Calculate effective atomic number of Fe2⊕ in [Fe(CN)6]4⊖ [ Given : (Z = 26) ](ii) Draw neat and labelled diagram of lead accumulator.(iii) Write two advantages of hydrogen – oxygen fuel cell.

Answer: (i) EAN = 36. (ii) Diagram described below. (iii) Advantages below.

(i) EAN = (number of electrons in the metal ion) + (number of electrons donated by the ligands).

Fe²⁺: 26 − 2 = 24 electrons. Six CN⁻ ligands each donate a pair: 6 × 2 = 12 electrons.

EAN = 24 + 12 = 36 (equal to the atomic number of the noble gas krypton, so the complex is stable).

(ii) Lead accumulator (lead storage battery) – diagram described in words:

• A rectangular container (hard rubber or plastic) filled with dilute H₂SO₄ (about 38% by mass, density 1.2 g mL⁻¹) as electrolyte.
• Alternate vertical grids dipping in it: anode = grids of lead packed with spongy lead (Pb), connected to the negative terminal (−); cathode = lead grids packed with lead dioxide (PbO₂), connected to the positive terminal (+).
• Thin insulating separators placed between the plates; terminals labelled + and − on top.

Reactions during discharge: anode Pb_(s) + SO₄²⁻ → PbSO_4(s) + 2e⁻; cathode PbO_2(s) + 4H⁺ + SO₄²⁻ + 2e⁻ → PbSO_4(s) + 2H₂O. Cell e.m.f. about 2 V.

(iii) Advantages of the H₂–O₂ fuel cell (any two):

1. It is pollution free: the only product is water.
2. It has high efficiency (about 70%) as it converts chemical energy directly into electrical energy.
3. It gives a continuous supply of electricity as long as H₂ and O₂ are supplied; the water formed can be used for drinking by astronauts.