Author’s note: I do not claim to have coined this terminology or its’ meaning. I am simply relaying the information taught to me. I found it made redox chemistry a piece of cake – I hope you do too!
I have been inspired to write this post as a few of my students studying redox have never heard of the saying: AN OIL RIG CAT, which was taught to me by my chemistry teacher. It makes redox simple and easy and I cannot believe that educators out there either don’t know about this fabulous key, or are not relaying it to their students to ease their chemistry journey. But let’s start at the start at the start shall we?
Redox is the study of chemical reactions that undergo oxidation and reduction. In redox, oxidation cannot occur without reduction, and reduction cannot occur without oxidation. This all essentially means that elements lose or gain electrons to change their oxidation state (or oxidation number). Please see my previous post on assigning oxidation numbers.
Here’s how the key works:
Think about AN OIL RIG CAT separated into two halves:
AN OIL / RIG CAT
OIL: Oxidation Is Loss of electrons
Read: Increase in oxidation state
RIG: Reduction Is Gain of electrons
Read: Decrease or REDUCTION of oxidation state.
AN OIL: Oxidation occurs at the ANode
RIG CAT: Reduction occurs at the CAThode
Let’s illustrate how this works. Consider the ionic equation below, with assigned oxidation states as shown:
0 +2 +2 0
Fe(s) + Cu2+(aq) => Fe2+(aq) + Cu(s)
Iron is going from an oxidation state of 0 to +2, and therefore is losing 2 electrons to become more positive:
Fe(s) => Fe2+(aq) + 2e-
Iron is undergoing oxidation.
Copper is going from an oxidation state of +2 to 0, and therefore must gain 2 electrons to become neutral:
Cu2+(aq) + 2e- => Cu(s)
Copper is undergoing reduction.
The concept of oxidation/reduction and oxidising/reducing agents (or oxidants/reductants) also seems to cause students grief. When determining these
Elements undergoing oxidation are REDUCING AGENTS (reductants).
Elements undergoing reduction are OXIDISING AGENTS (oxidants).
In our example above,
Iron is undergoing oxidation, and is the reductant.
Copper is undergoing reduction, and is the oxidant.
Application to a galvanic cell
So how does this all tie in with the concept of a galvanic cell with anodes and cathodes? Well, let’s use our example. Each half equation for Iron and Copper as above makes up two half-cells for a galvanic cell. A blank schematic of a galvanic cell looks like this:
Below each beaker (which represents our half cell), write your equation. Each long rectangle in the beakers represents our electrodes, which are solid metal, and the solution will contain our ions for that half cell. See annotations added below:
Because oxidation occurs at the anode, and reduction occurs at the cathode, we can label the electrodes as being either the anode or the cathode. If you wish to simplify this, the anode has a negative charge, and the cathode has a positive charge, which can be denoted just as (-) or (+) if you wish. In this example I have included both descriptions:
Now we can show the direction of our charged particles. For the external circuit between the anode and the cathode, electrons flow from the anode to the cathode, which can be represented as below.
For the salt bridge (usually composed of KNO3), the K+ ions and the NO3- ions move in opposite directions to balance out the charge difference between the two half cells. The positive ions (K+) always move towards the cathode, and the negative ions (NO3-) always move towards the anode.
Finally (this step is optional), is to show which electrode is being eaten away (solid metal is being oxidised to aqueous ions), and which one is being coated (aqueous ions are being reduced to solid metal). This is tricky to show in a diagram, but here is my attempt below:
And there you have it:
Redox simplified to four simple words: AN OIL RIG CAT.