I am a bit confused. My openstax textbook states:
"When the half-cell X is under standard-state conditions, its potential is the standard electrode potential, E°X. Since the definition of cell potential requires the half-cells function as cathodes, these potentials are sometimes called standard reduction potentials."
I would greatly appreciate if someone can explain to me why the half cell has to be a cathode if the measured reduction table has both positive and negative values.
In aqueous solution the potential of the standard hydrogen electrode is defined as 0. volts. The standard conditions are: all activities, fugacities, molarities equal to one [1.]. By DEFINITION in a cell Oxidation is at the Anode and Reduction is at the Cathode. Now comes the dilemma How do we write a standard redox equation?
Na +1/2H3O+ = Na+ + 1/2H2 + H2O; E = ~+2.8V oxidation at the anode. Or
Na+ +1/2H2 + H2O = Na + H3O+ E = ~-2.8V reduction at the cathode!
Way back when, the Europeans liked the first and the Americans liked the second. As usual they fought it out and decided on the second [It was preferred that Sodium ions did not react with water than that Sodium did react explosively with water.] but you can still reverse everything when necessary.
Physicists are still confused because they consider electric current as the flow of +charge not as [we know it] the flow of electrons.
Sodium reacts violently with water. It is oxidized. the oxidation potential is positive [+]. Sodium ion is reduced with difficulty in water. The reduction potential is negative [-].
Finally, it is the nature of aqueous solution that the potentials for H2 are "central". Some species [Na+, Ca++ etc.] are reduced with difficulty and have negative reduction potentials. Others such as O2, F2, Cl2, MnO4- etc. are reduced easily and have positive reduction potentials. There is a large range in between. Of course, if one prefers to work with oxidation potentials everything is reversed.
