Why won't water molecules dissociate?

In summary, pure water does not act as an electrolyte because the hydrogen and oxygen atoms do not dissociate into charged particles. However, there is a water ion product that describes the dissociation of H+ and OH- ions in small clusters. The concentrations of these ions are low, giving pure water a high specific resistance. Protons can be solvated by multiple water molecules, resulting in a series of cations with the general formula H2n+1On+. The difference between H+ and H3O+ is that the proton is covalently bound to a water molecule in H3O+, while larger clusters are bound through hydrogen bridges. This bonding becomes more asymmetrical as the cluster size increases.
  • #1
Lim Y K
26
0
Pure water is not an electrolyte. This is because the hydrogen atoms and oxygen atom will not dissociate into individual charged particles. Why won't they dissociate?
 
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  • #2
They do dissociate, as described by a water ion product. However, concentrations of H+ and OH- thus produced are quite low, so the specific resistance of a pure water is quite high (18 MΩ).
 
  • #3
Besides, that's not two atoms'( ##\text{H}## and ##\text{O}## ) dissociation, but ##\text{H}^+## and ##\text{OH}^-.##
 
  • #4
tommyxu3 said:
Besides, that's not two atoms'( ##\text{H}## and ##\text{O}## ) dissociation, but ##\text{H}^+## and ##\text{OH}^-.##
Isn't the actual ionic equilibrium ##2H_2 O (l) ⇔ H_3 O^{+} (aq)+ OH^{-} (aq) ## ? If I remember correctly, H+ ions are just used instead of hydronium ion for simplification of the equation, but the actual equation is the one above.
 
  • #5
Protons can be solvated by several water molecules (so there exists whole series of cations of the general formula H2n+1On+). H+ is about as good as H3O+ IMHO - works OK as a symbol, doesn't reflect the reality. Add to that fact that OH- is not isolated in the solution, but solvated as well (yielding H2n+1On+1- anions), and you should start to see the picture :wink:
 
  • #6
Borek said:
Protons can be solvated by several water molecules (so there exists whole series of cations of the general formula H2n+1On+). H+ is about as good as H3O+ IMHO - works OK as a symbol, doesn't reflect the reality. Add to that fact that OH- is not isolated in the solution, but solvated as well (yielding H2n+1On+1- anions), and you should start to see the picture :wink:
Ahh, I see. But is it safe to say that n is not large? I mean after all, you'll eventually crowd up the entire area around the H+ ion so that the repulsion between the lone pair of electrons on the O atoms of water molecules solvating the ion and the lone pairs on the O atoms of 'unlinked' water molecules is greater than any attractive force the 'unlinked' water molecules might feel toward the H+ ion, preventing these 'unlinked' molecules from solvating the H+ ion as well.
P.S. I can't find a replacement word for 'unlinked' from my Chemistry verbiage memory bank right now :P
 
  • #8
  • #9
Borek said:
Protons can be solvated by several water molecules (so there exists whole series of cations of the general formula H2n+1On+). H+ is about as good as H3O+ IMHO - works OK as a symbol, doesn't reflect the reality. Add to that fact that OH- is not isolated in the solution, but solvated as well (yielding H2n+1On+1- anions), and you should start to see the picture :wink:

I think the difference is that in H3O+, the proton is covalently bounded to a water molecule while the additional water molecules in the larger clusters are bound via hydrogen bridges.
 
  • #10
DrDu said:
I think the difference is that in H3O+, the proton is covalently bounded to a water molecule while the additional water molecules in the larger clusters are bound via hydrogen bridges.

I would expect the change to be gradual - for small n (like 2 or 3) I would guess the cluster to be quite symmetrical, and all water molecules to be equivalent. Once the structure gets larger the outer layers will be definitely bonded differently.
 
  • #11
The only symmetric hydrogen bond I know about is the FHF-. So even in small water clusters, you can uniquely define which one is the hydronium ion.
 

1. Why don't water molecules dissociate easily?

The reason why water molecules do not dissociate easily is because of the strong bonds between the oxygen atom and the two hydrogen atoms. This bond, known as a covalent bond, is very stable and requires a significant amount of energy to break. Additionally, the small size of the hydrogen atoms makes it difficult for them to break away from the oxygen atom.

2. What factors affect the dissociation of water molecules?

The dissociation of water molecules can be affected by a number of factors such as temperature, pressure, and the presence of other substances. Higher temperatures can provide the energy needed to break the bonds between the hydrogen and oxygen atoms, while increased pressure can push the molecules closer together, making it easier for them to dissociate. Other substances, such as acids and bases, can also influence the dissociation of water by interacting with its molecules.

3. Can water molecules ever dissociate?

Yes, under certain conditions, water molecules can dissociate. For example, when an electric current is passed through water, it can cause the water molecules to split into positively charged hydrogen ions (H+) and negatively charged hydroxide ions (OH-). This process is known as electrolysis. However, this dissociation is only temporary and the ions will quickly recombine to form water molecules again once the current is removed.

4. Why is water considered a stable molecule?

Water is considered a stable molecule because it has a very low tendency to dissociate under normal conditions. The strong covalent bonds between the hydrogen and oxygen atoms make it difficult for the molecule to break apart. Additionally, the shape of the water molecule, with the two hydrogen atoms bonded to one side of the oxygen atom, also contributes to its stability.

5. What are the implications of water's stability on life?

The stability of water molecules is crucial for life on Earth. If water was easily dissociated, it would not be able to form the hydrogen bonds that are essential for many biological processes, such as protein folding and DNA replication. Additionally, the stable nature of water allows it to act as a solvent, providing an ideal environment for chemical reactions to occur in living organisms.

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