This cheat sheet helps students remember which electrode has oxidation and which electrode has reduction in electrochemical cells. The key memory aid is AN OX, meaning anode oxidation, and RED CAT, meaning reduction cathode. This is useful when labeling galvanic cells, electrolytic cells, half-reactions, electron flow, and electrode signs.
It keeps the most tested rules in one clear reference for chemistry problem solving.
The anode is always where oxidation occurs, so electrons are produced there in a half-reaction such as . The cathode is always where reduction occurs, so electrons are consumed there in a half-reaction such as . In a galvanic cell, the anode is negative and the cathode is positive, while in an electrolytic cell, the anode is positive and the cathode is negative.
Electrons flow through the wire from anode to cathode, even though ions move through the solution or salt bridge to keep charge balanced.
Key Facts
- AN OX means oxidation always happens at the anode.
- RED CAT means reduction always happens at the cathode.
- Oxidation is loss of electrons, shown by a half-reaction such as .
- Reduction is gain of electrons, shown by a half-reaction such as .
- Electrons flow through the external wire from anode to cathode.
- In a galvanic cell, the anode is and the cathode is .
- In an electrolytic cell, the anode is and the cathode is .
- The cell voltage for a galvanic cell is found by .
Vocabulary
- Anode
- The electrode where oxidation occurs in any electrochemical cell.
- Cathode
- The electrode where reduction occurs in any electrochemical cell.
- Oxidation
- A process in which a substance loses electrons and its oxidation number increases.
- Reduction
- A process in which a substance gains electrons and its oxidation number decreases.
- Galvanic cell
- An electrochemical cell that uses a spontaneous redox reaction to produce electrical energy.
- Electrolytic cell
- An electrochemical cell that uses electrical energy to drive a nonspontaneous redox reaction.
Common Mistakes to Avoid
- Saying the anode is always negative is wrong because the anode is negative only in a galvanic cell and positive in an electrolytic cell.
- Saying the cathode is always positive is wrong because the cathode is positive only in a galvanic cell and negative in an electrolytic cell.
- Reversing electron flow is wrong because electrons leave the anode and travel through the wire to the cathode.
- Calling oxidation gain of electrons is wrong because oxidation is electron loss, as in .
- Choosing the electrode by charge before identifying the cell type is wrong because electrode signs depend on whether the cell is galvanic or electrolytic.
Practice Questions
- 1 In the half-reaction , does the reaction occur at the anode or cathode?
- 2 In the half-reaction , does the reaction occur at the anode or cathode?
- 3 For a galvanic cell with and , calculate using .
- 4 Why does the phrase AN OX, RED CAT still work for both galvanic and electrolytic cells even though the electrode signs change?
Understanding Which electrode undergoes which reaction (AN OX, RED CAT) Memory Aid
A useful way to understand electrode reactions is to track what happens to atoms, not just memorise labels. When a metal atom loses electrons, it becomes a positive ion and may leave the metal strip. The strip can slowly lose mass.
At the other electrode, positive metal ions in solution gain electrons and form solid metal atoms. This can make that electrode gain mass or become coated with metal. These visible changes are evidence for the half reactions.
The electrons written in half reactions must cancel when the two reactions are added. They do not build up in the overall cell equation.
Charge balance is essential for a working cell. If electrons leave one container, that solution can become too positive because metal ions enter it. If positive ions are removed from the other container, that solution can become too negative.
A salt bridge prevents this charge buildup from stopping the reaction. Negative ions from the bridge move toward the container where positive charge is increasing. Positive ions move toward the container where positive ions are being used up.
The salt bridge completes the circuit through ion movement. It does not carry electrons through the solution in the same way that a metal wire does.
The signs on electrodes cause many mistakes because they depend on the type of cell. In a battery that produces electrical energy on its own, the electrode supplying electrons is negative. The other electrode receives those electrons and is positive.
An external power supply can force a nonspontaneous reaction in an electrolytic cell. Its positive terminal pulls electrons from one electrode, while its negative terminal pushes electrons to the other electrode. This reverses the electrode signs compared with a galvanic cell.
The reaction locations do not change. Focus first on electron loss and electron gain, then decide the signs from the cell type.
Students meet these ideas in batteries, metal plating, corrosion, and electrolysis. Rusting involves oxidation of iron, while another substance is reduced nearby. Electroplating uses electrical energy to deposit a thin layer of metal onto an object.
The object being coated is connected so that metal ions can gain electrons on its surface. In calculation questions, use reduction potential values carefully. Tables usually list both half reactions as reductions.
Keep the cathode value as written, subtract the anode reduction value, and do not multiply a potential by a coefficient when balancing electrons. A positive cell voltage under standard conditions indicates a spontaneous galvanic reaction. Check each diagram by tracing the wire, the ion paths, and the changing masses before choosing an answer.