Edexcel A Level Chemistry:复习笔记6.1.3 Measuring Standard Electrode Potential

Measuring Standard Electrode Potential

 

  • There are three different types of half-cells that can be connected to a standard hydrogen electrode to measure standard electrode potential
    • 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

5.4.2-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)        Eꝋ = + 0.80 V

2H+ (aq) + 2e- ⇌ H2 (g)        Eꝋ = 0.00 V

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

2H+ (aq) + 2e- ⇌ H2 (g)        Eꝋ = 0.00 V

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

 

5.4.2-Non-Metal_Non-Metal-Ion-Half-Cell

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)       Eꝋ = +1.52 V

2H+ (aq) + 2e- ⇌ H2 (g)       Eꝋ = 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 Ecellꝋ is Ecellꝋ = (+ 1.09) - (0.00) = + 1.09 V

5.4.2-Ion_-Ion-Half-Cell

Ions in solution half cell

 

Electromotive Force

Standard cell potential

  • Once the Eꝋ 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 (Ecellꝋ) can be calculated by subtracting the less positive Eꝋ from the more positive Eꝋ value
    • The half-cell with the more positive Eꝋ value will be the positive pole
      • By convention this is shown on the right hand side in a conventional cell diagram, so is termed  Erightꝋ

       

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

       

     

Ecellꝋ = Erightꝋ - Eleftꝋ

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

Ecellꝋ = Ereductionꝋ - Eoxidation

Worked Example

Calculating the standard cell potential

Calculate the standard cell potential for the electrochemical cell below and explain why the Cu2+ / Cu half-cell is the positive pole. The half-equations are as follows:

Cu2+(aq) + 2e- ⇌ Cu(s)      Eꝋ = +0.34 V

Zn2+(aq) + 2e- ⇌ Zn(s)      Eꝋ = −0.76 V

 

5.4-Electrochemistry-Calculations-Electrochemical-Cell

 

Answer

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

Ecellꝋ = Erightꝋ - Eleftꝋ

Ecellꝋ = (+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 Eꝋ is more positive than the Eꝋ value of the Zn2+ / Zn half-cell

Exam Tip

A helpful mnemonic for remembering redox in cells

5.4.2-Lio-the-Lion

 

Lio the lion goes Roor!

Lio stands for 'Left Is Oxidation' and he is saying ROOR because that is the order of species in the cell:

Reduced/Oxidised (salt bridge) Oxidised/Reduced

 

转载自savemyexams

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