Determination of electrode potentials

Connecting half-cells together

• The two half-cells are setup, under standard conditions

• The two electrodes are connected together usually with a voltmeter

• The two solutions are connected with a salt bridge

• A salt bridge allows ions to flow

• Salt bridges are usually made of a strip of filter paper soaked in concentrated solution of a salt that won’t react with either half-cell solution

  • For example KNO₃(aq) is commonly used

Different types of half-cells

• Metal and solution of the metal ions

  • The metal is used as the electrode

  • And the metal ion solution at 1.00 moldm^-3

• Non-metal and solution of non-metal ions

  • A gaseous non-metal and its aqueous solution

  • This is set up per the standard hydrogen electrode

• Ions of the same element but in different oxidation states

  • All ions are present in the same solution at 1.00 moldm^-3

  • A platinum electrode is used, because it is inert

Calculating cell potentials

• Compare the two half equations, the more positive E^o half-cell will go forwards (reduced) the least positive will go backwards (oxidised)

• Cu²⁺(aq) + 2e^- ⇌ Cu(s) E⁰ = +0.34V (forwards, reduced)

• Zn²⁺(aq) + 2e^- ⇌ Zn(s) E⁰ = -0.76V (backwards, oxidised)

• E^o_cell = E^o_reduced - E^o_oxidised

• +0.34 - -0.76 = +1.10V

Representing cells

• Al(s) | Al³⁺(aq) || Cu²⁺(aq) | Cu(s) E⁰_cell = +2.00V

• A solid vertical line indicates a change in phase e.g. the change from a solid to a solution

• A double vertical line shows a salt bridge

• The species with the highest oxidation state is show next to the salt bridge

• The right hand half-cell dictates the sign of the E⁰_cell, as the copper half cell has a more positive E⁰ then the value in this case is positive

• Remember electrons will flow from the least positive E⁰ half-cell (oxidised) to the most positive (reduced) half-cell.

Anodes and cathodes

• The polarity of the anode and cathode is different between electrolysis (when electricity drives the chemistry) to electrochemical cells (when chemistry produces electricity)