Here's a set of videos on finding oxidation states and redox reactions (18 min, 18min, 14min): Khan Academy: Introduction to Oxidation States, on YouTube, Khan Academy: More on Oxidation States, on YouTube, Khan Academy: Redox Reactions, on YouTube
Here's another video discussion of redox (11 min): CrashCourse Chemistry, Redox Reactions, on YouTube
Want to learn and practice this material in a more interactive way? Check out my new project, Chemiatria!
Redox reactions are reactions in which electrons shift allegiance. Allegiance means loyalty or commitment to a group, like your allegiance to your family or your country. If you decide to leave your home and become a citizen of a new country, you have shifted allegiance. Of course, you might be able to share your allegiance between your old country and your new country by being a citizen of both countries at the same time. This is similar to what electrons do. They have allegiance to a nucleus, and sometimes during a reaction they will shift allegiance to a different nucleus. They might shift allegiance completely, if the new nucleus is much better for them than the old one, or they might shift partially, and share their loyalty between the two. (Also, electrons, like people, can spend some time near nuclei they don't belong to, just like you can visit countries where you don't live, and then go back near their main nucleus, although electrons' trips are much shorter than ours!)
All neutral atoms have a population of electrons equal to their number of protons. Atoms, just like countries, always have elite citizens that they treat really well (power, money...) and these elite citizens are way too confortable to shift allegiance unless they get kidnapped or something. Unlike countries, atoms always have only a small number of non-elite citizens, between 0 and 8, roughly, but usually no more than 3 or 4. These non-elite electrons will easily shift allegiance if they get a better offer.
What are the good parts of the periodic table? Fluorine is like heaven for electrons, they will basically never leave. If fluorine is accepting immigrants, electrons will leave anywhere else to move to F. Oxygen is second best. (Fluorine seems nasty to us because we don't want our citizen electrons to leave us for fluorine. We have jobs for them to do!) The noble gases are aloof nations, their populations of electrons hardly ever change. Their citizens don't want to leave, and they don't want any troublesome immigrants either. In general, the upper right corner of the periodic table are good comfortable nations to live in. The bottom left is such a bad place to live that electrons will destroy property in their rush to leave. (Watch this video for an example) This shows "standard of living" across the periodic table. Red elements will tend to keep their own electrons and attract electrons from other nuclei; yellow elements will tend to lose their non-elite electrons.
Ionic substances are like alliances between nations with really different standards of living. The non-elite electrons of the metal will try to shift allegiance to the non-metal. In molecular substances, also called covalent, the electrons will have dual citizenship, although they might have slightly greater loyalty to one nuclei than another, because these are alliances between similarly comfortable non-metal nuclei.
With all these dual citizen electrons, with partial loyalty to several nuclei, it can get hard to count the population of electrons at each nucleus. Oxidation number is one procedure for counting. In this procedure, electrons are counted as belonging completely to the nuclei they feel the greatest loyalty to. The number of electrons that feel primary loyalty to each nucleus is counted, and subtracted the number of protons in that nucleus. But there are shortcuts to make this faster using valence rules. For example, in CO2, oxygen has a normal valence of 2. This means that it can accept 2 extra citizen electrons per nucleus, so it has an oxidation number of -2. Therefore, because there are 2 O nuclei, carbon loses 4 citizen electrons, who transfer their primary loyalty to O. Thus, carbon has an oxidation number of +4, because it has 4 fewer truly loyal electrons than protons.
How to count oxidation states:
Oxidation refers to a process in which something loses electrons, and has its oxidation number increase. This usually happens to compounds that react with oxygen gas, which is why it is called oxidation. Reduction refers to a process in which something gains electrons, and its oxidation number is reduced. Actually this is not quite where the word came from. When a metal oxide is reduced to the elemental metal and an elemental non-metal or non-metal compound, the mass of solid decreases because the non-metal usually leaves as a gas. This reduction in quantity is where the word comes from. When you think about it, for one thing to be oxidized, another thing must be reduced (because electrons can't appear out of nowhere), which is why we often use the combination word redox.
Many of the reaction types we've already seen involve redox. For
instance, combination of elements is a redox reaction. Decomposition
reactions are often redox reactions. Combustion reactions are redox
reactions. (Dissolution/precipitation and acid-base are not redox
reactions.) The examples of redox you've already seen involve
electrons shifting loyalty at the same time that new bonds are made
between nuclei. It's also possible for electrons to abandon one
nucleus for another separate nucleus that isn't bonded to the first
one. For example, if an elemental metal is placed in a solution of
salt or acid, the metal can lose some electrons to the cation in the
solution. The metal forms cations, and the former cation usually
leaves the solution, forming hydrogen gas or elemental metal. Here
are some examples:
Zn(s) + 2HBr(aq) → ZnBr2(aq) + H2(g)
Fe(s) + Pb(NO3)2(aq) →
Fe(NO3)2(aq) + Pb(s)
All that happened in these reactions is that the disloyal electrons
left a less-comfortable home country for a more comfortable
one. These are called displacement reactions
because one cation is displaced (replaced) by another. To predict
what reactions will occur on this pattern, you can use the activity series, which is like a ranking of "worst
places to live". It's pretty good for making predictions, because in
general electrons will move to places lower on the list, but it
doesn't take into account all the specific circumstances, like
current population pressures. You'll learn more reliable ways to
make predictions later. The activity series is: