Oxidation Numbers: An Ecological Perspective

Assigning oxidation numbers for the purposes of predicting behavior is one the key concepts in chemistry. Although there are countless benefits to teaching kids basic literacy and numeracy, if it was all done solely for them to understand oxidation numbers, it would still be worth it!

It initially appears strange to students that an oxidation number is not always grounded in reality. In certain cases, however, they are the same as ionic charges, which are real. And whether real or imaginary, oxidation numbers are extremely useful.

Let’s begin with ions. The nucleus of atoms has a positive charge, which exerts an attractive force for oppositely charged electrons. The nucleus contends with distance between itself and electrons and with screening by other electrons. After all that, some atoms fall into categories of extremes. There are some that are really good at exerting a strong force between its nucleus and its most distant, so called valence electrons, and those that are fairly feeble. Put together a member of each different group, and it doesn’t get much to generate a reaction. Examples include table salt which you can make by gently heating the low-melting sodium metal and placing it in a beaker filled with the yellow-green and poisonous chlorine gas. Or you can flatten soft potassium metal into a disk and use it to cover a few crystals of pure iodine solid. A gentle blow with a hammer will form the other additive of table salt, potassium iodide, which is included in the salt we buy to prevent goiter.

LeotheLionIn each of the reactions, the metal atom loses an electron to the non-metal, either chlorine or iodine. The metal in the newly formed salt goes from being neutral to +1, the charge resulting from having lost a negative electron. Because it mimics what substances do in oxygen’s presence, we say that the sodium has been oxidized from 0 to +1. Another way of viewing this is that neutral atoms have an equal number of protons(+) and electrons(-). But after losing one electron, the sodium atom, for example, now has 11 protons and only 10 electrons. The sum of those charges, 11 + (-10) = +1. That is the new oxidation number of sodium in NaCl or of potassium in the salt potassium iodide. For the chloride or iodide ions, the opposite is true. The excess of electrons gives each of their atoms a charge of -1. We say that the chlorine has been reduced, in this case form 0 to -1.

In our story so far, we have covered two rules of assigning oxidation numbers. An atom in its neutral or free (not bonded to another kind of atom) state is assigned an oxidation number of zero. Metal ions and non metal ions, the members of the extreme groups, have oxidation numbers equal to their respective charges.

Why does it matter? When most metals go from a free state to a charged one the process of bonding to a nonmetal takes them to a more favorable thermodynamic state. What does that mean? In reacting, atoms strive for both disorder and for the bottom of the hill in terms of potential energy, which is associated with the attractive force we described. They can’t always get both, but if at least one of the favorable conditions offsets an unfavorable one, it will still make the reaction spontaneous. In this case, making a crystalline salt doesn’t create disorder. But the act of combining an intermediate gaseous metal ion with a gaseous non metal releases an awful lot of heat, causing the overall reaction to be a downhill process.

To make this less abstract, let us use a practical example: the rust that forms on railroad tracks, supporting beams or cars. The main compound involved is Fe2O3. The oxidation number of oxygen in compounds is usually -2. Since the entire compound, Fe2O3 is neutral, solving this simple equation 2Fe +3(-2) = 0 reveals iron’s oxidation state to be +3. Both the +2 and +3 states of iron in compounds are more stable than that of the zero state found in the free iron of steel. To move from zero to +3, all that free iron has to do is lose electrons to an atom who has the opposite problem. That atom is oxygen.

P1090737Free oxygen only exists on earth because plants use the energy of sunlight to produce sugars. In so doing they use pigment-centers that lose electrons when absorbing light, but neutrality is restored when water molecules split up to return electrons.In so doing, H2O molecules not only create a proton gradient that’s used to invest the energy of sunlight, but they also produce free oxygen. But each atom in the oxygen molecule can accommodate two more electrons in its valence shell—hence oxygen’s oxidation number of -2 —hence the fact that oxygen created either by stars or photosynthesis ends up as either water or in the main compounds of the earth’s crust: silicates, iron oxide and aluminum oxide.

When we create iron from ore in the planet’s crust we chase out the oxygen with heat and coal and return electrons to the the iron ions. To prevent it from losing electrons again afterwards—and to delay it from reaching its thermodynamic destiny— we have to either paint or wax its surface in an almost vain attempt to block out oxygen. A more durable alternative is to mix it with other metals whose oxidized coating seriously slows down oxygen’s intrusion. Such metals include nickel, chromium, and zinc.

If we let too much iron rust, we get caught up in having to produce more of the free element. The starting materials, rust and ore, will not run out soon, but the heat and carbon required for its reduction depend on fossil fuels, which when consumed, yield carbon dioxide. If we consume too much stainless steel, their production also has repercussions since nickel, chromium and zinc all have to be reduced as well. Their ores are all oxides or sulfide equivalents.

h2o symmNow we come to assigning oxidation numbers to covalent compounds, where electrons are shared; they are not lost and taken as they are in ionic compounds such as salt and rust. The -2 oxidation number is assigned to the oxygen in CO2, even though it doesn’t represent a true charge. Consistent with that is an oxidation number of +4 for carbon. It turns out that the creation of CO2 through combustion, respiration or decomposition puts the carbon in a molecule that’s in a gaseous and higher state of entropy than any solid or liquid form containing carbon. Moreover, the formation of a pair of tight C=O bonds found in the molecule also releases a fair amount of energy. Figuratively speaking, producing CO2 is a free ride. But CO2 is a greenhouse gas whose concentration jumps dramatically when we oxidize fossil fuels on a massive scale!

Over millions of years, in the absence of the electron thief, oxygen, it took lots of energy in the form of geological pressure to form compounds like CH4(methane), C8H18(octane),  and C-aggregates(coal). The oxidation number of carbon in those compounds is -4 , -9/4(an average), and 0, respectively, all lower than the +4 in carbon dioxide. To raise the oxidation state, it takes the same strategy that works for neutral iron: yield electrons to oxygen, even though this time the atoms engaged share the electrons. The end-result is that when burnt, the molecules “gladly” return heat and CO2 in proportion to how many carbon atoms are in each molecule, with methane having the least and coal being cursed with the most.

I was demonstrating to my students that the Mn in potassium permanganate (KMnO4) is in a highly oxidized state, +7 to be exact. This makes it a proficient electron thief. In the presence of glycerin, C3H8O3, beholder of a carbon in a low oxidation state relative to CO2, a reaction usually takes place within a minute that the two are in contact. But this time the reaction was so slow to activate that it actually came to fruition in another class, one hour later.

The reason for this strange occurrence might have boiled down to the fact that the KMnO4 powder I used was far finer than normal. In principle coarse powder reacts more slowly. But my reaction was far slower than what I usually observed from powder that had less surface area per volume. If a powder is too fine, its higher surface area might imply that the additional exposed atoms could obtain their electrons beforehand by snatching them from moisture in the air inside the container. (Something similar occurs if you try reacting old powdered zinc with acid. The reaction is slower than what’s obtained with zinc nuggets!) This feat of having a oxidation-reduction reaction with H2O, rare among man-made chemicals, is possible because KMnO4 is a stronger oxidizing agent than oxygen. Oxygen is created from the oxidation of water in a reaction similar to what occurs in photosynthesis.

To verify this theoretical hypothesis, my wife suggested that I grind the already fine powder with a mortar and pestle just prior to adding the glycerin. This would get some of the moisture-reduced coating off the powder and expose more pure KMnO4. Sure enough, within seconds the same powder and glycerin that had taken so long to react now erupted and produced a beautiful violet-colored flame. (The color is created by the presence of potassium ion.)

In the same way that I was motivated to reduce manganese, we need more members of society to work together with algae and terrestrial plants. Inadvertently, in our absence they succeed in balancing the amount of carbon dioxide in the atmosphere. They do so by reducing the oxidation state  of carbon , converting CO2 into sugars and fatty, nucleic and amino acids, which make other forms of life possible. They work against volcanic eruptions, lightning-induced fires, respiration and decomposition, all of which serve to place more of the oxidized form of carbon into the atmosphere. What photosynthesizers cannot do, because there aren’t enough of them, is offset our overzealous oxidation of carbon. We have to live less greedily and use technology more wisely in converting energy in the absence of combustion.

 

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Two Offbeat Questions About Greenhouse Gases

In science, something fruitful can arise from innocent or atypical questions.  One of many examples of this was a recent query in Quora, ” Can chlorine be potentially a greenhouse gas? Why or why not?

Here’s what I wrote, but I’ve since added a little more detail.

Short answer , no.  🙂

Now here’s why….

An infrared spectrum is used to check if specific molecules are good absorbers of infrared energy, which qualifies them as “greenhouse gases”. Some chlorine-containing gases like CFCs fall into that category, but diatomic chlorine(Cl2) or monoatomic chlorine(Cl) do not. How come?

In order for molecular vibrations to absorb IR energy, the vibrational motions along the bond must change the dipole moment CO2IRof the molecule. A dipole occurs when pulling forces within a molecule do not cancel out. Why are there forces in the first place? These exist because of the differences in “greediness” for electrons between different atoms. So Cl by itself has no bonds so it can’t even vibrate. Diatomic chlorine does not meet the criteria either because, like O2 and N2 in air, it has the same atom pulling on each side of the bond, and equally important, any type of stretching or bending will not cause or lead to a change in net force between the bonded atoms.
The neat and thing about CO2 is that it doesn’t have a permanent dipole moment, but it can experience a net force during an asymmetric stretch. This happens when one oxygen gets squished towards the carbon in the middle of the molecule while the other oxygen atom stretches away, leading to a change in dipole. That’s what makes carbon dioxide a greenhouse gas. If the oxygen atoms each pull away from the carbon, the net force will still equal zero. With no net change, the symmetric stretch cannot intensify and cannot absorb infrared energy.

co2 asymm
CO2’s asymmetric stretch, which leads to strong absorption of infrared Source for 3 gifs.
co2 symm
This symmetric stretch cannot intensify with incoming infrared red absorption.

There is also some absorption in another part of the infrared spectrum, although not as intense but still due to a change in dipole. It happens when the CO2 molecule bends.

co2 bend
A bending vibration of the CO2 molecule. The change in dipole occurs, implying that it can intensify by absorbing infrared.

At the rish2o symmk of belaboring the idea, let’s point look at the H2O molecule, a strong and more abundant greenhouse gas than CO2.  The latter has a linear shape. Due to  water’s angular shape, water already has an overall dipole.  Even with a symmetric stretch a change in dipole will result. Specifically, the movement shown increases the net pull of oxygen. As a result such a stretch can also be amplified when it absorbs infrared.

By the way, why don’t we focus any attention on our emissions of water vapor? Simplistically, one might attribute it to relative numbers. Air on average is about 2% H2O gas. (The percentage breakdowns for atmospheric composition that are normally given are for dry air.) So although the combustion of petroleum and natural gas emits H2O, relative to CO2, the water produced causes very little change in the overall percentage of water vapor. Moreover, coal, which produces the most CO2 per kWh, produces little water when burnt because hydrogen only makes up about 5% of coal.  There is a net movement of water from land to ocean of 37 trillion tons of water per year. Of this total, about 12 trillion tons of water is in the air at any one time because water vapor only stays in the air for an average of 10 days. That’s 12 000 gigatons, of which we add about 3 to 5 gigatons(Gton) annually, a change of 0.03%. In contrast, we add 9 Gtons of carbon dioxide annually. After an increased withdrawal by oceans and land, there is a net annual  input of 4 Gtons to a pool of 840 Gtons, or a 0.5% increase, more than 15 times bigger than that of water.

But far more importantly,  in the real world’s dynamics, CO2 and other non-H2O greenhouse gases, are the major limiting factor in the greenhouse effect.

Without non-condensing greenhouse gases[such as CO2], water vapor and clouds would be unable to provide the feedback mechanisms that amplify the greenhouse effect.

488311main_feedback-forcings

Other references for the above research:

Andrew Lacis. NASA Goddard Institute Space Studies. CO2: The Thermostat that Controls Earth’s Temperature 2010

Lacis, Hansen and al. NASA GISS  The role of long-lived greenhouse gases as principal LW control knob that governs the global surface temperature for past and future climate change 2013

….

I encountered the second question on a NASA blog. It’s based on a common misconception, but again it shows how even someone without the erroneous idea can still benefit from reading a thorough answer, such as the one given below by Rebecca Lindsey. The ozone hole and global warming are separate problems, but some of the minor connections between the two are rarely discussed in big media.

Are the ozone hole and global warming related ?

 By Rebecca LindseySeptember 14, 2010

The ozone hole and global warming are not the same thing, and neither is the main cause of the other.

The ozone hole is an area in the stratosphere above Antarctica where chlorine and bromine gases from human-produced chlorofluorocarbons (CFCs) and halons have destroyed ozone molecules.

Global warming is the rise in average global surface temperature caused primarily by the build-up of human-produced greenhouses gases, mostly carbon dioxide and methane, which trap heat in the lower levels of the atmosphere.

There are some connections between the two phenomena.

For example, the CFCs that destroy ozone are also potent greenhouse gases, though they are present in such small concentrations in the atmosphere (several hundred parts per trillion, compared to several hundred parts per million for carbon dioxide) that they are considered a minor player in greenhouse warming. CFCs account for about 13% of the total energy absorbed by human-produced greenhouse gases.

The ozone hole itself has a minor cooling effect (about 2 percent of the warming effect of greenhouses gases) because ozone in the stratosphere absorbs heat radiated to space by gases in a lower layer of Earth’s atmosphere (the upper troposphere). The loss of ozone means slightly more heat can escape into space from that region.

Global warming is also predicted to have a modest impact on the Antarctic ozone hole. The chlorine gases in the lower stratosphere interact with tiny cloud particles that form at extremely cold temperatures — below -80 degrees Celsius (-112 degrees Fahrenheit). While greenhouse gases absorb heat at a relatively low altitudes and warm the surface, they actually cool the stratosphere. Near the South Pole, this cooling of the stratosphere results in an increase in polar stratospheric clouds, increasing the efficiency of chlorine release into reactive forms that can rapidly deplete ozone.

  1. References:

  2. Allen, Jeannie. (2004, February 10). Tango in the Atmosphere: Ozone and Climate Change. Earth Observatory. Accessed: September 14, 2010.

  3. Baldwin, M.P., Dameris, M., Shepherd, T.G. (2007, June 15). How will the stratosphere affect climate change?Science, 316 (5831), 1576-1577.

  4. Intergovernmental Panel on Climate Change, (2007). Summary for Policymakers. In Climate Change 2007: The Physical Science Basis. Contribution of Working Group I to the Fourth Assessment Report of the Intergovernmental Panel on Climate Change. [Solomon, S., D. Qin, M. Manning, Z. Chen, M. Marquis, K.B. Averyt, M. Tignor, and H.L. Miller (eds.)]. Cambridge, United Kingdom, and New York, New York: Cambridge University Press.

  5. Ozone Hole Watch. Accessed: September 14, 2010.

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