chemical bonding help (1 Viewer)

[abdooooo!!!]

ok when you dissolve NaCl in water, does it actually become Na+ ion and Cl- ion detached from each other?

why does this happen though? shouldn't it just be single NaCl molecules that are not attached to other NaCl in a ionic lattice. ie NaCl(aq). instead of Na+ and Cl-.

so assuming if it does split up into ions in water then what causes this effect? i can't see water being able to break up the intramolecular forces that is holding NaCl together as a single molecule. so somehow the water molecule is able to hold together, but NaCl break up. what causes the so called ionic bonded molecules (since ionic and covalent bond is classified by an arbitary number chosen for the electronegativity cutoff at 2.1) weak intramolecular forces???

thanks.

 

[CM_Tutor]

NaCl solid consists of an infinite array of alternating Na⁺ and Cl^- ions. It has no molecules, so phrases like "NaCl molecules" should be avoided in assessments - they suggest a weak understanding of bonding.

To answer your question, when NaCl_(s) dissolves in water, the lattice is disrupted (broken up), and the individual Na⁺ and Cl^- ions separate (or dissociate). This process would be written in an equation as:

NaCl_(s) ---H₂O---> Na⁺_(aq) + Cl^-_(aq)

As you suggest, the breaking of the ionic bonds requires an amount of energy (called the lattice enthalpy) to be supplied, but this is offset by the formation of new ion-dipole interactions between the ions and the polar water molecules - a process that releases energy (called the hydration energy). Each ion will be surrounded by a hydration shell consisting of several water molecules, so many more ion-dipole interactions are formed than are ionic bonds broken. Overall, depending on the relative size of the lattice enthalpy and the hydration energy, this process may be exothermic, as it is for sodium hydroxide:

NaOH_(s) ---H₂O---> Na⁺_(aq) + OH^-_(aq) deltaH = -45 kJmol^-1

or endothermic, as it is for potassium nitrate:

KNO_{3 (s)} ---H₂O---> K⁺_(aq) + NO₃^-_(aq) deltaH = +36 kJmol^-1

In either case, the reaction will still be spontaneous, as it is the increase in disorder that occurs upon dissolution, rather than enthalpy change, that drives the process. Note that this explanation is going beyond HSC chemistry, but you did ask.

 

[abdooooo!!!]

yea i know that... the second law determines what is possible and what is not. so if that does happen it must be entropy favoured.

i think you misunderstood my question...

to illustrate my point:

does HF (a compound which has a electronegativity difference on the border of 2.1) exists as the so called ionic lattice where by definition no molecules are existent as they are all ions, or a covalent molecule like water?

so some how at this magical number of 2.1 the chemicals turn from molecules to ions and vice versa. this must be wrong. the number 2.1 is just set based on 50-50 property, where compounds contain equally both ionic and covalent properties.

Originally posted by CM_Tutor
NaCl solid consists of an infinite array of alternating Na⁺ and Cl^- ions. It has no molecules, so phrases like "NaCl molecules" should be avoided in assessments - they suggest a weak understanding of bonding.

i would think they suggest a strong understanding depending on how you say it don't you think.

i mean NaCl molecule must exist, at least some or even one in a million LOL. my understanding is that it contains more ions than molecules. hence the name ionic compound. same with water, as it contains more molecules than ions so its called a covalent molecule, not because they are 100% molecular or ionic. as a true 100% of anything does not exist in the physical world... it is only a metaphor we use in math. but my question is why ions form? why the ionization process?

edit: do you get my question? to make it even clearer i'll just say what is the cause of the so called ionic bonding (the molecules to split into ions) compared to covalent bonding in terms of the bonding theory at atomic level.

oh and another thing, textbooks would lead me to believe that when you heat a ionic compound such as NaCl to boiling point molecules of NaCl is formed, and not ions like when it is in water. thats why my original question was about the water situation.

 

[Xayma]

Originally posted by abdooooo!!!
does HF (a compound which has a electronegativity difference on the border of 2.1) exists as the so called ionic lattice where by definition no molecules are existent as they are all ions, or a covalent molecule like water?

No Hydrogen Fluoride exists as diatomic molecules. However they are polar and will arrange themselves as determined by dipole dipole attraction.

Ionic substances have one atom stripped of its outer electrons and another one gaining it. This creates relatively strong negative and positive charge. So multiple ions (eg Cl-) arrange around another ion (ie Na+).

I don't know the specifics of how water breaks the bonds (because they are pretty strong considering the melting point), but the polar molecules seperate them.

The ionisation process I think has something to do with the distance of an electron from the nucleus (that is why the lower left metals are more reactive and top-right non-metals).

The outer electron on sodium is a relatively far distance away from the nucleus, however the outer electrons on chlorine are reatively close, hence to the outer electron chlorine is more positive and it jumps over there (due to the unlike charges attracting and the inverse square law).

Non-metal's in that situation I think still have some degree of control over the electron and still partially share it (since it wasn't that far from the nucleus) and metalloids are just stupid and shouldn't be considered. *Waits for CM_Tutor to correct the multitude of mistakes

 

[CM_Tutor]

Originally posted by abdooooo!!!
i think you misunderstood my question...

to illustrate my point:

does HF (a compound which has a electronegativity difference on the border of 2.1) exists as the so called ionic lattice where by definition no molecules are existent as they are all ions, or a covalent molecule like water?

Abdooooo!!!, you are asking some interesting questions, and I'm happy to discuss them, but you should be careful trying these sorts of arguments in an assessment. Remember that many teachers do not have a strong chem background - 1st year Uni only, in some cases - so they may not be as familiar with the theory as I am. Furthermore, even if they are, there is the open question as to whether they'll credit you as showing additional insight, or think that you don't understand what you're talking about.

To answer the direct question you asked, I will quote Xayma

Originally posted by Xayma
No Hydrogen Fluoride exists as diatomic molecules. However they are polar and will arrange themselves as determined by dipole dipole attraction.

This is absolutely true, although hydrogen bonding will also be a factor in determining arrangement. You will no doubt wonder, given the electronegativity difference, why this is a clear cut issue. The answer is that Chemistry is an experimental science, and thus experiments always rule over theory. The theories of bonding have only been developed as models to help us understand reality. So, let's look at the experimental data.

HF. MP = -83 C, BP = 19 C, enthalpy of fusion 4 kJmol^-1 - absolutely consistent with a substance being held together by weak intermolecular interactions - solubility of 1950 g/100 g H₂O, dipole moment 6.1 * 10^-30 Cm - absolutely consistent with a substantial molecular dipole - and slight electrical conductivity in aqueous solution, suggesting partial ionisation (in keeping with HF being a weak acid).

By conrtast:

NaCl. MP = 801 C, BP = 1465 C, enthalpy of fusion 28 kJmol^-1 - absolutely consistent with a substance being held together by strong interaction interactions, but the enthalpy of sublimation (235 kJmol^-1) is too low for a netwrok covalent solid, thus it has an ionic structure (confirmed by x-ray crystallography) - solubility of 36 g/100 g H₂O - surprisingly low for a soluble ionic solid - dipole moment 30.0 * 10^-30 Cm in gas phase - suggesting ion pairing, rather than covalent bonding, in gas phase - and high electrical conductivity in aqueous solution, consistent with the dissociation of an ionic solid.

so some how at this magical number of 2.1 the chemicals turn from molecules to ions and vice versa. this must be wrong. the number 2.1 is just set based on 50-50 property, where compounds contain equally both ionic and covalent properties.

edit: do you get my question? to make it even clearer i'll just say what is the cause of the so called ionic bonding (the molecules to split into ions) compared to covalent bonding in terms of the bonding theory at atomic level.

Good point, there is a certain arbitrariness in where to draw the ionic / covalent line, and there are compounds that illustrate this point, but HF and NaCl do not. Try, instead, a compound like copper(II) bromide, CuBr₂ - much lower melting point, decomposes before reaching a boiling point, electronegativity difference less than 1, overall properties much less clear cut than either of the above. However, again I advise caution, as none of this stopped the 1987 HSC Chemistry exam begining a 5 mark question with the statement "Copper(II) bromide is an ionic salt".

i would think they suggest a strong understanding depending on how you say it don't you think.

i mean NaCl molecule must exist, at least some or even one in a million LOL. my understanding is that it contains more ions than molecules. hence the name ionic compound. same with water, as it contains more molecules than ions so its called a covalent molecule, not because they are 100% molecular or ionic. as a true 100% of anything does not exist in the physical world... it is only a metaphor we use in math. but my question is why ions form? why the ionization process?

In relation to NaCl in the solid state, there is zero evidence for molecules in the conventional sense of that term. The x-ray crystallographic data is absolutely clear - NaCl in solid state has discrete Na⁺ ions, each surrounded octahedrally by 6 Cl^- ions, each at the same distance. Each Cl^- ion is, in turn, surrounded octahedrally by 6 Na<suo>+</sup> ions. In the gas state, there is evidence of ion pairing, that is one Na<suo>+</sup> ion and one Cl^- ion moving together like a 'molecule', but the dipole moment is way too high to consider this bond as having any significant character.

By contrast, water undergoes self-ionisation, and thus there is a genuine equilibrium between an uncharged molecular form and an ionic form, ie:

2H₂O_(l) <---> H₃O⁺_(aq) + OH^-_(aq)

Incidentally, there is such a thing as a perfectly covalent bond, in compounds such as F₂, but you are correct in saying that there is no perfectly ionic bond.

Your question about why we get ionic bonding is a perfectly reasonable one - we have theoretical chemists who look at questions like this - but it seems to me you have the sequence backwards. You seem to be saying that you need to know why there is ionic bonding in order to accept it. To a chemist, the sequence goes the other way - experiment shows there is such a thing, now we need to figure out why. Do you see the distinction?

oh and another thing, textbooks would lead me to believe that when you heat a ionic compound such as NaCl to boiling point molecules of NaCl is formed, and not ions like when it is in water. thats why my original question was about the water situation.

The testbook is over simplifying, avoiding a discussion of ion pairing in gas phase.

Originally posted by Xayma
*Waits for CM_Tutor to correct the multitude of mistakes

Xayma, I don't mean to make you nervous that I'll jump in and correct you Do you want me to expand on the ionisation and dissolution processes?

 

[Xayma]

Originally posted by CM_Tutor
Xayma, I don't mean to make you nervous that I'll jump in and correct you Do you want me to expand on the ionisation and dissolution processes?

You didn't make me nervous, but what I typed was just from my memory and resoning, so I wasn't 100% sure it was right (or near it)

 

[abdooooo!!!]

okays.

i can't accept something seriously if i don't know the basis of its truth, i need logical connections. all this talk of bonding seems contradicting when you apply them to different situations. its like they switch models to explain different situations where the other model failed, there must be something majorly wrong in the concepts which they are build upon.

oh, can you confirm for me this, does ionic molecules contain a long bond length as in little to none overlap compared to covalent molecules which has significant overlap?

hsc does not matter to me anymore after i started to appreciate what i am learning instead of stupid marks which means so little.

is there such a thing as just theorotical chemistry major where you use mathematical models to depict chemistry? im only interested in the concepts cause i think inside my head all the time, other stuffs like experiments are just not for me. i might not even take chemistry in uni cause its just too much lab work for my liking... meh... i'll just stick to math.