A button cell used in watches function as following.
Zn_(s) + Ag₂O_(s) + H₂O_(l) $$\rightleftharpoons$$ 2Ag_(s) + Zn²⁺_(aq) + 2OH^$$-$$_(aq)

If half cell potentials are
Zn²⁺_(aq) + 2e^$$-$$ $$ \to $$ Zn_(s);  E^o = $$-$$0.76 V
Ag₂O_(s) + H₂O_(l) + 2e^$$-$$ $$ \to $$ 2Ag_(s) + 2OH^$$-$$_(aq), E^o = 0.34 V

The cell potential will be

A hydrogen gas electrode is made by dipping platinum wire in a solution of HCl of pH = 10 and by passing hydrogen gas around the platinum wire at one atm pressure. The oxidation potential of electrode would be

Standard reduction potentials of the half reactions are given below :
F_2(g) + 2e^$$-$$ $$ \to $$ 2F^$$-$$_(aq) ;   E^o = + 2.85 V
Cl_2(g) + 2e^$$-$$ $$ \to $$ 2Cl^$$-$$_(aq) ;   E^o = + 1.36 V
Br_2(l) + 2e^$$-$$ $$ \to $$ 2Br^$$-$$_(aq) ;   E^o = + 1.06 V
I_2(s) + 2e^$$-$$ $$ \to $$ 2I^$$-$$_(aq) ;  E^o = + 0.53 V

The strongest oxidising and reducing agents 23 respectively are

The Gibb's energy for the decomposition of Al₂O₃ at 500^oC is as follows
$${2 \over 3}$$ Al₂O₃ $$ \to $$ $${4 \over 3}$$ Al + O₂
$$\Delta $$_rG = +960 kJ mol^$$-$$1
The potential difference needed for the electrolytic reduction of aluminium oxide (Al₂O₃) at 500^oC is at least