AN OX, RED CAT is a compact way to remember where oxidation and reduction happen in an electrochemical cell. AN OX means the anode is the site of oxidation, and RED CAT means reduction happens at the cathode. This rule matters because many cell problems begin by identifying which half-reaction loses electrons and which half-reaction gains electrons.
Once the electrodes are labeled correctly, electron flow, ion movement, and cell voltage become easier to track.
Understanding Chemistry: Which electrode undergoes which reaction (AN OX, RED CAT)
A useful next step is to connect electron transfer with changes in charge. When a metal atom leaves a solid electrode and enters solution, it becomes a positive ion because electrons are left behind in the metal. This can make that electrode lose mass over time.
At the other electrode, positive metal ions in solution can take electrons from the solid and form neutral metal atoms. These atoms may build up as a coating, so that electrode gains mass. Oxidation numbers give another way to track the same change.
An oxidation number becomes more positive during oxidation and more negative during reduction. This method is especially helpful when no solid metal is visible.
A complete cell needs a path for more than electrons. The wire carries electrons, but ions must move through the solutions to prevent charge from building up. A salt bridge often contains an inert salt dissolved in a gel.
Negative ions move toward the compartment where positive ions are being produced. Positive ions move toward the compartment where positive ions are being removed. Without this ion movement, one solution would become too positive and the other too negative.
The electric force caused by this imbalance would soon stop further electron transfer. The salt bridge does not supply the electrons used in the reaction. Its job is to keep each solution close to electrically neutral.
Students often confuse electrode names with electrode signs. The reaction names do not change, but the signs can change depending on the type of cell. In a galvanic cell, a spontaneous chemical reaction produces electrical energy.
The electrode releasing electrons is negative because it has an excess of electrons available for the wire. The electrode receiving electrons is positive. In an electrolytic cell, an external power supply forces a nonspontaneous reaction to occur.
The power supply pulls electrons away from one electrode and pushes them toward the other. This reverses the signs compared with a galvanic cell, even though the same reaction rule for each electrode still applies.
When solving a cell problem, write the two half reactions separately before trying to combine them. Check which substance changes into ions and which ions change into a substance. Balance atoms first, then balance charge by adding electrons.
Multiply an entire half reaction when needed so the number of electrons lost equals the number gained. Do not multiply electrode potentials when multiplying a half reaction. Voltage is an energy value per unit charge, so it is not changed by the amount of reaction written.
Batteries, metal plating, corrosion, and electrolysis all use these ideas. Careful labels for phases, charges, and electron direction prevent most mistakes.
Key Facts
- AN OX means oxidation occurs at the anode.
- RED CAT means reduction occurs at the cathode.
- Oxidation is loss of electrons: M(s) -> M^n+(aq) + ne^-.
- Reduction is gain of electrons: M^n+(aq) + ne^- -> M(s).
- In a galvanic cell, electrons flow through the wire from anode to cathode.
- For a Daniell cell: Zn(s) -> Zn^2+(aq) + 2e^- at the anode, and Cu^2+(aq) + 2e^- -> Cu(s) at the cathode.
Vocabulary
- Anode
- The electrode where oxidation occurs in any electrochemical cell.
- Cathode
- The electrode where reduction occurs in any electrochemical cell.
- Oxidation
- A reaction in which a substance loses electrons and its oxidation number increases.
- Reduction
- A reaction in which a substance gains electrons and its oxidation number decreases.
- Salt bridge
- A connection between half-cells that allows ions to move and keeps the solutions electrically neutral.
Common Mistakes to Avoid
- Reversing AN OX and RED CAT is wrong because oxidation is always at the anode and reduction is always at the cathode.
- Thinking the anode is always negative is wrong because electrode charge depends on whether the cell is galvanic or electrolytic, but anode oxidation never changes.
- Drawing electrons moving through the salt bridge is wrong because electrons travel through the external wire while ions move through the salt bridge.
- Labeling electrodes only by metal identity is wrong because the anode and cathode are determined by the half-reactions, not by the names of the metals alone.
Practice Questions
- 1 In a galvanic cell, zinc is oxidized according to Zn(s) -> Zn^2+(aq) + 2e^-. Which electrode is zinc, and how many moles of electrons are produced when 3.0 mol of Zn reacts?
- 2 Copper ions are reduced according to Cu^2+(aq) + 2e^- -> Cu(s). If 0.50 mol of Cu^2+ is reduced, how many moles of electrons are consumed, and is the copper electrode the anode or cathode?
- 3 A student says the anode must be the electrode where electrons enter the cell because anode sounds like attraction. Explain why this reasoning is incorrect using AN OX, RED CAT.