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?)
• The?standard electrode potential?(E?)?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??value

Br2(l) + 2e–?? 2Br–(aq)? ? ? ??E??= +1.09 V

2H+(aq) + 2e–?? H2(g)? ? ? ??E??= 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??value

Na+?(aq) + e–?? Na(s)? ? ? ??E??= -2.71 V

2H+?(aq) + 2e–?? H2(g)? ? ? ??E??= 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??= + 0.80 V

2H+?(aq) + 2e-?? H2?(g)? ? ? ??E??= 0.00 V

• Since the Ag+/ Ag half-cell has a more positive?E??value, this is the?positive pole?and the H+/H2?half-cell is the?negative?pole
• The?standard cell potential (Ecell?) is?Ecell??= (+ 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??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??= +1.09 V

2H+?(aq) + 2e-?? H2?(g)? ? ? ??E??= 0.00 V

• The Br2?/ Br-?half-cell is the?positive pole?and the H+?/ H2?is the?negative?pole
• The?Ecell??is:?Ecell??= (+ 1.09) - (0.00)?= + 1.09 V
• The Br2?molecules are more likely to get?reduced?than H+?as they have a greater?E??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??= +1.52 V

2H+?(aq) + 2e-?? H2?(g)? ? ? ?E??= 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?Ecell??is?Ecell??= (+ 1.09) - (0.00)?= + 1.09 V

Ions in solution half cell

Standard cell potential

• Once the?E??of a half-cell is known, the?potential difference?or?voltage?or?emf?of an?electrochemical cell?made up of any two half-cells can be calculated
  • These could be?any?half-cells and neither have to be a standard hydrogen electrode

   

• The?standard cell potential?(Ecell?) can be calculated by?subtracting?the?less?positive?E??from the?more positive?E??value
  • The half-cell with the more positive?E??value will be the?positive?pole
    • By convention this is shown on the right hand side in a conventional cell diagram, so is termed?Eright?

     

  • The half-cell with the less positive?E??value will be the?negative?pole
    • By convention this is shown on the left hand side in a conventional cell diagram, so is termed?Eleft?

     

   

Ecell??=?Eright??-?Eleft?

• • Since oxidation is always on the left and reduction on the right, you can also use this version

Ecell??=?Ereduction??-?Eoxidation

Answer

Step 1:?Calculate the standard cell potential. The copper is more positive so must be the right hand side.

Ecell??=?Eright??-?Eleft?

Ecell??= (+0.34) - (-0.76)

= +1.10 V

The voltmeter will therefore give a value of +1.10 V

Step 2:?Determine the positive and negative poles

The Cu2+?/ Cu? half-cell is the?positive?pole as its?E??is more positive than the?E??value of the Zn2+?/ Zn half-cell