Introduction to Oxidation-Reduction

As described in the opening segment to this chapter, electrochemistry plays a very big role in many real world processes. It accounts for the way a your cell phone works, or your calculator, even your car. Electrochemistry can be very complex to understand because there are so many reactions that lie in the background of it and to understand it, we will need to learn a little bit of the background of electrochemistry which involves a type of chemical reaction know as oxidation-reduction reactions. Remember that when atoms interact with one another, there is a transfer of electrons that occurs as well as shared. Now looking at the bigger picture, when molecules interact with one another and reactions occur, there is a transfer of electrons. These reactions where there are one or more electrons transferred are known as oxidation-reduction reactions.

You can see this reaction occur when you mix solid sodium and chlorine gas. Solid sodium contains neutral sodium atoms and chlorine gas is made up of diatomic neutral compound as well.

The reaction above shows that two neutral compounds, when combined together will create a solid material which contains Na+ and Cl - ions. Notice here that those atoms could not have gotten those charges, if it was not for the transferring of electrons that occurred in the reaction.

None of the products formed are ionic in nature but for the products to be formed, there had to be transfer of some electrons from the carbon atom to the oxygen atom. To explain this, we need to go over oxidation states.

Oxidation States

Now that you know the basic rules for assigning oxidation states, lets look at some examples.

Example 7.1.1

Let’s assign some oxidation states for the following compounds

i). SO2

ii.) PCl5

If we are going to assign oxidation states to everything, the methane is neutral in charge so hydrogen would have a +1 for each atom totaling +4, meaning then that carbon has a -4 oxidation state. O2 is elemental oxygen in its diatomic state so it has an oxidation state of zero. CO2 is neutral as well, meaning that since O2 has an oxidation state of -4 (-2 for each O atom), carbon must have one of +4. H2O is the same way, its neutral so each hydrogen atom has a oxidation state of +1, while oxygen has it at -2.

In this reaction, there are no ionic compounds but everything can still be characterized with the flow and transfer of electrons. Notice here how the carbon changes from a -4 to a +4 oxidation state when being converted from CH4 to CO2. This means that there were 8 electrons that were transferred over.

Image 1

Oxygen goes from oxidation number 0 in O₂ to −2 per oxygen atom in the combustion products. For methane combustion, CH₄+2O₂→CO₂+2H₂O, four oxygen atoms each gain two electrons in the bookkeeping, for eight electrons total. The total electron transfer is eight; no individual oxygen has oxidation number −8.

Image 2

Important to note here that hydrogen doesn’t change oxidation states so it is not involved in the transfer of electrons.

Since we have a better understanding of this now, it is important to learn some new terms. Oxidation refers to an increase in oxidation state, so when an atom has in increase in oxidation state (losing electrons), it is normally said to be oxidized. Reduction is attributed to a decrease in oxidation state (gaining electrons) so atoms are therefore reduced.

Now we’re going to let you in here in a little secret on how to remember that little trick. Actually, its not really a trick because we’re sure you learned it one way or another. If this is new to you, then pay attention, cuz it’ll help!

You read that right, a lion named LEO, goes GER, not grrrrrr, but GER. Well the sound and actual action of a lion saying “grrr” is correct, but for acronym purposes, GER is better for us.

So what does it mean? Well here is a nice breakdown:

Image 3

So when electrons are lost, the unit is oxidized. When electrons are gained, they are reduced. So, every time you forget which is which, just remember LEO the lion goes GER.