The cell potential (E cell) is calculated by determining the difference in reduction potentials between the cathode (where reduction occurs) and the anode (where oxidation occurs).
Understanding Cell Potential
Cell potential, often denoted as E cell, signifies the potential difference between the two half-cells of an electrochemical cell. This difference is a crucial factor in determining the direction and spontaneity of a redox reaction.
Key Concepts
- Reduction Potential: A measure of the tendency of a chemical species to gain electrons and be reduced. More positive reduction potentials indicate a greater tendency for reduction.
- Oxidation Potential: The tendency of a chemical species to lose electrons and be oxidized.
- Cathode: The electrode where reduction occurs.
- Anode: The electrode where oxidation occurs.
Calculating E cell
The calculation for cell potential is straightforward:
E cell = E (cathode) - E (anode)
Or, as stated in the provided reference:
E cell = E (reduced) - E(Oxidised)
Here's how to use this formula:
- Identify the Half-Reactions: Determine the oxidation and reduction half-reactions occurring in the electrochemical cell.
- Find Standard Reduction Potentials: Look up the standard reduction potential (E°) for each half-reaction. These values are typically found in a standard reduction potential table.
- Apply the Formula: Plug the values into the formula E cell = E°(cathode) - E°(anode). Remember, the reduction potential of the cathode is always the one with a more positive value.
Example Calculation
Let’s consider a simple example of a zinc-copper galvanic cell:
Half-Reactions:
- Reduction (at the Cathode, Copper): Cu2+(aq) + 2e- → Cu(s) E° = +0.34 V
- Oxidation (at the Anode, Zinc): Zn(s) → Zn2+(aq) + 2e- E° = -0.76 V
Calculation:
E cell = E°(cathode) - E°(anode)
E cell = (+0.34 V) - (-0.76 V)
E cell = +1.10 V
The positive E cell value indicates that the reaction is spontaneous under standard conditions.
Practical Insights
- Positive E cell: A positive E cell value indicates a spontaneous reaction, meaning the reaction will proceed in the forward direction, and the cell is able to produce electrical energy (galvanic or voltaic cell).
- Negative E cell: A negative E cell indicates a non-spontaneous reaction. To force such a reaction to occur, external energy must be applied (electrolytic cell).
- Standard Conditions: Standard cell potentials are usually determined under standard conditions (1 M concentration, 298 K, 1 atm pressure). However, the Nernst equation is used to determine the cell potential at non-standard conditions.
Summary
The cell potential (E cell) is a measure of the overall tendency of a redox reaction to occur. A more positive reduction potential indicates that a species is more easily reduced. By calculating E cell using the standard reduction potentials, we can determine whether a reaction is spontaneous under standard conditions. The reference clarifies that E cell = E (reduced) - E(Oxidised) which is crucial for calculating the cell potential.
