Why Do Electrons Matter in Electrochemistry?
In any redox reaction, oxidation and reduction happen simultaneously:
- Oxidation is the loss of electrons.
- Reduction is the gain of electrons.
Tip
To remember this, use the mnemonic OIL RIG:
- Oxidation Is Loss (of electrons).
- Reduction Is Gain (of electrons).
In electrochemical cells, these processes are separated into two locations: the anode and the cathode. Let’s explore how this works.
Oxidation at the Anode
The anode is where oxidation occurs. At this electrode:
- A species loses electrons, which are then transferred through an external circuit.
- The electrons flow away from the anode toward the cathode.
Example
In a zinc–copper voltaic cell:
- At the zinc anode: $$Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$$
- Zinc is oxidized, releasing electrons.
Common Mistake
- Students often confuse the anode as always being "negative."
- This is true for voltaic cells but not for electrolytic cells.
- Always consider the type of cell before assigning polarity.
Reduction at the Cathode
- The cathode is where reduction occurs.
- At this electrode a species gains electrons from the external circuit.
Example
In the same zinc–copper voltaic cell:
- At the copper cathode: $$Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$$
- Copper ions are reduced to solid copper.
Electrode Polarity: Voltaic vs. Electrolytic Cells
The polarity of the electrodes depends on the type of electrochemical cell. Let’s compare the two:
Voltaic (Primary) Cells
Voltaic cells generate electrical energy from spontaneous redox reactions.
Example
In a Daniell cell (zinc–copper primary cell):
- Anode (oxidation): $Zn(s) \rightarrow Zn^{2+}(aq) + 2e^-$ (negative).
- Cathode (reduction): $Cu^{2+}(aq) + 2e^- \rightarrow Cu(s)$ (positive).
Electrolytic Cells
Electrolytic cells use electrical energy to drive non-spontaneous redox reactions.
Example
In the electrolysis of water:
- Anode (oxidation): $2H_2O(l) \rightarrow O_2(g) + 4H^+(aq) + 4e^-$ (positive).
- Cathode (reduction): $4H^+(aq) + 4e^- \rightarrow 2H_2(g)$ (negative).
Note
In electrolytic cells, the anode is positive because it is connected to the positive terminal of the power supply, which attracts anions.
Self review
- In which type of cell is the cathode positive?
- What happens at the anode of an electrolytic cell?
- Why does the polarity of electrodes differ between voltaic and electrolytic cells?
