Which of the following, on treatment with KCN, will give cyanogen gas? — Qualitative and Quantitative Analysis Chemistry Question

💡 Solution & Explanation

Step 1: Cu2+ is a moderately strong oxidizing agent, while CN- is a strong ligand and a reducing agent. Step 2: When [Cu(NH3)4]2+ is treated with KCN, the copper(II) is reduced to copper(I) by cyanide, forming stable [Cu(CN)4]3- and liberating cyanogen gas, (CN)2: 2[Cu(NH3)4]2+ + 10CN- -> 2[Cu(CN)4]3- + (CN)2↑ + 8NH3. Step 3: Consequently, the tetraamminecopper(II) complex gives cyanogen gas on treatment with KCN, matching option (b).