electrodes

1)

reversible electrodes are the electrodes which constitute a reversible cell

when we say that the quinhydrone electrode is reversible wrt H+ ions we mean that hey form a reversible cell

an example of reversible cell is rechargeable batteries

that is once they are discharged, they can again be recharged

flow of electrons can take place in both the directions according to the need

3)

the reaction that takes place is

Pb+CaC2O4=PbC2O4+Ca⁺⁺+2e

electrodes of this kind which employ a single concentration cell are useful in determining the activity of the mettalic ions, the metals of which cannot be used as the electrodes of first or second kind owng to their reactions with the aqueous solutions

2) In such a half cell the metal is coated with its insoluble salt and is placed in contact with a solution containing the anion of the insoluble salt

it is just another type of cell