Standard reduction potentials Knowledge-Based Study

Study notes

1. What is Standard Reduction Potential?

   In electrochemistry, the tendency of a chemical species to acquire electrons and undergo reduction is quantified by its reduction potential. When measured under standard thermodynamic conditions, this value is termed the standard reduction potential, denoted as $E^\circ$.

   Standard conditions require:

   • Solute concentrations of exactly 1 M for all aqueous species.
   • A partial pressure of 1 atm (or 1 bar) for all gases involved.
   • A reference temperature of 298.15 K (25 °C).
   • Pure solids and pure liquids in their standard states.

   By international convention established by IUPAC, all standard electrode potentials are reported as reduction half-reactions:

   $\text{Oxidized form} + n\text{e}^- \rightarrow \text{Reduced form}$

   The Reference Zero

   Because an isolated half-reaction cannot occur independently, an absolute electrode potential cannot be measured directly. Instead, all standard reduction potentials are determined relative to a universally accepted reference: the Standard Hydrogen Electrode (SHE).

   The SHE consists of a platinized platinum electrode immersed in an acidic solution with a hydronium ion concentration of 1 M, over which hydrogen gas is bubbled at a pressure of 1 atm at 25 °C. The standard reduction potential of the SHE is defined as exactly zero volts at all temperatures:

   $2\text{H}^+(aq) + 2\text{e}^- \rightarrow \text{H}_2(g) \quad E^\circ = 0.00\text{ V}$

   When any half-cell is paired with the SHE under standard conditions, the measured cell potential directly yields the standard potential of that half-cell.

   Relative Oxidizing and Reducing Strengths

   Standard reduction potentials are compiled into an electrochemical series. Interpreting the magnitude and sign of $E^\circ$ values provides direct insight into chemical reactivity:

   • High positive $E^\circ$ values: Indicate a strong thermodynamic tendency to gain electrons. Species with very positive reduction potentials (such as $\text{F}_2$ with $E^\circ = +2.87\text{ V}$) are powerful oxidizing agents.
   • Negative $E^\circ$ values: Indicate that the species is less easily reduced than $\text{H}^+$. Instead, the corresponding reduced form has a strong tendency to lose electrons. Species with large negative values (such as $\text{Li}^+$ with $E^\circ = -3.04\text{ V}$) correspond to reduced forms (e.g., $\text{Li}(s)$) that act as powerful reducing agents.

   Key Rule: A species on the left side of a higher reduction half-reaction can spontaneously oxidize a species on the right side of a lower reduction half-reaction under standard conditions.

   Predicting Galvanic Cell Voltage

   In an electrochemical cell, oxidation takes place at the anode and reduction takes place at the cathode. The standard cell potential, $E^\circ_{\text{cell}}$, represents the difference between the reduction potentials of the two half-cells:

   $E^\circ_{\text{cell}} = E^\circ_{\text{cathode}} - E^\circ_{\text{anode}}$

   When using this equation, both $E^\circ_{\text{cathode}}$ and $E^\circ_{\text{anode}}$ are substituted directly as standard reduction potentials from the table; the sign of $E^\circ_{\text{anode}}$ is not inverted beforehand.

   Important Properties:

   • Intensive Property: Standard reduction potential does not depend on the amount of substance reacting. Multiplying stoichiometric coefficients in a half-reaction by an integer does not change the value of $E^\circ$.
   • Spontaneity Criterion: A positive standard cell potential ($E^\circ_{\text{cell}} > 0$) corresponds to a thermodynamically spontaneous reaction under standard state conditions ($\Delta G^\circ < 0$).