Integrated SequencePhysics Chemistry Organic Biology

Web Resources

Purdue University - Oxidation Numbers
Good treatment of the rules for assigning oxidation numbers.



  click if a link is broken



Special points of emphasis

Electricity

Periodic Properties

The Chemical Bond

Thermochemistry

Oxidation-Reduction

I was thinking about the oxidation number system a few days ago when I was helping my son learn long division. Long division is a 'method' that gets you to the correct answer, but it represents a step in learning mathematics, where the operation of solving the problem has almost completely detached from the conceptual underpinnings. You move the decimal point here. You keep your columns straight. You follow the steps, and don't have to really think about what is going on underneath. You get the answer.

Oxidation-reduction is analogous to long division within chemistry, and few students are ever really challenged to see the underpinnings of the system, even though to do so makes redox much more coherent. What, at heart, is going on in a chemical reaction? Defamiliarize your thoughts. Look at it in terms of the barest essentials. On one side of the reaction you have charge densities, nuclei and electrons, in one form, the reagents. On the other side of the reaction, these charge densities are in a different form, the products. As we have discussed, you can imagine the electrostatic potential energy change from reagents to products by imagining pulling apart the reagents and letting them fall together as the products. If the volume and temperature are constant, this electrostatic potential energy change maps directly onto enthalpy change.

If the chemical bonds in the reagents are weak, it does not take a great deal of energy to break them apart, and if the chemical bonds in the products are strong, a great deal of energy is released as the nuclei and electrons fall together into a deep potential energy well. Such a reaction would have large negative enthalpy change. What does this have to do with oxidation-reduction? Oxidation-reduction is a systematic way that chemists have developed, which allows you to easily predict the enthalpy change of the reaction in a simple step like way. When an element with a high reduction potential (high electronegativity) forms a new chemical bond, it pulls the electron density within the bonding orbital inwards towards its electron greedy nucleus. Elements like oxygen or fluorine have a big powerful nucleus shielded by only a thin layer of electrons, so when they form new bonds, there is a potential energy decrease not only because of molecular orbital formation, but also because they are 'siezing electron control', i.e. pulling negative charge inwards. You would have to do a lot of work to get the electrons away from such an element, so very electronegative, high reduction potential elements form strong, low energy bonds. By assigining the elements reduction potentials, and then keeping track of shifts in electron control using the oxidation numbers, oxidation-reduction gives one an easy way to account for the energy changes within a chemical reaction as a narrative of electron control. The oxidation state or oxidation number of an element is a shorthand way of accounting for the shifts (or transfers) of electron density from one atom to another when compounds form. When the electron density is shifted toward the more electronegative element, that atom is said to have control over the electrons in redox terms, and its oxidation number decreases; the element having lost electron control in the compound gets a positive change to oxidation number.








The WikiPremed MCAT Course is a free comprehensive course in the undergraduate level general sciences. Undergraduate level physics, chemistry, organic chemistry and biology are presented by this course as a unified whole within a spiraling curriculum.

Please read our policies on privacy and shipping & returns.  Contact Us.
MCAT is a registered trademark of the Association of American Medical Colleges, which does not endorse the WikiPremed Course.


Creative Commons License
The work of WikiPremed is published under a Creative Commons Attribution Share Alike 3.0 License. There are elements of work here, such as a subset of the images in the archive from WikiPedia, that originated as GNU General Public License works, so take care to follow the unique stipulations of that license in printed reproductions. You can use the resources here for commercial or non-commercial purposes, but please give attribution and a link to the production credits and edit history of the resource. For the works here which began as my individual work, please attribute "John Wetzel, an author at wikipremed.com".