2.III: Dissociation of water - Biology

2.III: Dissociation of water - Biology

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2.III: Dissociation of water

WATER and pH

A water molecule is an irregular and irregular slightly skewed tetrahedron with an oxygen atom at its center. Water is a solvent for a wide range of organic molecules. Water molecules can dissociate into hydroxide ions(OH-) and protons (H+).

Structure of water

Water is a dipole molecule. Water molecules have inter and intra linked hydrogen bonds. Normally water is in liquid form at room temperature. The boiling point of water is 100°C and the freezing point is 0°C.
Water molecules are V-shaped with an angle of 104.5°. water is polar because of the high electronegativity of oxygen. A molecule of water consists of two hydrogen atoms and one oxygen atom that is attached by two sigma bonds and has two lone pairs of electrons. The geometry of water molecules is stated as bend or angular with sp3 hybridization.

Oxygen is highly electronegative in comparison to Hydrogen so the sharing of electron between oxygen and hydrogen is unequal. The electrons are more often in the vicinity of the oxygen rather than hydrogen. This results in developing a partial positive charge in each Hydrogen. This is due to the presence of Hydrogen bonds. Hydrogen bonds are relatively weak, and the energy required to break the bond(Bond dissociation energy) of about 23KJ/mol.

Water dissociation constant

  • Keq value determined by the electrical conductivity measurement of pure water and that is 1.8 X 10-16.

The product of [H+][OH-] at 25°C in the aq. The solution is always 1.0 X 10-14 M2. . there is exactly equal concentrations of H+ and OH- in pure water, so the solution is at neutral pH.

Dissociation of Water

When water dissociates, one of the hydrogen nuclei leaves its electron behind with the oxygen atom to become a hydrogen ion, while the oxygen and other hydrogen atoms become a hydroxide ion. Since the hydrogen ion has no electron to neutralize the positive charge on its proton, it has a full unit of positive charge and is symbolized as H+. The hydroxide ion retains the electron left behind and thus has a full unit of negative charge, symbolized by OH-. The hydrogen ion (proton) does not wander long by itself before it attaches to the oxygen atom of a second un-ionized water molecule to form a hydronium ion (H3O +)

In any sample of water, very few of the molecules are dissociated at any one time: in fact, only about one in 550 million. There is, however, a constant change as one hydrogen ion reattaches to a hydroxide ion to form a water molecule, another water molecule dissociates to replace the hydrogen ion and the hydroxide ion in solution.

Problem: Hydroxylamine, NH2OH, is a weak base. The following is the equilibrium equation for its reaction with water:NH2OH(aq) + H2O(l) ⇌ NH3OH+(aq) + OH - Kb = 9.1 x 10 -9What is the pOH of a 2.37 M NH 2OH solution?a) 1.15b) 3.83c) 4.99d) 6.72e) 8.13

We’re being asked to calculate the pOH of a 2.37 M NH2OH solution .

Since NH2OH has a low Kb value, it’s a weak base. Remember that weak bases partially dissociate in water and that bases accept H + from the acid (water in this case). The dissociation of NH2OH is as follows :

NH2OH(aq) + H2O(l) ⇌ OH – (aq) + NH3OH + (aq) Kb = 9.1 × 10 –9

From this, we can construct an ICE table. Remember that liquids are ignored in the ICE table.

The Kb expression for NH2OH is:

Note that each concentration is raised by the stoichiometric coefficient: [NH2OH], [OH – ] and [NH3OH + ] are raised to 1 .

Problem Details

Hydroxylamine, NH2OH, is a weak base. The following is the equilibrium equation for its reaction with water:

NH2OH(aq) + H2O(l) ⇌ NH3OH+(aq) + OH - Kb = 9.1 x 10 -9


  • For any neutral solution, (pH + pOH = 14.00) (at 25°C) and (pH = ce <1/2>pK_w).
  • Ion-product constant of liquid water: [K_w = [H_3O^+][OH^&minus] onumber ]
  • Definition of (pH): [pH = &minuslog_<10>[H^+] onumber] or [[H^+] = 10^ <&minuspH> onumber ]
  • Definition of (pOH): [pOH = &minuslog_<10>[OH^+] onumber] or [[OH^&minus] = 10^ <&minuspOH> onumber ]
  • Relationship among (pH), (pOH), and (pK_w): [pK_w= pH + pOH onumber ]

Water is amphiprotic: it can act as an acid by donating a proton to a base to form the hydroxide ion, or as a base by accepting a proton from an acid to form the hydronium ion ((H_3O^+)). The autoionization of liquid water produces (OH^&minus) and (H_3O^+) ions. The equilibrium constant for this reaction is called the ion-product constant of liquid water ((K_w)) and is defined as (K_w = [H_3O^+][OH^&minus]). At 25°C, (K_w) is (1.01 imes 10^<&minus14>) hence (pH + pOH = pK_w = 14.00). The process is endothermic, and so the extent of ionization and the resulting concentrations of hydronium ion and hydroxide ion increase with temperature. For example, at 100 °C, the value for (K_ce) is approximately (5.1 imes 10^<&minus13>), roughly 100-times larger than the value at 25 °C.

When you say that the water is dirty even though it has high resistivity (Is that 10 Meg ohm-cm?), I guess you mean it has some dust in it.
You can measure dust levels by shining a cheap little red laser into the water in a clear glass cuvette and measuring the amount of light scattered. You can judge by eye or do that more systematically using a photodiode (also cheap and sturdy) together with an optical filter matched to the laser wavelength. Dust can be filtered out with membrane filters available from several suppliers.

Properly distilled water or reverse-osmosis purified water should generally be cleaner than water that's just been run through an ion exchange resin. I hate to give technical advice on something where I have no direct experience, but I can't think of any reason why you should avoid distilled or reverse-osmosis water. Is de-ionized water cheaper?

A step closer to a hydrogen-fueled economy using an efficient anode for water splitting

Schematic representation of the water dissociation process at low overpotential of about 32 mV using NiSx nanowires stuffed into C3N4 scabbard as anode for water oxidation. Credit: Niigata University

In the recent past, there has been a paradigm shift towards renewable sources of energy in order to address the concerns pertaining to environmental degradation and dwindling fossil fuels. A variety of alternative green energy sources such as solar, wind, hydrothermal, tidal, etc., have been gaining attention to reduce global carbon footprints. One of the key challenges with these energy generation technologies is that they are intermittent and are not continuously available.

"We cannot use solar energy at night and wind energy when the wind is not blowing. But we can store the generated electricity in some other forms and utilize it whenever required. That is how water splitting bridges the gap and has emerged as a very promising energy storage technology," said Professor Masayuki Yagi, who conducts research on energy storage materials and technology at the Department of Materials Science and Technology, Faculty of Engineering/Graduate School of Science and Technology, Niigata University. Water splitting is one of the promising energy storage solutions that would potentially drive the world towards a hydrogen-fueled economy.

The water dissociation process, alternatively known as artificial photosynthesis, traditionally employs electricity to split the water molecule through two half reactions in an electrochemical cell. The hydrogen evolution reaction occurs at the cathode where hydrogen fuel is generated and the water oxidation occurs at the anode where breathable oxygen is released. Although water is a simple molecule that is constituted by only three atoms, the process of dissociating it is quite intense and challenging.

The initial energy, known in scientific terms as the overpotential, plays a crucial role in influencing the progress of the reaction. For the materials explored so far, the initial energy required to trigger the hydrogen evolution at the cathode and oxygen evolution at the anode is so high that the process escalates the overall cost of the reaction, thereby, adversely affecting its commercial utilization. This is particularly a major concern at the anode because the oxygen evolution reaction involves the transfer of four electrons which demands a higher initial energy as compared to the reaction at the cathode.

Prof. Yagi's research team at the Niigata University, in association with research collaborators at the Yamagata University, are investigating on the electrocatalytic water splitting and to address the key shortcomings. They have been successful in developing an efficient water dissociation process using nickel based nano-compounds as anodes, which has been published in as a scientific article in Energy & Environmental Science on 20 May.

In this study, Prof. Yagi's team have observed that the nickel sulfide nanowires based anode has supported the reduction of initial energy that is required for the oxygen evolution reaction. "We have fabricated the anode using a unique motif of nickel sulfide nanowires stuffed into carbon nitride scabbards. The carbon nitride scabbards prevent the core region of NiSx rods from transforming to their oxide, thereby protecting them from further degradation. On the surface of the nickel sulfide nanowires, a thin oxide film is formed due to the contact with the electrolyte solution, which facilitates the oxygen evolution reaction," explained Prof. Yagi.

The research team has observed, with the aid of advanced microscopy techniques and electrochemical measurements, that the fabricated anode aids in reducing the initial energy, which accelerates the four-electron transfer process in the oxygen evolution reaction. The research finding of Prof. Yagi's team has immense potential in improving the long-term performance and stability of the electrochemical cell.

This research study is an important milestone towards improving the efficiency of the water splitting technology. Prof. Yagi said, "This result is a great breakthrough in the electrocatalytic water splitting system and could undoubtedly contribute to realize the de-carbonized human society in near future."


Using molecular dynamics simulations and methods of importance sampling, we study the thermodynamics and dynamics of sodium chloride in the aqueous premelting layer formed spontaneously at the interface between ice and its vapor. We uncover a hierarchy of time scales that characterize the relaxation dynamics of this system, spanning the picoseconds of ionic motion to the tens or hundreds of nanoseconds associated with fluctuations of the liquid–crystal interface in their presence. We find that ions distort both local interfaces, incurring restoring forces that result in the ions preferentially residing in the middle of the layer. While ion pair dissociation is thermodynamically favorable, these structural and dynamic effects cause its rate to vary by over an order of magnitude through the layer, with a maximum rate significantly depressed from the corresponding bulk value. The solvation environment of ions in the premelting layer is distinct from that in a bulk liquid, being dominated by slow reorganization of water molecules and a water structure intermediate between ice and its melt.


The dissociation of the hydrotrioxy (HOOO) radical to OH and O2 has been studied theoretically using coupled-cluster methods. The calculated dissociation energy for the trans-HOOO isomer is 2.5 kcal mol −1 including zero-point corrections. The minimum energy path to dissociation has been explored and an exit barrier has been revealed, which may help to rationalize the apparent disagreement between theory and experiment on the magnitude of the bond energy.


We put this question to Luca Montabone, Atmospheric, Oceanic and Planetary Physics dept., Oxford University.

On the Earth, water can exist in all three forms namely as a solid, liquid, or gas. Evaporation transforms liquid water into water vapour which can then freely move in the atmosphere as a gas.

Now, atmospheric molecules, including water vapour molecules, are in perpetual motion in all directions. Without the gravitational field of the Earth, those moving away from the planet would be lost. Even with the gravitational field, in the upper thin part of the atmosphere, a molecule moving outwards has little chance of colliding with another and would therefore be able to escape if it has sufficient speed.

The average speed of the gas, for example water vapour, depends on its temperature. The conditions of temperature at the altitude from which water molecules are able to escape indicate the earth can retain water vapour over geological time scales, that is, over several billion years.

The retention of water vapour on our planet is also favoured by the fact that it can condense, form clouds at an altitude well below the one from which water molecules can escape and precipitate back to the ground as rain or snow.

Adding to all these, we have to remember that water is also introduced in the hydrological cycle from the interior of the planet, for example, every time that a volcanic eruption occurs.

So, to summarize, even if a few water molecules are continuously lost to space, the average level remains fairly constant over geological times, which is what we want!

Watch the video: Έργασια για το (May 2022).


  1. Nyles

    I confirm. This was and with me.

  2. Hadrian

    I believe that you are making a mistake.

  3. Ailean

    I don't quite understand what you mean?

  4. Fedal

    Please tell in more detail.

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