Redox can be a confusing topic for VCE Chemistry students. It’s also taught right at the end of the year, when students are tired and some teachers are rushing their lessons so they can finish the course before the end of Term 3. Student motivation levels are at their lowest time of the year, which means that students often finish the course with an incomplete understanding of Redox.
Fortunately, there are six universal principles that are always true in Redox no matter what type of cell is being studied.
First, here’s a reminder of the types of cells you need to have studied in this course.
- Primary (can’t be recharged)
- Secondary (can be recharged)
- Fuel Cells (reactants are supplied continuously)
- Electroplating Cells (no overall reaction)
- Electrolytic Cells (non-spontaneous reaction)
- Commercial Cells (usually molten electrolytes)
- Recharge reaction of a secondary cell (non-spontaneous)
Now, here are the six universal Redox principles.
1. The strongest oxidant at the cathode reacts with the strongest reductant at the anode (SOC SRA)
To predict which species will react with each other, circle all the species present at the cathode on the electrochemical series. The highest species on the left will always react. Now, circle all the species present at the anode… the lowest species on the right will react.
2. The half-reaction with the highest E° value is always positive
In all cells, the half-equation with the highest electrode potential (also called ‘reduction potential’ or E° value) always occurs at the positive electrode. Similarly, the half-equation with the lowest electrode potential (E°) will always occurs at the negative electrode.
3. OIL RIG
Oxidation is loss of electrons. Reduction is gain of electrons.
4. ←AN OIL RIG CAT→
Anode reaction (oxidation reaction) is whichever reaction is happening to the left in the electrochemical series.
Cathode reaction (reduction reaction) is whichever reaction is happening to the right in the electrochemical series.
5. Electrons always flow in this order (RACO)
Reductant → anode → cathode → oxidant
6. In the internal circuit, cations always flow to the cathode, and anions always flow to the anode.
The internal circuit might be an electrolyte or a salt bridge that contains soluble weak oxidants and reductants such as KNO3(aq) (potassium nitrate). Either way:
- cations always flow to the cathode; and
- anions always flow to the anode.
Keep practicing redox questions by completing past papers, Checkpoints and Lisachem questions. If you need more help, contact me via the Get a Tutor button in the site’s menu bar. Students learn much faster with a tutor than on their own.