why does standard hydrogen electrode acts cathod when connected to zinc electrode half cell and as anode when connected to copper electrode half cell?
i have read the answer of this in books which explains in the terms of electrode reduction potential but i find it confusing please explain it mechanically.
Electrochemical series is the arrangement of metals based on the tendency of each metal to donate electrons. Greater the tendency to donate electrons, higher up is its place in the electrochemical series and greater is its electropositive character, and vice versa. The standard reduction potential actually quantifies such tendency in respect of a reference system, say, standard hydrogen electrode.
So, when a voltaic cell is constructed by placing two redox systems from different positions in the electrochemical series, the metal located higher donates electrons, thereby accumulating them on its surface. Thus it becomes the negative terminal or anode of the cell and the metal located lower accepts these electrons, thereby acting as the positive terminal or cathode of the cell.
Considering Zn/Zn2+, Cu/Cu2+ and H+/H2 redox systems, zinc is more electropositive followed by hydrogen and copper comes last in the electropositivity count (you can get it from their relative position in the electrochemical series, or from their general chemical reactivities, or from their displacement abilities etc.). Therefore for reasons discussed above, hydrogen will act as cathode when coupled with a Zinc half-cell, and as anode, when coupled with a Copper half-cell.