What is the Difference Between Cell Potential and EMF?

The crucial difference between cell potential and EMF (electromotive force) lies in whether or not the electrochemical cell is actively supplying current to an external circuit. EMF is the theoretical maximum potential difference when no current flows, while cell potential is the actual voltage measured when current is flowing.

Detailed Explanation

• Electromotive Force (EMF):

  • EMF represents the driving force behind the flow of electrons in an electrochemical cell. It's the maximum potential difference between the two electrodes (anode and cathode) when no current is being drawn from the cell. Think of it as the "ideal" voltage the cell could provide.
  • EMF is often denoted as E°cell under standard conditions (298K, 1 atm pressure, 1 M concentration).
  • It's determined by the difference in standard electrode potentials of the half-cells involved.
  • Measurement: EMF is measured using a potentiometer under zero current conditions.
  • Analogy: Imagine a water tank at a certain height. The EMF is like the potential energy of the water due to its height, representing the potential to do work if a tap is opened.

• Cell Potential:

  • Cell potential (also sometimes referred to as terminal voltage) is the actual voltage reading obtained when the electrochemical cell is connected to an external circuit and current is flowing.
  • It is always less than the EMF due to various factors such as:
    • Internal Resistance (r): The electrochemical cell itself offers some resistance to the flow of current. This causes a voltage drop within the cell (IR drop).
    • Polarization: Electrode polarization effects cause a decrease in cell potential.
    • Concentration Changes: As the cell operates, reactant concentrations decrease, and product concentrations increase, shifting the equilibrium and lowering the cell potential (as described by the Nernst equation).
  • Analogy: Continuing the water tank analogy, cell potential is like the actual water pressure at the tap when the tap is open and water is flowing. It will be less than the ideal potential energy because of friction in the pipes and the rate at which water is drawn.

Key Differences Summarized:

In short, EMF is the theoretical maximum voltage a cell can deliver, while cell potential is the actual voltage observed when the cell is working and supplying current. The difference arises from internal resistance and other factors that impede the flow of current within the cell itself.