5.5.3 Electrode Potentials

Standard electrode potential

• The position of equilibrium and therefore the electrode potential depends on factors such as:
  • Temperature
  • Pressure of gases
  • Concentration of reagents

• So, to be able to compare the electrode potentials of different species, they all have to be measured against a common reference or standard
• Standard conditions also have to be used when comparing electrode potentials
• These standard conditions are:
  • Ion concentration of 1.00 mol dm^-3
  • A temperature of 298 K
  • A pressure of 100 kPa

• Standard measurements are made using a high resistance voltmeter so that no current flows and the maximum potential difference is achieved

• The electrode potentials are measured relative to a standard hydrogen electrode
• The standard hydrogen electrode is given a value of 0.00 V, and all other electrode potentials are compared to this standard
• This means that the electrode potentials are always referred to as a standard electrode potential (E^θ)
• The standard electrode potential (E^θ) is the potential difference ( sometimes called voltage) produced when a standard half-cell is connected to a standard hydrogen cell under standard conditions
• For example, the standard electrode potential of bromine suggests that relative to the hydrogen half-cell it is more likely to get reduced, as it has a more positive E^θ value

Br₂(l) + 2e^– ⇌ 2Br^–(aq)        E^θ = +1.09 V          

2H⁺(aq) + 2e^– ⇌ H₂(g)        E^θ = 0.00 V

• The standard electrode potential of sodium, on the other hand, suggests that relative to the hydrogen half-cell it is less likely to get reduced as it has a more negative E^θ value

Na⁺ (aq) + e^– ⇌ Na(s)        E^θ = -2.71 V

2H⁺ (aq) + 2e^– ⇌ H₂(g)        E^θ = 0.00 V

 

• The standard hydrogen electrode is a half-cell used as a reference electrode and consists of:
  • Hydrogen gas in equilibrium with H⁺ ions of concentration 1.00 mol dm^-3 (at 100 kPa)

2H⁺ (aq) + 2e^- ⇌ H₂ (g)

• • An inert platinum electrode that is in contact with the hydrogen gas and H⁺ ions

• When the standard hydrogen electrode is connected to another half-cell, the standard electrode potential of that half-cell can be read off a high resistance voltmeter

The standard electrode potential of a half-cell can be determined by connecting it to a standard hydrogen electrode

• There are three different types of half-cells that can be connected to a standard hydrogen electrode
  • A metal / metal ion half-cell
  • A non-metal / non-metal ion half-cell
  • An ion / ion half-cell (the ions are in different oxidation states)

Metal / metal-ion half-cell

Example of a metal / metal ion half-cell connected to a standard hydrogen electrode

• An example of a metal/metal ion half-cell is the Ag⁺/ Ag half-cell
  • Ag is the metal
  • Ag⁺ is the metal ion

• This half-cell is connected to a standard hydrogen electrode and the two half-equations are:

Ag⁺ (aq) + e^- ⇌ Ag (s)        E^ꝋ = + 0.80 V

2H⁺ (aq) + 2e^- ⇌ H₂ (g)        E^ꝋ = 0.00 V 

• Since the Ag⁺/ Ag half-cell has a more positive E^ꝋ value, this is the positive pole and the H⁺/H₂ half-cell is the negative pole
• The standard cell potential (E_cell^ꝋ) is E_cell^ꝋ = (+ 0.80) - (0.00) = + 0.80 V
• The Ag⁺ ions are more likely to get reduced than the H⁺ ions as it has a greater E^ꝋ value
  • Reduction occurs at the positive electrode
  • Oxidation occurs at the negative electrode

Non-metal / non-metal ion half-cell

• In a non-metal / non-metal ion half-cell, platinum wire or foil is used as an electrode to make electrical contact with the solution
  • Like graphite, platinum is inert and does not take part in the reaction
  • The redox equilibrium is established on the platinum surface

• An example of a non-metal / non-metal ion is the Br₂ / Br^- half-cell
  • Br₂ is the non-metal
  • Br^- is the non-metal ion

• The half-cell is connected to a standard hydrogen electrode and the two half-equations are:

Br₂ (aq) + 2e^- ⇌ 2Br^- (aq)        E^ꝋ = +1.09 V

2H⁺ (aq) + 2e^- ⇌ H₂ (g)        E^ꝋ = 0.00 V   

• The Br₂ / Br^- half-cell is the positive pole and the H⁺ / H₂ is the negative pole
• The E_cell^ꝋ is: E_cell^ꝋ = (+ 1.09) - (0.00) = + 1.09 V
• The Br₂ molecules are more likely to get reduced than H⁺ as they have a greater E^ꝋ value

Example of a non-metal / non-metal ion half-cell connected to a standard hydrogen electrode

Ion / Ion half-cell

• A platinum electrode is again used to form a half-cell of ions that are in different oxidation states
• An example of such a half-cell is the MnO₄^- / Mn²⁺ half-cell
  • MnO₄^- is an ion containing Mn with oxidation state +7
  • The Mn²⁺ ion contains Mn with oxidation state +2

• This half-cell is connected to a standard hydrogen electrode and the two half-equations are:

MnO₄^- (aq) + 8H⁺ (aq) + 5e^- ⇌ Mn²⁺ (aq) + 4H₂O (l)       E^ꝋ = +1.52 V

2H⁺ (aq) + 2e^- ⇌ H₂ (g)       E^ꝋ = 0.00 V   

• The H⁺ ions are also present in the half-cell as they are required to convert MnO₄^- into Mn²⁺ ions
• The MnO₄^- / Mn²⁺ half-cell is the positive pole and the H⁺ / H₂ is the negative pole
• The E_cell^ꝋ is E_cell^ꝋ = (+ 1.52) - (0.00) = + 1.52 V

Ions in solution half cell

Conventional Representation of Cells

• Chemists use a type of shorthand convention to represent electrochemical cells
• In this convention:
  • A solid vertical (or slanted) line shows a phase boundary, that is an interface between a solid and a solution
  • A double vertical line (sometimes shown as dashed vertical lines) represents a salt bridge
    • A salt bridge has mobile ions that complete the circuit
    • Potassium chloride and potassium nitrate are commonly used to make the salt bridge as chlorides and nitrates are usually soluble
    • This should ensure that no precipitates form which can affect the equilibrium position of the half cells

  • The substance with the highest oxidation state in each half cell is drawn next to the salt bridge
  • The cell potential difference is shown with the polarity of the right hand electrode

• The cell convention for the zinc and copper cell would be

Zn (s)∣Zn²⁺ (aq) ∥Cu²⁺ (aq)∣Cu (s)                  E _cell = +1.10 V

• This tells us the copper half cell is more positive than the zinc half cell, so that electrons would flow from the zinc to the copper
• The same cell can be written as:

Cu (s)∣Cu²⁺ (aq) ∥Zn²⁺ (aq)∣Zn (s)                  E _cell = -1.10 V

• The polarity of the right hand half cell is negative, so we can still tell that electrons flow from the zinc to the copper half cell

Answer

Al (s)∣Al³⁺ (aq) ∥ Zn²⁺ (aq)∣Zn (s)                  E _cell = +0.94 V

It is also acceptable to include phase boundaries on the outside of cells as well:

∣ Al (s)∣Al³⁺ (aq) ∥ Zn²⁺ (aq)∣Zn (s) ∣                  E _cell = +0.94 V