WAEC Chemistry 2019 Theory — Question 18

Model answer

(i) A redox reaction is one in which a chemical species undergoes reduction while another species simultaneously undergoes oxidation (i.e. electron transfer occurs between reactants). (ii)/(iii): I. 2Na+Cl2→2NaCl — REDOX (Na is oxidized: 2Na→2Na⁺+2e–; Cl2 is reduced: Cl2+2e–→2Cl–) II. AgCl+2NH3→[Ag(NH3)2]Cl — NOT redox (it is a complex-formation/ligand-substitution reaction; oxidation states of all atoms remain unchanged throughout) III. C2H2+H2→C2H4 — REDOX (an addition/hydrogenation reaction in which C2H2 is reduced by the addition of hydrogen while H2 is oxidized) IV. 2FeCl3+2KI→2FeCl2+2KCl+I2 — REDOX (Fe is reduced from +3 to +2: 2Fe3++2e–→2Fe2+; I is oxidized from –1 to 0: 2I–→I2+2e–) (iv) Cl2+2e–→2Cl– (reduction) and 2Na→2Na⁺+2e– (oxidation); or 2Fe3++2e–→2Fe2+ (reduction) and 2I–→I2+2e– (oxidation).