STANDARD ELECTRODE POTENTIAL SEXPLAIN THE MEASUREMENTS OF THE STANDARD ELECTRODE POTENTIALS
DEFINITION• the potential of a redox system to lose or gain electrons when compared to the standard hydrogen electrode - assigned a value of 0 volts.• Def of standard electrode potential- potential diff between a standard hydrogen electrode and a metal which is immersed in a solution containing metal ions at 1 mol dm-3 conc. at 298 K and 1 atm pressure
• In any reduction-oxidation half equation the electrons are gained by the species on the left hand side: Cu2+ + 2e Cu• This is an equilibrium and so if a more powerful reducing agent is allowed enters into electrical contact with the above system it can force the copper ions to accept electrons and push the equilibrium to the right hand side.• Conversely, if a weaker reducing agent is brought into contact with the above equilibrium then the copper can force it to accept electrons allowing its own equilibrium to move to the left hand side.
• The electrode potential measures the tendency of electrons to flow away from or towards a redox equilibrium. They are always measured with respect to the standard hydrogen electrode (which is assigned a value of zero volts).• Equilibrium redox systems with the reduced side (usually a metal) more reactive than hydrogen have a negative electrode potential, i.e. they can lose electrons more easily than hydrogen.• Equilibrium redox systems with the reduced side less reactive than hydrogen have a positive electrode potential, i.e. they can lose electrons less easily than hydrogen.
EXAMPLE• Zinc has a standard electrode potential of - 0.76 volts• Consequently the equilibrium... Zn Zn2+ + 2e• has more of a tendency to move to the right hand side than the equilibrium... H2 2H+ + 2e• Hence if the two equilibria are brought into electrical contact using an external wire and a salt bridge, the electrons will be pushed from the zinc equilibrium to the hydrogen equilbrium with a force of - 0.76V (the negative sign simply indicates the direction of flow - from zinc to hydrogen ions)
• The two equations then may be summed together to give the reaction occuring in the whole cell.• Zn Zn2+ + 2e• 2H+ + 2e H2• overall cell reaction• Zn + 2H+ Zn2+ + H2
Representing the cell• The whole cell can be represented by showing the half cells in order of phase (solid, | solution, |salt bridge | solution | solid)