't react with water. It has an affinity for electrons, but water has no free electrons to give to natrium.

Neither has O2 to O+ but you still get O3 in the Earth's upper atmosphere.

There's also the observation that when you dissolve table salt, NaCl, in water the solution still tastes salty.

But if the NaCl has broken up into separate ions in solution how can the solution still taste of salt?

The basic philosophy of the situation surely doesn't add up? I mean what do Na+ ions taste like in isolation and what do Cl- ions taste like in isolation? But apparently if you put these two flavours together you get the taste of salt!

Does this make any sense?

Originally Posted by galexander

There's also the observation that when you dissolve table salt, NaCl, in water the solution still tastes salty.

But if the NaCl has broken up into separate ions in solution how can the solution still taste of salt?

I'm not sure of the how but perhaps your implied assumption that we can actually taste NaCl when it is a solid might be wrong.

Try this experiment:

What happens when you put solid NaCl into your mouth? It dissolves, yet the salty taste doesn't go away when it does. So, you definitely taste dissolved Na+ and Cl- ions (or at least one of them, anyway).

What happens when you lick a block of solid salt? It tastes salty. Why? I think it is because your saliva dissolves the salt and, again, you are tasting the dissolved ions not the solid NaCl.

In that context, the salty taste of water containing dissolved NaCl makes sense.

But, are you tasting both the Na+ and Cl- ions, or only one of them? I'm not sure, but If you've ever tasted potassium chloride, KCl (which dissolves into separate ions in an aqueous solution, just like NaCl does), it has a different taste so perhaps it is the sodium ion that has produces the familiar "salty" taste of table salt, more than the chloride ion.

Originally Posted by galexander

Neither has O2 to O+ but you still get O3 in the Earth's upper atmosphere.

Why do you keep implying that ozone has ionic link in itself. As far as I am informed, and excuse me if I am wrong, the formation of ozone is purely covalent.

Originally Posted by jsloan

Originally Posted by galexander

There's also the observation that when you dissolve table salt, NaCl, in water the solution still tastes salty.

But if the NaCl has broken up into separate ions in solution how can the solution still taste of salt?

I'm not sure of the how but perhaps your implied assumption that we can actually taste NaCl when it is a solid might be wrong.

Try this experiment:

What happens when you put solid NaCl into your mouth? It dissolves, yet the salty taste doesn't go away when it does. So, you definitely taste dissolved Na+ and Cl- ions (or at least one of them, anyway).

What happens when you lick a block of solid salt? It tastes salty. Why? I think it is because your saliva dissolves the salt and, again, you are tasting the dissolved ions not the solid NaCl.

In that context, the salty taste of water containing dissolved NaCl makes sense.

But, are you tasting both the Na+ and Cl- ions, or only one of them? I'm not sure, but If you've ever tasted potassium chloride, KCl (which dissolves into separate ions in an aqueous solution, just like NaCl does), it has a different taste so perhaps it is the sodium ion that has produces the familiar "salty" taste of table salt, more than the chloride ion.

But it still isn't a pleasant thought that when you add salt to your cooking you are actually adding Na+ and Cl- ions.

Doesn't really sound too appetising does it?

Originally Posted by Sindrato

Originally Posted by galexander

Neither has O2 to O+ but you still get O3 in the Earth's upper atmosphere.

Why do you keep implying that ozone has ionic link in itself. As far as I am informed, and excuse me if I am wrong, the formation of ozone is purely covalent.

Ozone forms from oxygen molecules and oxygen radicals.



There are no ions involved.

Originally Posted by drowsy turtle

Originally Posted by Sindrato

Originally Posted by galexander

Neither has O2 to O+ but you still get O3 in the Earth's upper atmosphere.

Why do you keep implying that ozone has ionic link in itself. As far as I am informed, and excuse me if I am wrong, the formation of ozone is purely covalent.

Ozone forms from oxygen molecules and oxygen radicals.



There are no ions involved.

Am I right in observing drowsy turtle that you are a geologist and not a chemist?

And secondly where in your diagram is it explained that no ions are involved? Please indicate.

I am a geology student, yes.

Oxygen undergoes homolytic fission in the Stratosphere to produce two free radicals - essentially, oxygen as atoms rather than as diatomic molecules. These radicals attack the covalent bond between the atoms in molecules of diatomic oxygen, resulting in triatomic ozone.

In the diagram, it is specifically stated that oxygen atoms are produced when the oxygen molecule splits, rather than anions and cations. Oxygen molecules will always tend to split into radicals rather than ions, because the bond has no polarity (the electrons in the bond are equally spaced between the two nuclei).

Electrophilic or nucleophilic interactions would not result in the formation of ozone, as the molecule would retain an overall charge.

Further reading

Originally Posted by galexander

Am I right in observing drowsy turtle that you are a geologist and not a chemist?

And secondly where in your diagram is it explained that no ions are involved? Please indicate.

It does not take a chemist to understand a simple chemistry diagram.

And it is clear from the diagram that there are no ions involved. The words 'atom' and 'molecule' are used, but not 'ion.'

Originally Posted by drowsy turtle

I am a geology student, yes.

Oxygen undergoes homolytic fission in the Stratosphere to produce two free radicals - essentially, oxygen as atoms rather than as diatomic molecules. These radicals attack the covalent bond between the atoms in molecules of diatomic oxygen, resulting in triatomic ozone.

In the diagram, it is specifically stated that oxygen atoms are produced when the oxygen molecule splits, rather than anions and cations. Oxygen molecules will always tend to split into radicals rather than ions, because the bond has no polarity (the electrons in the bond are equally spaced between the two nuclei).

Electrophilic or nucleophilic interactions would not result in the formation of ozone, as the molecule would retain an overall charge.

Further reading

As a matter of fact the liberated oxygen atoms which react with molecular O2 are in the D state making them highly reactive in breaking down atmospheric gases.

So my point still stands. Ions ARE highly reactive since they contain more latent potential energy than atoms in the D state.

Originally Posted by galexander

So my point still stands. Ions ARE highly reactive since they contain more latent potential energy than atoms in the D state.

Was that your point? I wasn't adressing it, I was just briefly explaining to sindrato and yourself how ozone forms in the stratosphere through homolytic fission and radical reactions.

Most ions are fairly unreactive, since they form by gaining or losing electrons to create a more stable electron configuration.

Originally Posted by drowsy turtle

Originally Posted by galexander

So my point still stands. Ions ARE highly reactive since they contain more latent potential energy than atoms in the D state.

Was that your point? I wasn't adressing it, I was just briefly explaining to sindrato and yourself how ozone forms in the stratosphere through homolytic fission and radical reactions.

Most ions are fairly unreactive, since they form by gaining or losing electrons to create a more stable electron configuration.

To a chemist, maybe, but to physicist ions have a high affinity for electrons because the number of protons to electrons is now out of balance. This is why they have charge.

Chemists would have it of course that neutral atoms are able to chemically react but charged atoms would be even more reactive.

On the subject of the reactivity of ions again consider the difference between a covalent and an ionic bond.

In a covalent bond electrons are shared but in an ionic bond it is the electronic attraction between two ions of opposite charge which hold the molecule together. In other words a positive ion will stick to a negative ion and become bonded.

By definition therefore ions must be chemically reactive, there is simply no way of avoiding it.

Wait, what are you trying to prove? Nobody ever said that ions are not reactive. We just said that Na and Cl ions have nothing to react with in water. Period.

Originally Posted by galexander

To a chemist, maybe, but to physicist ions have a high affinity for electrons because the number of protons to electrons is now out of balance. This is why they have charge.

Reactivity is, essentially, how readily something changes oxidation state. Ions such as sodium ions tend to remain in the +1 oxidation state almost indeffinitely, as the 3s electron has a very low ionisation energy.

Originally Posted by galexander

Chemists would have it of course that neutral atoms are able to chemically react but charged atoms would be even more reactive.

You make it sound like anions and cations should readily react to produce atoms; this is not the case. For instance, taking the example of sodium chloride, even when the ions do interact and form a solid, they remain as ions in the solid as an ionic lattice.

Originally Posted by Sindrato

Wait, what are you trying to prove? Nobody ever said that ions are not reactive. We just said that Na and Cl ions have nothing to react with in water. Period.

So you are suggesting that a cloud of positive charge from H2O surrounds the Na+ ions and that a cloud of negativity from the O- pole of H2O surrounds the Cl- ions?

Pretty clever these dipolar water molecules aren't they?

And these same water molecules are mightily strong also, they can cleave an Na-Cl bond no problem.

What else can this supposedly chemically inert liquid H2O also do?

Originally Posted by galexander

What else can this supposedly chemically inert liquid H2O also do?

Who has claimed water is inert? You are raising a monumental strawman. If you are going to attack stupid ideas why not make them someone elses rather than your own?

Originally Posted by galexander

So you are suggesting that a cloud of positive charge from H2O surrounds the Na+ ions and that a cloud of negativity from the O- pole of H2O surrounds the Cl- ions?

No. Water molecules have a dipole, with partially positive hydrogen atoms and partially positive oxygen atoms, due to the polarity of the H-O bond.

Originally Posted by galexander

Pretty clever these dipolar water molecules aren't they?

No. They simply follow physical laws.

Originally Posted by galexander

And these same water molecules are mightily strong also, they can cleave an Na-Cl bond no problem.

The strenght of the Na-Cl "bond", or the lattice enthalpy, can be determined by comparing the vaporisation enthalpy for NaCl with the enthalpy of solution. It is actually pretty weak as lattice enthalpies go.

If you then look at the Maxwell-Boltzmann curve for water molecules at room temperature, you will find that a fairly high proportion of the molecules have kinetic energy equal to or greater than the strength of the Na-Cl "bond"; and hence, salt dissolves in water at room temperature.

Originally Posted by galexander

What else can this supposedly chemically inert liquid H2O also do?

Water is not inert. It can easily be made to react, for instance with group 1 and 2 metals.

Originally Posted by galexander

So you are suggesting that a cloud of positive charge from H2O surrounds the Na+ ions and that a cloud of negativity from the O- pole of H2O surrounds the Cl- ions?

Except that you got positive and negative mixed up, yes.

Pretty clever these dipolar water molecules aren't they?

Things aren't "clever" just because they obey the laws of physics.

And these same water molecules are mightily strong also, they can cleave an Na-Cl bond no problem.

Salt dissolving in water has more to do with the associated entropy increase than it does with charge stabilization between the water and the ions. To put it another way, the dissolving process is mostly entropically driven rather than enthalpically driven. There are plenty of salts that will spontaneously dissolve even though it actually increases the enthaply of the salt+water system.

What else can this supposedly chemically inert liquid H2O also do?

Given how much of you body chemistry involves water reacting with things, you wouldn't be able to exist if water was inert.

Originally Posted by drowsy turtle

Originally Posted by galexander

So you are suggesting that a cloud of positive charge from H2O surrounds the Na+ ions and that a cloud of negativity from the O- pole of H2O surrounds the Cl- ions?

No. Water molecules have a dipole, with partially positive hydrogen atoms and partially positive oxygen atoms, due to the polarity of the H-O bond.

Originally Posted by galexander

Pretty clever these dipolar water molecules aren't they?

No. They simply follow physical laws.

Originally Posted by galexander

And these same water molecules are mightily strong also, they can cleave an Na-Cl bond no problem.

The strenght of the Na-Cl "bond", or the lattice enthalpy, can be determined by comparing the vaporisation enthalpy for NaCl with the enthalpy of solution. It is actually pretty weak as lattice enthalpies go.

If you then look at the Maxwell-Boltzmann curve for water molecules at room temperature, you will find that a fairly high proportion of the molecules have kinetic energy equal to or greater than the strength of the Na-Cl "bond"; and hence, salt dissolves in water at room temperature.

Originally Posted by galexander

What else can this supposedly chemically inert liquid H2O also do?

Water is not inert. It can easily be made to react, for instance with group 1 and 2 metals.

drowsy turtle said the following:

If you then look at the Maxwell-Boltzmann curve for water molecules at room temperature, you will find that a fairly high proportion of the molecules have kinetic energy equal to or greater than the strength of the Na-Cl "bond"; and hence, salt dissolves in water at room temperature.

Okay, fine. But that does not therefore mean that NaCl will split up into ions in aqueous solution.

Originally Posted by galexander

drowsy turtle said the following:

If you then look at the Maxwell-Boltzmann curve for water molecules at room temperature, you will find that a fairly high proportion of the molecules have kinetic energy equal to or greater than the strength of the Na-Cl "bond"; and hence, salt dissolves in water at room temperature.

Okay, fine. But that does not therefore mean that NaCl will split up into ions in aqueous solution.

Actually, that's exactly what it means. The kinetic energy of the water molecules is easily sufficient to seperate the ionic lattice into seperate ions.

I don't know why we're trying to explain basic concepts of chemistry to you when resources such as wikipedia are available:

Ionic bonds

Lattice energy

Electrolytes

Originally Posted by drowsy turtle

Originally Posted by galexander

drowsy turtle said the following:

If you then look at the Maxwell-Boltzmann curve for water molecules at room temperature, you will find that a fairly high proportion of the molecules have kinetic energy equal to or greater than the strength of the Na-Cl "bond"; and hence, salt dissolves in water at room temperature.

Okay, fine. But that does not therefore mean that NaCl will split up into ions in aqueous solution.

Actually, that's exactly what it means. The kinetic energy of the water molecules is easily sufficient to seperate the ionic lattice into seperate ions.

I don't know why we're trying to explain basic concepts of chemistry to you when resources such as wikipedia are available:

Ionic bonds

Lattice energy

Electrolytes

Okay then drowsy turtle, are you sure air molecules don't also have enough kinetic energy to break open the Na-Cl bond?

Originally Posted by galexander

Okay then drowsy turtle, are you sure air molecules don't also have enough kinetic energy to break open the Na-Cl bond?

They certainly do. I would speculate that this is why you can smell salt; chloride ions in the air. However, there aare two reasons why a pile of salt doesn't "dissolve" into the air; firstly, air is not very dense. There are far fewer interactions between air and the ions per second than there are between salt and water (something in the order of millions less, at a guess). And secondly, but related to the first point, the molecules in air are not packed closely together, and are largely non-polar (nitrogen, oxygen, argon are all non-polar molecules), so they will not allign around and surround the sodium or chloride ions. So whenever the Na-Cl bond is broken, it will tend to instantaneously reform because of the electrostatic attraction between the freed ion and the oppositely charged region it has just left, and no significant attractive forces between it and the molecules in the air.

By the way, Galexander, are you actually reading the content of my posts and following up the links, or are you simply picking out individual sentances that you can try to attack to "disprove" me?

If your intention is to attack my knowledge of chemistry (or in the thread in Earth Sciences, geology), rather than to actually try and learn from the discussion, then I shall save my time by not replying to you.

Originally Posted by drowsy turtle

Originally Posted by galexander

Okay then drowsy turtle, are you sure air molecules don't also have enough kinetic energy to break open the Na-Cl bond?

They certainly do. I would speculate that this is why you can smell salt; chloride ions in the air. However, there aare two reasons why a pile of salt doesn't "dissolve" into the air; firstly, air is not very dense. There are far fewer interactions between air and the ions per second than there are between salt and water (something in the order of millions less, at a guess). And secondly, but related to the first point, the molecules in air are not packed closely together, and are largely non-polar (nitrogen, oxygen, argon are all non-polar molecules), so they will not allign around and surround the sodium or chloride ions. So whenever the Na-Cl bond is broken, it will tend to instantaneously reform because of the electrostatic attraction between the freed ion and the oppositely charged region it has just left, and no significant attractive forces between it and the molecules in the air.

So you are able to smell salt are you?

And it smells of chlorine?

Hmm.........yum, yum!

Chloride actually. Chlorine is a diatomic molecules, whereas chloride is a simple anion.

Originally Posted by galexander

Okay, fine. But that does not therefore mean that NaCl will split up into ions in aqueous solution.

If anything you've said in this thread was remotely true, you'd be in a state of flaccid paralysis right now and incapable of living or even breathing.

Maybe this will help you to understand, Mr. Physicist.

Since you are a physicist, you must know that Newton's Second Law of Thermodynamics states that ∆Suniverse must always increase. Thus, whatshisface's explanation that the dissociation of sodium and chloride ions is an entropy-driven reaction should make sense to you. You seem to think that ions should react to form neutral atoms. In this case, that would mean Na+ would have to gain an electron and Cl- would have to lose one. Unfortunately, Na+ cannot steal an electron because electron repulsion from the 2p subshell would prevent other electrons from entering the 3s subshell (that counteracts the sodium nucleus' relatively weak affinity for electrons). Secondly, the oxygen of the water molecule and the chloride ion are also far too electronegative to allow sodium to "steal" one of their electrons. Thirdly, the receptors on the cells of your tongue bind to ions. That's why you can taste salt in solution. When you lick solid table salt, the saliva on your tongue causes the solid salt to dissociate and you are tasting the ions. Finally, it's already been made clear that ozone formation does not involve oxygen ions.

I'd also like to point out the interesting fact that no compound is 100% ionic in character.