Why Does a Battery Run Down?

2011 April 22
by roma

Why does a battery run down?

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 K+ ions in the salt bridge migrate toward the copper electrolyte 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 electrolyte. The electrons then pass through the SO42- ions, into the electrolyte and eventually back into the wires.

What causes the battery to run down?

2 Responses leave one →
  1. roma permalink*
    April 25, 2011

    I still haven't resolved this problem. I posted the question on openstudy.com, and the answer I received was the way that my chemistry book describes it. 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 ions 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?

    Still searching for the answer. I will try posting this on allaboutcircuits.com, of which I recently became a member.

  2. roma permalink*
    April 25, 2011

    Received a pretty good response at the allaboutcircuits.com forum.

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