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Why is water so mysterious?

I am a relatively new teacher in Chemistry, and every year I seem to have the same conceptual struggles with the bonding properties of water.  My students may feel confident in the concept the water is covalently bonded with a dipole nature that allows it to create hydrogen bonds and exhibit unique properties like surface tension and adhesion.  Then we start introducing the self-ionization properties of water and acid-base chemistry concepts, and they seem to go into mental melt-down. How is water capable of both covalent and ionic bonding . . . and are there other molecules that do this as well?  Does anyone have any reliable teaching tools or descriptive activities that can help my students (and myself) better understand these mysterious properties of water?? I would gladly take any resources being offered!

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New Contributor

Re: Why is water so mysterious?

The ability of water to form hydrogen bonds is well understood now. Almost every molecule can do similar intra- and inter molecular bonds. Water can do this under ambient conditions that we live on whereas H2S does it at much lower temperatures. You may look at some of these references where I have discussed these in more details.

http://pubs.rsc.org/en/content/articlehtml/2015/fd/c4fd00167b

Dynamics of the chemical bond: inter- and intra-molecular hydrogen bond - Faraday Discussions (RSC P...

Hydrogen bonding vs dipole-dipole interactions

http://www.ias.ac.in/resonance/Volumes/19/08/0704-0712.pdf

Hydrogen bonding vs van der Waals interactions

http://dx.doi.org/10.1039/B907708A

And some perspective on intermolecular bonds

http://dx.doi.org/10.1039/C4CP02585G

and a news story

http://www.currentscience.ac.in/Volumes/105/07/0892.pdf

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Re: Why is water so mysterious?

The ionization of water is a chemical reaction. It’s a dynamic process, with an activation energy and a free energy change that determines the equilibrium constant, which is very small! Nonetheless, the forward and reverse rates are very fast because the concentration (in liquid water) is high and the activation barrier is low, especially in the reverase direction.

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Re: Why is water so mysterious?

Dear Rochelle,

I did a short stint as a chemistry teacher two years ago and I understand your frustration with teaching water! Water is truly one of the most interesting and unique molecules in chemistry and yet students often lack an appreciation of the complexity of its chemistry because it is so common place. Bonding interactions are all about electrons and we teach that the structure of nucleus of the atom is unchanged by bonding interactions whether they be ionic or covalent. We also usually teach ionic and covalent bonding as individual and separate things when in fact every bond has degrees of ionic or covalent nature- we just teach examples where either the ionic or covalent nature is dominant and so easier to understand. (this link sums this up nicely: Compounds With Ionic and Covalent Bonds ) If you look at the electronegativity values, the difference between Oxygen and Hydrogen is 1.3 which puts it towards the ionic side of the "polar covalent" bonds, so there is a higher degree of ionization in that covalent bond than say, in a C-O bond (difference of 0.89) or a N-O bond (difference of 0.4).

When we are dealing with water, however, its ionization can also be viewed as either either gaining or losing a single proton (hydrogen nucleus). The hydrogen nucleus is unique in that it is a single proton with no neutrons. Its transfer as H+ is therefore somewhat analogous to the transfer of an electron, although the energies involved in the transfer (since the proton is so much more massive than an electron) is much larger and therefore slower and more difficult. This is the key insight of Bronsted and Lowry in defining acid/base chemistry as the transfer of protons rather than the usual visualization of chemical reactions as the transfer of electrons.

Thinking about the H+ ion this way has helped me to explain some of the unique behavior of hydrogen and its centrality to acid/base discussions. I see that your posting of the question is 4 years old so you may have come to these insights yourself in the meantime!

Karen

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