Standard Cell Potential: Calculations, Electron Flow & Feasibility (Cambridge International A-Level Chemistry)

Standard Cell Potential Calculations

• Once the E^ꝋ 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 ( E*_cell^ꝋ) can be calculated by subtracting the less positive E^ꝋ from the more positive E*^ꝋ value

  • The half-cell with the more positive E^ꝋ value will be the positive pole
  • By convention this is shown on the right-hand side in a conventional cell diagram, so is termed **E_right^ꝋ

• The half-cell with the less positive E^ꝋ value will be the** negative pole

  • By convention this is shown on the left-hand side in a conventional cell diagram, so is termed E_left^ꝋ

E_cell^ꝋ= E_right ꝋ - E_left^ꝋ

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

E_cell^ꝋ= E_reduction ꝋ - E_oxidation^ꝋ

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^ꝋ = +0.34 V

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

Answer

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

E_cell^ꝋ = E_right^ꝋ - E_left^ꝋ

E_cell^ꝋ = (+0.34) - (-0.76) = +1.10 V

• Step 2: Determine the positive and negative poles

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

Exam Hint

• Students often confuse the redox process that take place in voltaic cells and electrolytic cells.
• An easy way to remember is the phrase RED CATS: REDuction takes place at the CAThode. OR AN OX. OXidation takes place at the ANode

Electrochemical Series

• The E^ꝋ values of a species indicate how easily they can get oxidised or reduced

• In other words, they indicate the relative reactivity of elements, compounds and ions as oxidising agents or reducing agents

• The electrochemical series is a list of various redox equilibria in order of decreasing E^ꝋ values

• More positive(less negative) E^ꝋ values indicate that:

  • The species is easily reduced
  • The species is a better oxidising agent

• Less positive(more negative) E^ꝋ values indicate that:

  • The species is easily oxidised
  • The species is a better reducing agent

An example electrochemical series

In this example electrochemical series the equilibria are arranged in order of decreasing Eꝋ values. These values can then be used to identify the strongest and weakest reducing / oxidising agents.