Why does an electrochemical cell drain?
Imagine zinc at the anode and copper at the cathode. The zinc forms a Zn2+ electrolyte and the electrons travel through the circuit. They wind up at the copper cathode and are then transferred into the electrolyte that contains positive copper ions.
A K2SO4 salt bridge connects the Zn2+ and Cu2+ electrolytes. The traditional explanation is that the salt bridge acts as a source of negative charges (sulfate anion) to the anode, and a source of positive charges (potassium cation) to the cathode. In this explanation, if it weren't for this replenishment, the anode would start to lose its negative charge as the electrons are sent through the wire to the cathode. The cathode would in turn start to lose its positive charge as the stream of electrons would neutralize its charge. The voltage between the anode and cathode would decrease and current would stop flowing. Even with the salt bridge, of course, the ion concentrations in the salt bridge eventually diminish and the battery eventually drains.
The problem with this explanation is that it (seemingly) ignores the main requirement for a circuit to be complete in order for current to exist. If nothing besides the ion exchange described above occurs in the salt bridge, then how is the circuit closed? Without a closed circuit, there is voltage, but there is no current and so a lamp would not light up even for a millisecond if it were placed in the circuit.
Another explanation that seemed to make sense to me was that the K+ ions in the salt bridge migrate toward the copper cathode side and accept the electrons that are coming into the electrolyte from the circuit through a porous membrane. These electrons are transferred to the SO42- side of the salt bridge. The SO42- ions have migrated to this side because they are attracted by the zinc cations in the anode elctrolyte. The electrons then pass through the SO42- ions, into the electrolyte and eventually back into the wire. The problem with this explanation is that it doesn't account for how the battery eventually drains.
Please help
Imagine zinc at the anode and copper at the cathode. The zinc forms a Zn2+ electrolyte and the electrons travel through the circuit. They wind up at the copper cathode and are then transferred into the electrolyte that contains positive copper ions.
A K2SO4 salt bridge connects the Zn2+ and Cu2+ electrolytes. The traditional explanation is that the salt bridge acts as a source of negative charges (sulfate anion) to the anode, and a source of positive charges (potassium cation) to the cathode. In this explanation, if it weren't for this replenishment, the anode would start to lose its negative charge as the electrons are sent through the wire to the cathode. The cathode would in turn start to lose its positive charge as the stream of electrons would neutralize its charge. The voltage between the anode and cathode would decrease and current would stop flowing. Even with the salt bridge, of course, the ion concentrations in the salt bridge eventually diminish and the battery eventually drains.
The problem with this explanation is that it (seemingly) ignores the main requirement for a circuit to be complete in order for current to exist. If nothing besides the ion exchange described above occurs in the salt bridge, then how is the circuit closed? Without a closed circuit, there is voltage, but there is no current and so a lamp would not light up even for a millisecond if it were placed in the circuit.
Another explanation that seemed to make sense to me was that the K+ ions in the salt bridge migrate toward the copper cathode side and accept the electrons that are coming into the electrolyte from the circuit through a porous membrane. These electrons are transferred to the SO42- side of the salt bridge. The SO42- ions have migrated to this side because they are attracted by the zinc cations in the anode elctrolyte. The electrons then pass through the SO42- ions, into the electrolyte and eventually back into the wire. The problem with this explanation is that it doesn't account for how the battery eventually drains.
Please help