On The Production of

Hydrogen Gas by The

Electrolysis of Water

Johnston

In the diagram (Figure 1) is a vessel filled with water and an electrolyte. I chose sulfuric acid (H2SO4) as the electrolyte because it seemed to be Faraday's favorite. When the electrolyte (which could also be represented as SO3, before hydration) is placed into the water it links up with water molecules to become H2SO4 (sulfuric acid). It apparently does this by attaching /attracting to itself the oxygen atom from a water molecule.

Figure 2:

As you can see, even with three oxygen atoms attached to the sulfur atom (SO3), there are two "holes" remaining in the valence shell of the sulfur. All three oxygen atoms have their shells entirely filled though. This being accomplished by each oxygen atom sharing two of the sulfur's electrons. So then, next the oxygen atom from an H2O molecule is attracted into the SO3 atom to fill those last two holes in the shell of the sulfur atom.

This happens because the oxygen is more strongly attracted to the sulfur atom than it is to the hydrogen atoms. Interestingly enough the ionization potentials of both hydrogen and oxygen are an identical 13.6 electron volts. "The oxidation potential of an atom is an expression of the tightness with which an atom holds on to an electron and is defined as the energy required to pull an electron off an atom. One electron volt (3.8 x 10 to the negative 20th power,cal) is equivalent to 23 kcal per Avogadro number of atoms."(1)(Section 3.14, Page 66)

The result of this bonding creates a problem for the hydrogen atoms, which are still attached to the oxygen atom, which has now linked into the SO3 (now SO4) molecule. They (the H2 atoms) would now have to "share" their single electrons with the oxygen, which is also sharing two of it's electrons with the sulfur atom (which has an ionization potential of 10.4e.v.). This double sharing is apparently the force which weakens the H2 bond to the oxygen enough to ultimately break it. "In other words, the formation of a chemical bond indicates that the resulting molecule represents a state of lower energy than the isolated atoms." (1)(Section 4.1, Page 76) And also; "Since oxygen is the more electronegative (in a water molecule), the shared electron pair is not shared equally but belongs more to the oxygen than to the hydrogen. In the course of combining H2(g) with O2(g) there is a change in electron sharing." (1)(Section 5.4, Page 104)

Part of the reason for this is that the oxygen, coupled to the sulfur atom now has a complete octet (8) of electrons in it's outer shell, as does the sulfur. "A shell of eight electrons, the so-called octet is especially difficult to break up. It is especially hard to pull an electron off an atom having eight electrons in it's outermost shell." (1)(Section 3.14, Page 68) But, conversely, the oxygen atom while in the water molecule also had a full octet shell by sharing one of it's electrons with each hydrogen atom, which in turn each shared it's one electron with the oxygen atom; "By sharing electrons, the oxygen atom gets a complete octet of electrons in it's outer shell, and each hydrogen atom gets two electrons, all that it's valence shell can accomodate. Since the molecule as a whole is electrically neutral and since all valence shells are filled with shared electrons, no other atoms can bind to the molecule. The valence is saturated." (1)(Section 4.7, Page 90) So it is aparrently easier to for the oxygen atom to pull the hydrogen's electrons away from them, when it joins the SO3 molecule than it would be for the hydrogen to take them back from the oxygen.

The electrons that originally belonged to the hydrogen atoms now seem to be more strongly attached to the oxygen atoms due to their being shared than they are to the hydrogen atoms to whom they initially belonged. This, by necessity, due to the octet rule, forces the two H atoms (minus their electrons) away from the complete SO4 molecule and, in so doing, creates two H+ ions, which are the two hydrogen atoms without their single electrons. Am I trying to say that these H+ ions (which amount to bare protons) now go racing off through the solution on their own? No. Chemistry invokes something called "Water of Hydration" to take care of that problem. "The ionization of a solute by water can be considered a chemical reaction and can be described by a chemical equation such as:

HCl + H2O ------> H3O+ and Cl-"

"Since the hydronium ion, H3O+, can be considered as a hydrated proton, and since water of hydration is often omitted from chemical equations, the above reaction can be written more simply as:

HCl-----> H+ and Cl-"

"with the tacit understanding that all species are hydrated."

"The practice of omitting water of hydration from chemical equations is dangerous unless we constantly bear in mind that, in aqueous solution, water is always associated with any dissolved species and may affect it's properties. The danger is greatest in the case of the hydrogen ion, because the hydrogen ion is nothing but a bare proton (nucleus of H atom). Whereas H+ is essentially of zero size, H3O+ has a volume which is about 10 to the fifteenth power as great and is comparable in size to other ions." (1)(Sections 10.6 & 10.7, page 204)

Note: The above example assumes that the HCl which is added to the water is already formed. To form HCl, Cl gas can be dissolved in water where it breaks apart water molecules to form HCl and an OH- ion. So technically the above example ignores the presence of the OH- ion. In practice, the gasses evolved in a cell containing water and HCl will be H2 at the cathode and either O2 or Cl2 at the anode with the evolution of Chlorine gas being the most common result, indicating that Cl is more easily oxidized than the OH- ion.mj

Very simply then, H+ ions, once separated from their original water molecule by the action of the electrolyte (in this case the oxidation of the sulfur atom in the SO3 molecule by the oxygen atom in the water molecule), are then picked up and carried by sympathetic H2O molecules. So then; "The only way by which electrons can be shifted away from an atom is for them to be pulled toward another atom. In this process (oxidation) the oxidation number of the first atom increases, and the oxidation number of the second atom decreases. Oxidation and reduction must always occur together and must just compensate each other. It should be evident that when a substance acts as a reducing agent, it itself must be oxidized in the process" (1)(Section 5.6, Page 108)

Ionic Bonds then can be described as bonds in which electrons are completely transferred from one atom to another. So the formation of an ionic bond can be thought of in three steps (using NaCl as an example):

Na• -----> Na+ and e-

Cl(with 7 electrons) + e- ------> CL(with 8 electrons)- (minus sign after "e" meaning one "extra")

Na+ plus Cl- ---------> Na+[Cl]-

Step (1) Requires energy equal to the ionization potential of sodium (5.1e.v.)

Step (2) Releases energy equal to the electron affinity of chlorine (3.75e.v.)

Step (3) Releases energy because of the attraction between positive and negative ions. The ionic bond is formed only because the energy released in steps (2) and (3) is greater than that required in step (1).

So when the oxygen links with the sulfur in the SO3 molecule it pulls the electrons completely away from the hydrogen atoms, creating the H+ ions. Please note that all of this happens when SO3 is mixed into water and that no addition of energy from an outside source is required to split the water molecules. Whether in an electrolysis cell or a primary (voltaic) cell, the splitting of water into it's component parts is accomplished by chemical reactions. Once all of the SO3 has reacted with available water molecules a state of equilibrium is reached and no more reaction occurs.

The fact that energy is used up in the process of adding a solute to a solvent can also be seen, on a more macroscopic level, by noting that; "When sugar is placed in a beaker of water, the sugar disappears. in the solution process, the sugar lattice is ripped apart, and the individual sugar molecules are distributed throughout the solution. The process can be envisioned as follows: First, a sugar molecule must be pulled from it's neighbors. Since molecules attract each other in the crystal lattice, work must be done against the attractions of the molecules remaining in the lattice; i.e., solute-solute attraction must be overcome. Second, the water molecules must be pushed aside to make a hole to accomodate the sugar molecule. This process also requires the expenditure of energy, since the molecules of water also attract each other: i.e., solvent-solvent attraction must also be overcome. Since both of these processes require energy, they of themselves cannot account for the fact that solution occurs.

There must be a third step which provides the energy that is needed. This third step is called solvation and arises from an attraction between the sugar molecules and the water molecules. Sugar-water attraction supplies almost enough energy to overcome the sugar-sugar attraction and water-water attraction. The last bit of energy required is supplied by a reduction in the average kinetic energy of all of the molecules. In this case solution is accompanied by a slight drop in temperature."

"The very fact that water dissolves sugar implies that H2O molecules exist preferentially in the solution state rather than in the pure water state."

"There are many cases in which the solution process is accompanied by the disassociation or breaking apart of molecules. The disassociated fragments are usually electrically charged, so that electrical measurements can show whether disassociation has occured. Charged particles, or ions, in solution carry electrical current, whereas water conducts electricity very poorly. Electrical conductivity requires charged particles. The greater the number of charges available for carrying electricity, the greater the conductivity observed. Strong electrolytes are essentially 100 per cent disassociated into ions, whereas weak electrolytes may be disassociated only a few percent."

For ionic substances, it is not surprising that there are charged particles in solution, because the solid itself is made of charged particles. The solvent rips the lattics apart into it's constitiuent pieces. Figure 10.10 (not reproduced) shows what is thought to happen when the ionic solid NaCl dissolves in water. Since the chloride ion is negative (like the SO4 -2 ion in sulfuric acid), the positive ends of water molecules cluster about the chloride (or SO4 -2) ion. The chloride ion surrounded by it's cluster of water molecules now moves off into the solution. It is now a hydrated chloride ion. The species is negatively charged because the chloride ion itself is negatively charged. At the sane time the sodium ion undergoes similar hydration (like the H+ ion in sulfuric acid), with the difference being that the negative or oxygen end of the water molecule faces the positive ion. Since the solution as a whole must remain electrically neutral, an equal number of hydrated sodium ions and hydrated chloride ions is formed. When positive and negative electrodes are inserted into this solution, the positively charged hydrated sodium ions (or H+ in sulfuric acid) are attracted to the negative electrode, the negatively charged hydrated chloride ions are attracted to the positive electrode. There is a net transport of electrical charge as the positive charge moves in one direction and the negative charge moves in the opposite direction." (1)(Section 10.6, Pages 200-203)

Or, as another example, consider electrically neutral HCL. The HCL molecules interact with the solvent to form ions;

HCL + H2O --------> H30Cl -------->H30+ and Cl-

HCL molecules collide with water molecules to give an unstable intermediate.This immediately forms H30+ and Cl- as shown.

 

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