Exams › JEE Advanced › Chemistry
Correct answer: I⁻
In this reaction, X⁻ is oxidized (X⁻ -> (1/2)X2 + e⁻) and Cu²+ is reduced to Cu⁺ (in Cu2X2). For this reaction to occur spontaneously, the reducing power of X⁻ must be sufficient to reduce Cu²+. Among the halogens: F⁻ is too stable (very poor reducing agent). Cl⁻ cannot reduce Cu²+ under normal conditions. Br⁻ also cannot reduce Cu²+ to Cu⁺. I⁻ is the only halide that spontaneously reduces Cu²+ to Cu⁺ giving the insoluble precipitate Cu2I2. This is a classic disproportionation/redox reaction used in iodometric titrations: 2Cu²+ + 4I⁻ -> Cu2I2 + I2.