Draw neat and labelled diagram of standard hydrogen electrode.(b) Calculate e.m.f. of the following cell.Mg | Mg++(aq) || Ag+(aq) | AgIf E°Mg = –2.37 Volt and E°Ag = 0.8 Volt.

Answer:

(a) Standard hydrogen electrode (SHE): [Diagram description: a glass tube (hood) dips into a beaker containing 1 M HCl (unit activity of H⁺ ions). Inside the tube is a platinum foil coated with platinum black, attached to a platinum wire sealed in glass that makes the electrical contact. Pure hydrogen gas at 1 bar pressure enters through a side inlet at the top of the tube and bubbles over the platinum foil; excess gas escapes through holes near the bottom of the tube. Temperature 298 K. Labels: H₂ gas (1 bar) inlet, glass tube, Pt wire, platinised Pt foil, 1 M HCl, H₂ bubbles.]

Representation: Pt | H_2(g) (1 bar) | H⁺ (1 M). Electrode reaction: 2H⁺_(aq) + 2e^– ⇌ H_2(g). Its potential is arbitrarily taken as 0.000 V at all temperatures.

(b) The left electrode (Mg) is the anode and the right electrode (Ag) is the cathode. Taking the ions at unit concentration (standard cell):

\(E^\circ_{cell} = E^\circ_{cathode} - E^\circ_{anode} = E^\circ_{Ag} - E^\circ_{Mg}\)

\(= 0.80 - (-2.37) = 3.17\) V

Cell reaction: Mg_(s) + 2Ag⁺_(aq) → Mg²⁺_(aq) + 2Ag_(s). The e.m.f. is +3.17 V, so the cell works spontaneously.