21.1    Standard Electrode Potential

Electrode potentials and half-cells

• • When a metal, M is immersed into water, there is a tendency that it will lose electrons and enter the water as metal ions, Mª⁺. Soon, the water becomes a solution of the metal ions. This leaves the electrons on the metal and the metal becomes more and more negative.
    M(s) → Mª⁺(aq) + ae⁻     , electrons are left behind the metal
  • The positive metal ions in the solution will be attracted towards the negative metal. Eventually some will accept the electrons and re-form the metal.
    Mª⁺(aq) + ae⁻ → M(s)    , ions from solution deposited

• When the rate of these two reactions becomes equal, an equilibrium is established. At this equilibrium, the metal goes into the solution at a rate exactly same as the ions depositing
  Mª⁺(aq) + ae⁻ ⇌ M(s)

• Different metals will have different tendencies of doing so. Some will lose electrons more readily than others. Reactive metals like magnesium prefer to stay as ions, therefore the position of equilibrium lies further to the left. Oppositely, unreactive metals like copper prefer to stay as metals, therefore the position of equilibrium lies further to the right

• This arrangement of a metal dipping into a solution of its ions is called a half-cell. The metal in a half-cell is called an electrode.

• Since there is a difference in charge between the negative metal and the positive metal ion solution, a potential difference exists between This potential difference is called the electrode potential,  E. Electrode potential is measured in volts, V.

 

• Electrode potential is also a numerical method to express the tendency of a metal to form ions.

 

• The bigger the difference between the negativeness and positiveness, the greater the electrode potential. However, this electrode potential is impossible to measure.

 

• However, the difference of electrode potentials between two half-cells is measurable. This can be done by connecting a wire between the two electrodes with a voltmeter between them

 

• Hence, to standardise, a standard half-cell must be chosen as the reference electrode so that all electrode potentials measured are relative to this

 

• This standard half-cell is called the standard hydrogen electrode, SHE.

 

Standard hydrogen electrode, SHE

• A standard hydrogen electrode has hydrogen gas in equilibrium with hydrogen The electrode potential of this half-cell is taken as 0 V.
  2H⁺(aq) + 2e⁻ ⇌ H2(g)          E = 0 V

 

• Since hydrogen is not a metal, platinum foil covered in porous platinum is used as the electrode. Platinum also catalyses the set up of the equilibrium

 

• Hydrogen gas at 100 kPa is bubbled over the On the surface of the platinum, the equilibrium is set up. The concentration of hydrogen ions is at 1 mol dm⁻³.

 

Standard electrode potential, E°

 

• Standard electrode potential, E° is the m.f. of a cell when a half-cell is connected to a standard hydrogen electrode under standard conditions.

• The standard conditions are:
  1. A pressure of 100 kPa(approximately atmospheric pressure).
  2. A temperature of 298 K or 25 °C.
  3. The ions at a concentration of 0 mol dm⁻³.

• A standard condition is required because all these factors will affect the position of equilibrium of the reaction, therefore the magnitude of E° will also be affected.

 

• The standard electrode potential of a cell can be measured by connecting a half-cell to a standard hydrogen electrode like this:

• Recall that electrode potential measures the tendency of a metal to lose its electrons. Standard electrode potential compares this tendency with the tendency of hydrogen to release its electrons
• • A negative value of E° implies that the metal loses electrons more readily than hydrogen does. Therefore the position of equilibrium lies further to the left.
  • A positive value of E° implies that the metal loses electrons less readily than hydrogen does. Therefore the position of equilibrium lies further to the right.
    

• Remember that standard electrode potential is all about A positive E° does not mean the position of equilibrium is exactly at the right. It is further to the right compared to the position of equilibrium of SHE.
• • The bigger the negative value, the further the position of equilibrium is to the left and the more reactive that metal is.
  • The bigger the positive value, the further the position of equilibrium is to the right, and the less reactive that metal is.

 

The electrochemical series

• The electrochemical series is built up by arranging various redox equilibria in order of their standard electrode potentials

 

• The figures above show some common redox equilibria arranged according to their standard electrode The full list can be found in the Data Booklet.

 

• From the top to the bottom, the position of equilibrium shifts from right to This is because the value of E° changes from the most positive to the most negative.

 

• This implies that the ease of losing electrons by the element increases from top to bottom(or the ease of gaining electrons by the ion decreases from top to bottom).
• • Ions at the top(with more positive E° value) gain electrons and get reduced more readily, hence they are good oxidising  agents.
  • Elements(metals) at the bottom(with more negative E° value) lose electrons and get oxidised more readily, hence they are good reducing agents.