Measuring Standard Electrode Potential (Edexcel International A Level Chemistry)

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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

Metal_Metal Ion Half-Cell, downloadable AS & A Level Chemistry revision notes

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

Non-Metal_Non-Metal Ion Half-Cell, downloadable AS & A Level Chemistry revision notes

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 = (+ 1.52) - (0.00) = + 1.52 V

Ion_ Ion Half-Cell, downloadable AS & A Level Chemistry revision notes

Ions in solution half cell

The Salt Bridge

  • A salt bridge has mobile ions that complete the circuit
  • Ions must be able to flow between the half-cells or solutions 
  • This should be made on metal wire, even if the metal is inert
    • Metal wire allows the flow of electrons but not the flow of ions
  • 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

Electromotive Force

Standard cell potential

  • Once the Eof 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 Efrom the more positive Evalue
    • The half-cell with the more positive Evalue 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 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  Eleft

Ecell = Erightꝋ Eleftꝋ   

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

Ecell = 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)      E= +0.34 V

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

Electrochemistry Calculations - Electrochemical Cell, downloadable AS & A Level Chemistry revision notes

Answer

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

EcellErightꝋ - Eleftꝋ   

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 Eis more positive than the Evalue of the Zn2+ / Zn half-cell

Examiner Tip

A helpful mnemonic for remembering redox in cells

Lio the Lion, downloadable AS & A Level Chemistry revision notes

 

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

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Richard

Author: Richard

Expertise: Chemistry

Richard has taught Chemistry for over 15 years as well as working as a science tutor, examiner, content creator and author. He wasn’t the greatest at exams and only discovered how to revise in his final year at university. That knowledge made him want to help students learn how to revise, challenge them to think about what they actually know and hopefully succeed; so here he is, happily, at SME.