[Andre_212 (OP)]

Hi,

In a water that contains manganese, iron and ammonium, I wondered what interaction they have with each other in terms of oxidation? I understand that they all oxidise but which element would oxidise first? Is it as simple as looking at the element and the shells to see which would lose an electron first?

Thanks

Regards,

Andre

[Kryptid]

If you know what the structure of the molecules are before a reaction and the structure after a reaction, then yes, you can know which elements were oxidized. Consider the reaction of oxygen with hydrogen to produce water. In dihydrogen, both hydrogen atoms are in an oxidation state of 0, because the electrons don't have a preference to be around either atom (both atoms have the same electronegativity as each other). The same is true of dioxygen.

When the chemicals react to form water, however, the hydrogen atoms bond to the more electronegative oxygen atoms (2.2 for hydrogen vs. 3.5 for oxygen). In this case, the electrons in the H-O bond prefer to be around the oxygen atoms, so the hydrogen atoms can be said to have "lost" electrons to the oxygen atoms they are bonded to. When an atom loses electrons, it has been oxidized and when it gains electrons, it has been reduced. Thus, one can say that the reaction between hydrogen and oxygen to form water results in the oxidation of hydrogen and the reduction of oxygen.

In your specific case, the reactant list seems incomplete. You can't simply add ammonium to water by itself because ammonium is a positive ion. You need a negative ion of some kind to balance out that excess charge. Also, are the manganese and iron in an ionized state or not?

[Andre_212 (OP)]

Thanks for the info around hydrogen and water. Now I know how it is formed.

The ammonium exists in groundwater as NH4+. Mn exists an Mn2+ however from basic chemistry I understand that this has 1 electron in the outer shell. So can I ask why this exists as Mn2+? Iron exists as Fe2+ or Ferrous Iron.
I have read that iron will oxidise first before Mn. Based on simple chemstry, Fe has two electrons on the outer shell and Mn has one. I would've thought the oxidation potential of Mn would be higher since it only has to lose 1 electron to form a complete outer shell. Ammonium NH4+, present in anaerobic groundwaters for examples also has one electron on the outer shell. But would this oxidise before iron?

[Kryptid]

The ammonium exists in groundwater as NH4+.

Ah, I suspect then that this ammonium either comes from dissolved ammonia or ammonium salts. Technically, ammonia becoming ammonium by dissolving in water does not represent a redox reaction as the nitrogen still "keeps" the electrons which it is using to bond with the hydrogen ion (electronegativity of nitrogen is 3.0 vs. 2.2 for hydrogen).

Mn exists an Mn2+ however from basic chemistry I understand that this has 1 electron in the outer shell. So can I ask why this exists as Mn2+? Iron exists as Fe2+ or Ferrous Iron. I have read that iron will oxidise first before Mn. Based on simple chemstry, Fe has two electrons on the outer shell and Mn has one. I would've thought the oxidation potential of Mn would be higher since it only has to lose 1 electron to form a complete outer shell.

Iron and manganese both have 2 electrons in their valence shells. The iron and manganese present in minerals are already in an oxidized state by bonding to oxygen, sulfur or some other anion. So iron and manganese in ground water simply come from dissolved minerals, not from direct dissolving of iron or manganese metal.

Ammonium NH4+, present in anaerobic groundwaters for examples also has one electron on the outer shell. But would this oxidise before iron?

Each of the hydrogen atoms in the ammonium has 2 electrons each, whereas the nitrogen has 8 electrons in its valence shell.

[chiralSPO]

Mn exists an Mn2+ however from basic chemistry I understand that this has 1 electron in the outer shell. So can I ask why this exists as Mn2+? Iron exists as Fe2+ or Ferrous Iron. I have read that iron will oxidise first before Mn. Based on simple chemstry, Fe has two electrons on the outer shell and Mn has one. I would've thought the oxidation potential of Mn would be higher since it only has to lose 1 electron to form a complete outer shell.

Iron and manganese both have 2 electrons in their valence shells.

Elemental manganese, Mn(0) has 7 valence electrons, therefore Mn(II), which has been oxidized by two electrons, has only 5 valence electrons. Fe(II) has 6 valence electrons. But both of these metals are late enough in the period that neither is likely to lose all its valence electrons, forming a "closed shell". Formally, the Mn atom in the permanganate ion, MnO₄^–, is in the Mn(VII) oxidation state, but this is an extremely powerful oxidizing agent (when dry, potassium permanganate will oxidize compounds like glycerol and ethylene glycol so vigorously that the mixture will spontaneously combust!). I am unaware of any Fe(VIII) compounds, but I would imagine such a species would be highly reactive!!!

Instead of thinking purely about the ionization energies (usually calculated or experimentally derived for single atoms/ions in the gas phase), you can think about the reduction potentials (experimentally determined values for ions in aqueous solution.) This is better because the situation is much more complicated than it might seem at first glance. The metal ions aren't just single atoms of iron or manganese--they will have coordinated water (H²O), hydroxide (OH^–) or oxo (O^2–) ligands in either tetrahedral or octahedral arrangement around the metal atom. One also has to consider the spin state of the ion. Fe(III) has 5 electrons, which can exist in multiple spin states: the high-spin state where all spins are parallel (spin = ⁵/₂) and the low-spin state, in which all but one of the electrons have paired up (spin = ½).

According to standard reduction tables, it would take about 1.5 V of driving force to remove an electron from Mn(II) to get to Mn(III), but only about 0.8 V to oxidize Fe(II) to Fe(III). One of the reasons for this is that there is a stability boost to having a half-filled shell (which Fe(III) has). This isn't as good as having a completely full or empty shell, but it is enough of an effect to make Fe(II) easily oxidized.

Also, the pH of the solution is important. It is much easier to oxidize ammonia (NH₃) than ammonium (NH₄⁺), largely because the added positive charge from the extra proton makes it harder to remove the negatively charged electron.

[Kryptid]

Elemental manganese, Mn(0) has 7 valence electrons

Oh, yes, I see where my mistake was now. It has been so long since I've read electron configurations that I was mistaking the two electrons in the 4s orbital as being the only valence electrons.

[Andre_212 (OP)]

That makes a lot more sense now as to how oxidation of the element occurs. Do you happen to know where I can get the table for standard reductions you mentioned?

You mention pH is important too. I know a higher pH is favorable for oxidation and more oxygen is present is due to the carbonate system. I also know what oxidation of metals reduces the pH as the oxidation reaction produces H+ ions and CO2. Additionally, bacterial activity for oxidation is favoured at higher pHs

What is the effect of temperature on oxidation? Is it a case of higher temperature leads to higher oxidation rates? I have read that oxygen diffusion from the air to water slows down at high temperatures.

[Kryptid]

What is the effect of temperature on oxidation? Is it a case of higher temperature leads to higher oxidation rates?

Chemical reactions in general are sped up by increases in temperature. Up to a certain limit, at least. Make a substance too hot and compounds break down again back into their component elements.

I have read that oxygen diffusion from the air to water slows down at high temperatures.

Yes, the solubility of gases decreases in liquids as the temperature rises. Alternatively, the solubility of solids in liquids increases as the temperature rises.

[Andre_212 (OP)]

Chemical reactions in general are sped up by increases in temperature. Up to a certain limit, at least. Make a substance too hot and compounds break down again back into their component elements

.

Yes I have read in studies that an increase in temperature increases the chemical oxidation reaction as well as promoting bacterial growth for biological oxidation.
This is all very interesting as I am looking at discolouration in water and the impact Mn has on it. From what I understand, it is the particulate Mn that causes discolouration and not the soluble Mn?

Yes, the solubility of gases decreases in liquids as the temperature rises. Alternatively, the solubility of solids in liquids increases as the temperature rises.

Is this now negligible because of the above? As I mentioned, studies have shown that an increase in temperature shows an increase in oxidation. I guess the solubility of gas law has a small impact perhaps.

[chiralSPO]

That makes a lot more sense now as to how oxidation of the element occurs. Do you happen to know where I can get the table for standard reductions you mentioned?

Google is your friend. I would search for "standard reduction potential" or "activity series" The image search might be faster...

What is the effect of temperature on oxidation? Is it a case of higher temperature leads to higher oxidation rates? I have read that oxygen diffusion from the air to water slows down at high temperatures.

Higher temperatures almost universally increase the rate of reaction. There are also cases in which some species are favored at one temperature and other species are favored at a higher temperature (which can make it appear that the reaction has reversed direction, but really both forward and reverse reactions have both increased in rate, just the reverse reaction increased more)

This is all very interesting as I am looking at discolouration in water and the impact Mn has on it. From what I understand, it is the particulate Mn that causes discolouration and not the soluble Mn?

I am guessing that you are dealing with MnO₂, which is essentially insoluble in water and very strongly colored (dark brown to black). I know some researchers who often produce this stuff during the course of their reactions (as an undesired side product of catalyst decomposition), and they complain all the time about MnO₂ sticking to their glassware. I don't know how they remove it, but I doubt the method is suitable for improving water quality...

[puppypower]

That makes a lot more sense now as to how oxidation of the element occurs. Do you happen to know where I can get the table for standard reductions you mentioned?

You mention pH is important too. I know a higher pH is favorable for oxidation and more oxygen is present is due to the carbonate system. I also know what oxidation of metals reduces the pH as the oxidation reaction produces H+ ions and CO2. Additionally, bacterial activity for oxidation is favoured at higher pHs

What is the effect of temperature on oxidation? Is it a case of higher temperature leads to higher oxidation rates? I have read that oxygen diffusion from the air to water slows down at high temperatures.

Here is a link to a table of standard reduction potentials. Oxidation potentials will be the backward reactions using the same table.

http://www.chemeddl.org/services/moodle/media/QBank/GenChem/Tables/EStandardTable.htm

In terms of oxidation and reduction, using oxygen as an example, oxygen will attempt to gain extra electrons to complete the octet; oxygen undergoes a reduction. It will do so by extracting electrons from other atoms; oxidize them. The final oxygen ion, with the completed octet, will now have two more electrons than protons; oxide.The question becomes, why is this more stable since it will increase the electrostatic repulsion of the oxygen atom due to two extra electrons compared to protons? The answer is, this new configuration makes up for the added electrostatic repulsion by means of a much more favorable magnetic addition.

The electromagnetic force; EMF, is a merger of electrostatic and magnetic force components. In terms of reduction of oxygen, there is a shift of the EM force away from the electrostatic side, toward the magnetic side of the EMF. In the case of iron+2 oxide ; FeO, both iron and oxygen contain electrostatic potential. This is compensated for, internally, by magnetic stability. In the case of loadstone, which is magnetic iron oxide, the remaining valence electrons of iron are induced into a higher energy level. This further which adds electrostatic potential, which is then compensated for with magnetism.

This atomic swinging of the EMF pendulum is very important, especially in water and life. The hydrogen bonding of water can exist as one of two states. One side of this binary switch favors the electrostatic side; polar, and the side of the binary favors the magnetic side of the EM force; covalent. The binary nature of hydrogen bonding allows water to span the needs of most chemical changes, by being able to shif the EMF pendulum back and forth without any degradation in the water; reversible.

The wild card the hydrogen atom. This atom is unique in that it only has one electron. It can also have no electrons as the hydrogen proton. In both cases it is skewed to the electrostatic side of the EMF. This is unique among atoms. In the pH affect of water, the hydrogen proton can form, with the oxygen gaining the extra electron density. In this case, the  oxygen dominates the magnetic contribution, while the hydrogen proton dominates the electrostatic contribution. Water can internally separate the EM force.

Hydrogen bonding of water, where hydrogen shares the outer electrons of oxygen, allows the hydrogen to share extra electron density, so it can participate in the magnetic side. Hydrogen bonding help hydrogen become more balanced. Cooperative hydrogen bonds are when hydrogen bonding begins to form a resonance type structure that can span a wide range of water molecules; enhanced magnetic impact on hydrogen.

This comes back to oxidation and reduction, since the range of water and hydrogen bonding impacts other atoms dissolved in water to help catalyze change. It also helps the enzymatic reactions of life, since protein surfaces are designed for enhance magnetic affects. Although harder to investigate, the cores of protein should polarized to the electrostatic side, for maximum EMF separation. 

[adianadiadi]

oxidation means removing electrons. Just look at which element is ready to give up electrons. That is said be oxidized. Metals usually undergo oxidation. That means we can remove electrons easily.