What follows are ideas I have along with established concepts.
Standard Hydrogen Electrode
The electronegativity level of a voltaic cell half cell is defined in reference with a universally used half cell called the Standard Hydrogen Electrode (SHE) (Brown, 839, 840). It’s a sealed container that consists of an electrically neutral platinum electrode that’s partly submerged in a solution of hydrogen cations, and there’s a concentration of hydrogen gas in the space above the ionic solution. The platinum electrode is connected to the other electrode by a platinum wire.
Platinum, a transition metal, is called a noble metal because of how unreactive it is. That is, most substances can’t take electrons from it resulting in it going into disequilibrium. So, it’s commonly used in voltaic cells as an electrode that doesn’t participate in the redox reactions but merely acts as a conductor of electrons. If the platinum electrode is dipped in an aqueous solution of itself, of a more electronegative noble metal, or of a less electronegative substance in a sufficiently high concentration, then electrons will be taken from it causing it to go into disequilibrium. Also, if the platinum wire is connected to a noble metal that’s more electronegative than it, then electrons will be pulled from the platinum wire. However, because the electrode is dipped in a substance that’s less electronegative than it and that’s not too concentrated, then it doesn’t really go into disequilibrium at all. This is due to how eager its atoms are to stay in a community instead of be isolated cations. Aqueous platinum cations are extremely eager to take electrons in order to become a part of a solid so that they can be in a community.
If the SHE is isolated, that is, not connected to another electrode, then the two different phases of the element hydrogen are in equilibrium with each other because the pressure created by the hydrogen cations in the ionic solution is the same as the pressure created by the hydrogen gas in the space above the ionic solution, although the concentration of hydrogen cations is less than the concentration of hydrogen gas (Silverthorn, 570; Brown, 523). This is called vapor pressure. Because hydrogen is less electronegative than platinum, the aqueous hydrogen cations can’t take electrons from the electrode. Also, the platinum electrode continually tugs on electrons belonging to the hydrogen gas molecules as they collide with it. The platinum electrode doesn’t as much pull on electron density belonging to the aqueous hydrogen cations since it’s harder to take electrons from an atom or molecule that’s already electron deficient than it is from an electrically neutral one.
Because the platinum electrode is already electrically neutral, there aren’t electron vacancies in electron niches in the surface layer. As explained previously, there would never be electron vacancies in the interior since the interior has to remain electrically neutral. So, instead of it being vacant electron niches doing the tugging which would suck electrons into the gaps like vacuum cleaners, it’s only the exterior sides of surface layer atoms which are doing the tugging. Since the disequilibrium caused by cations repelling each other is much greater than that of a cation by itself, then the pull of a vacant electron niche is much stronger than that of the exterior side of a surface layer atom.
It’s the case that the exterior sides of the surface layer atoms of a conductor are always tugging on electron density in the part of the interior of the conductor adjacent to them, that being the surface layer of atoms, but when there’s not a source or supply of electron density present or available beyond what’s found in the conductor itself, then there’s no electron density to spare to give to the exterior sides. However, when there is an outside source of electron density such as when the conductor is in physical contact with a less electronegative substance, then the conductor senses this, and the exterior sides can then take electron density from the interior since the interior is immediately replenished with electron density from the less electronegative substance. If a neutralizing amount of electron density is traveling through a conductor, then this is synonymous with or has the same effect as, it being stationary there.
The reason that the electrons travel through the interior instead of along the exterior side of the surface layer of atoms is due to electrons getting in each other’s way through like charge repulsions. Electrons are getting pulled from hydrogen gas particles onto the outside of the unsubmerged part of the platinum electrode simultaneously over this entire region. The direction of this pull is towards the interior, perpendicular to the surface. In order for excess electrons to travel along the exterior side of the conductor, they would have to get in the way of and interfere with the electrons being pulled from the gas particles bonding with the exterior sides of platinum surface layer atoms. Electrons in the process of being taken from gas particles don’t get in the way of electrons already taken and now involved in the process of establishing electrostatic equilibrium over the conductor when the latter travel through the conductor’s interior. This allows for maximum bonding between incoming electrons and the exterior sides of surface layer atoms and minimum like charge repulsions between these electrons and the electrons already in the process of uniformly distributing themselves over the conductor. It’s a mass movement of electrons that involves the entire conductor all simultaneously acting as a single body since a conductor behaves like a single atom.
However, the more distant parts of the exterior receive a smaller density of electron supply lines, meaning that they’re further apart from each other, due to the difficulty of electron supply lines maintaining their structural integrity amongst the thermal vibrations of the interior cations since all of them have to extend continuously from the junction. In fact, for this reason, it’s mainly nearby the junction that electron density comes to the surface from the interior, instead of only traveling along the exterior. After electrons are brought from the interior to the exterior, then they push each other around the exterior. Because of how much more difficult it is for the platinum to take electrons from aqueous hydrogen cations than from hydrogen gas particles, electrons go from the exterior of the unsubmerged part of the electrode through the interior to the exterior of the submerged part of the electrode. Due to the short range of electron interior travel, it’s mainly the case that electrons travel through the interior as far as the submerged part of the platinum electrode extends but beyond that only travel along the surface. Also, it’s the case that electron supply lines in the interior heading to the surface can’t bend too sharply since this causes them to more get in the way of other electron supply lines in the same way that automobiles on a public road slow down traffic and get in the way of other vehicles when they turn off the road into a business district. This causes like charge repulsions between electrons and slows down the momentum of electron travel. For this reason, it tends to be the case that for a thin conductor like the platinum wire, the exterior is really only ever populated with excess electrons by electron travel along the surface.
This process of the platinum electrode and wire populating their exteriors with electrons from the hydrogen gas continues until the like charge repulsions between the excess electrons with each other becomes so great that the pull of exterior sides of surface layer atoms is no longer strong enough to bring over any more electrons. It’s the case that only an extremely tiny percentage of the electron need of the exterior of the platinum electrode/wire conductor system is satisfied due to these like charge repulsions. This is indicated by the fact that with a Van de Graaff generator, which uses a much more aggressive method for populating the outside of a conductor with excess electrons, friction instead of the more passive mere tugging by cations, can seemingly indefinitely have excess electrons added to it because there are always surface layer exterior sides which lack sufficient electron coverage.
As electrons are taken from the hydrogen gas particles, the gas particles become hydrogen cations and go into the ionic solution. This decreases the amount of hydrogen gas and increases the amount of aqueous hydrogen cations. So, the vapor pressure equilibrium state between the hydrogen gas and the concentration of aqueous hydrogen cations only occurs after the platinum metal has ceased taking electrons from the hydrogen gas due to like charge repulsions between excess electrons on its exterior side.
However, when the SHE isn’t isolated, then there are two scenerios; the other electrode being more electronegative than the element hydrogen and the other electrode being less electronegative than it. If the other electrode is more electronegative than hydrogen, then hydrogen gas molecules in the SHE give up electrons to the platinum electrode and go into the ionic solution as hydrogen cations. This is because the hydrogen cations already in the ionic solution can’t themselves give up electrons to the electrode because they are already electron deficient.
If the other electrode is less electronegative than hydrogen, then hydrogen cations take electrons from the platinum electrode and leave the ionic solution as hydrogen gas. The hydrogen gas leaves the ionic solution because it’s nonpolar, and it wants to get away from the polar water molecules and ions of the ionic solution. Although the hydrogen gas molecules are as electronegative as hydrogen cations because they’re both the same element, it’s harder for gas molecules to take electrons because they are already electrically neutral, whereas hydrogen cations are electron deficient.
For example, if the platinum wire is connected to a copper electrode which is partly submerged in a solution with a concentration of copper cations that’s not too concentrated, then because copper is more electronegative than hydrogen and because platinum is more electronegative than both of them, although solid platinum tugs on electron density belonging to both the copper electrode and the hydrogen gas, where it’s specifically the platinum wire that pulls on electron density of the copper electrode and the platinum electrode that pulls on electron density of hydrogen gas molecules, the solid platinum tugs more on electron density belonging to the hydrogen gas. This is because it’s easier for solid platinum to pull on electron density belonging to the less electronegative substance. The greater amount of tugging on hydrogen gas electron density is represented by there being a greater concentration or density of electrons at the hydrogen gas junction, which is where the hydrogen gas molecules collide with the platinum electrode, than at the copper junction. Also, the electron density is pulled closer to the platinum at the hydrogen gas junction than at the other junction.
What happens first is that the platinum populates its exterior with electrons from both the other substances until the like charge repulsions between the excess electrons with each other prevents the platinum from being able to bring over any more electrons from these substances. The pulling over of electrons from the two reactants onto the platinum increasingly slows down and then stops. That is, a stalemate is reached between the platinum and each of the two other substances where the strength of the pull of each of the other two substances on electrons at its junction becomes the same as that of the platinum. However, due to the fact that at the junction with the more electronegative substance, the copper, the electron density is less concentrated and further from the platinum, true electrostatic equilibrium can’t be established over the platinum’s exterior since it’s not truly uniform. The platinum aggressively seeks to establish a uniform distribution of electron density over its exterior so that all of its surface atoms get equal electron coverage treatment. So, the platinum seeks to establish true electrostatic equilibrium over its exterior by causing electrons to move through the interior from the junction with the less electronegative reactant, the hydrogen, where there’s more electron density, to the junction with the more electronegative reactant, the copper, in order to try and make there be the same amount of electron density at both junctions. This causes there to be less electron density at the hydrogen gas junction than there was previously and more electron density at the copper junction than there was previously. So, electrons go from where they’re more concentrated to where they’re less concentrated. Because the surface layer is a system that’s separate from the interior since its atoms aren’t completely surrounded by other atoms as with interior atoms thereby giving it different properties, electron movement through the interior is simply entirely dependent on the concentration of electrons at the exterior. The amount of interior electron movement required to equalize the electron density at the two junctions represents how much more electronegative copper is than hydrogen.
Also, the platinum seeks to cause both its ends, one at each junction, to receive the same extent of tugging by the two reactants instead of a stronger pull by the more electronegative one, the copper. It does this by taking electron density from the less electronegative reactant, the hydrogen, specifically its gas form, which causes there to be a larger concentration of aqueous hydrogen cations, more excess positive charge, and consequently a stronger pull by the concentration of aqueous hydrogen cations on the platinum. Also, the platinum causes or allows the more electronegative reactant, the copper, to be able to take electron density from it, which diminishes the excess positive charge of the copper and therefore its tugging on the platinum. This is made possible because previously there was the stalemate between the platinum and the copper, but then the platinum transports electron density from one end of itself to the other, thereby disrupting the stalemate and creating a surplus of electron density at the copper junction. The copper promptly reestablishes the stalemate by taking the electron density surplus. Actually, the platinum doesn’t transport electron density from one end of itself to the other. It just moves electron density away from the hydrogen gas junction and moves electron density to the copper junction.
Essentially, what happens is that since the platinum is pulling on both reactants simultaneously in a net manner in opposite directions towards itself but is pulling harder on the less electronegative of the two, then the platinum cancels out with itself all of its pull on the more electronegative reactant, the copper, and what’s left is just a diminished pull by it on electron density belonging to the less electronegative reactant, the hydrogen gas. This is represented by the net movement of electron density through the interior from the anode to the cathode. The result is that the platinum causes its own valence electron environment to actually resemble and represent that of the copper after it’s taken electron density from the hydrogen gas in the nonvoltaic cell version of the redox reaction, which is where the copper and hydrogen gas are in physical contact with each other instead of being separated by the platinum. The amount of electron density that gets taken from the hydrogen gas junction, a relationship between hydrogen gas and platinum, causes the amount of electron density that remains there to be the same as that found at the junction between the hydrogen gas and the copper in the nonvoltaic cell version. Also, the amount that gets added to the copper junction, a relationship between copper and platinum, causes that junction to have the same amount as the hydrogen gas junction now does, which as already mentioned, now represents a relationship between hydrogen gas and copper instead of a relationship between hydrogen gas and platinum. After the dosage of interior electron flow that equalizes the two ends of the platinum occurs, since the platinum now represents copper metal that’s taken electron density from hydrogen gas as indicated by the amount of electron density at the copper junction, it’s simply a matter of diffusion through the same material from greater concentration to less concentration for electron density to go from the platinum into the actual copper. So, before this diffusion occurs, the platinum represents copper that has taken electron density from hydrogen gas whereas the actual copper represents copper that hasn’t yet taken electron density from hydrogen gas. So, due to the platinum cancelling with itself resulting in it no longer having any pull on the copper, at this point, the copper merely pulls on platinum electron density in a net manner in the same way that it would pull on hydrogen gas electron density if they were directly adjacent to each other.
That is, before electron movement through the interior, the amount of electron density at each of the two junctions represents how much more electronegative the platinum itself is than each of the two reactants separately and in no way represents a relationship between the two reactants with each other. It’s the electron movement through the interior that associates the two reactants with each other, and it amounts to what the copper would pull from the hydrogen gas.
This electron movement through the interior is assisted by the coating of excess electrons on the exterior of the platinum which acts like the plastic insulation casing around a wire by helping to keep the electrons inside the wire. Electrons in the interior are repelled by the excess electrons on the exterior and are thereby kept in the interior where the pressure and therefore electron mobility is greater. The excess electrons on the exterior are stationary at least as far as net movement is concerned. If it wasn’t for this excess electron coating on the exterior, then the momentum of interior electron flow would continually be disrupted by the continual tugging of the exterior sides of the surface layer atoms on interior electron density.
Works Cited:
(1) Brown, Theodore, et al. Chemistry: The Central Science, 12ed. Pearson, 2012.
(2) Silverthorn, Dee. Human Physiology: An Integrated Approach, 6e. Pearson.