An electrochemical cell (galvanic cell) stops working after some time because which of the following conditions is eventually reached between its electrodes and solutions?

Introduction / Context:
Electrochemical cells, such as galvanic or voltaic cells, convert chemical energy into electrical energy through redox reactions. Over time, however, such cells eventually stop delivering current, and the lamp or device they power goes out. This question asks you to identify the main reason why a cell stops working, focusing on the concept of electrode potentials and cell electromotive force (EMF). Understanding this helps link practical battery behaviour with theoretical electrochemistry.

Given Data / Assumptions:
- We are dealing with an electrochemical cell that initially produces a potential difference (EMF). - Over time, the cell stops working, meaning it can no longer drive a current through an external circuit. - Options mention electrode potentials becoming equal or zero, electrode disappearance, and reaction reversal. - We assume a typical galvanic cell with two half-cells and a salt bridge.

Concept / Approach:
The EMF of a galvanic cell arises from the difference in electrode potentials between the two half-cells. As the cell operates, reactants are consumed and products accumulate, gradually changing the concentrations in each half-cell. Eventually, the system approaches equilibrium, the forward and reverse reaction rates become equal, and the difference in electrode potentials falls to zero. At that point, there is no driving force for electron flow, and the cell can no longer deliver current. The correct approach is therefore to identify the option that states the electrode potentials equalise, eliminating net EMF.

Step-by-Step Solution:
Step 1: Recall that cell EMF is given by E_cell = E_cathode - E_anode. Step 2: As the cell discharges, ion concentrations in the two half-cells change and the Nernst equation shows that electrode potentials shift. Step 3: When the reaction reaches equilibrium, the cell EMF becomes zero because the driving force for further reaction disappears. Step 4: Mathematically, this corresponds to E_cathode = E_anode, so their difference is zero. Step 5: Option a states that the electrode potential of both electrodes becomes equal, so no net EMF remains. This matches the theoretical explanation. Step 6: Options b, c, and d either exaggerate or misrepresent the behaviour and do not correctly explain the underlying equilibrium condition.

Verification / Alternative check:
Electrochemistry textbooks explain that a galvanic cell functions as long as its overall redox reaction is thermodynamically favourable, which means it is not at equilibrium. Over time, as the reaction proceeds, the Gibbs free energy change approaches zero and so does the cell EMF. At equilibrium, the reaction quotient equals the equilibrium constant, and the Nernst equation yields zero EMF. This theoretical description aligns perfectly with the idea that the electrode potentials equalise, causing the cell to stop working.

Why Other Options Are Wrong:
Option b suggests that one of the electrodes completely vanishes, which may happen in some extreme cases over long periods, but a cell can stop working even before that happens, once equilibrium is reached. Option c claims that the reaction reverses its direction, but at equilibrium the forward and reverse reactions occur at equal rates, resulting in no net change, not continuous reversal with net current. Option d implies that both electrode potentials are zero from the start, which would mean the cell never worked; in reality, they only converge towards equality over time, not necessarily to zero. Hence, these explanations are either incomplete or incorrect.

Common Pitfalls:
A typical mistake is to imagine physical disappearance of an electrode as the main reason for cell failure, ignoring the crucial role of equilibrium. Another pitfall is thinking that the reaction somehow stops abruptly for no reason, without linking it to changes in concentration and electrode potential. Remembering that cell EMF is driven by a difference in electrode potentials, and that this difference goes to zero as equilibrium is approached, will help you understand and answer such questions correctly.

Final Answer:
An electrochemical cell stops working because the electrode potentials of both electrodes become equal and the net EMF falls to zero.