Which of the following compounds gives ethane upon electrolysis of its aqueous solution?

Predict the products of electrolysis in each of the following:
$(i)$ An aqueous solution of $AgNO_3$ with silver electrodes.
$(ii)$ An aqueous solution of $AgNO_3$ with platinum electrodes.
$(iii)$ $A$ dilute solution of $H_2SO_4$ with platinum electrodes.
$(iv)$ An aqueous solution of $CuCl_2$ with platinum electrodes.

In the process of electrolysis,the current is carried out inside the electrolyte by

During the electrolysis of brine,by using inert electrodes,

An electrolyte:

During the electrolysis of $NaCl$ solution,$H_2$ gas is liberated at the cathode when a $Pt$ electrode is used,whereas sodium amalgam is formed when a $Hg$ cathode is used. What is the reason for this?