Q.Explain the variation in E°(M³⁺/M²⁺) of 3d series.

Step 1. Recall the tabulated M³⁺/M²⁺ reduction potentials: Ti -0.37 V, V -0.26 V, Cr -0.41 V, Mn +1.51 V, Fe +0.77 V, Co +1.81 V.

Step 2. For Ti, V and Cr, the NEGATIVE values mean the higher (+3) oxidation state is thermodynamically preferred -- reducing these ions down to +2 is unfavourable, so a strong reducing agent (e.g. zinc, E° = -0.76 V) is needed to force the reduction.

Step 3. For Mn, Fe and Co, the POSITIVE values mean the lower (+2) oxidation state is preferred instead -- these M³⁺ ions are readily reduced to M²⁺. The jump is especially sharp at manganese (+1.51 V), because Mn³⁺ (3d⁴) is reduced to the extra-stable, half-filled Mn²⁺ (3d⁵), a strong thermodynamic driving force. …

Read the M³⁺/M²⁺ reduction-potential values across the series, identify the sign-change from Cr to Mn, and explain the pattern via …

• Describing the trend as smoothly rising or falling, rather than noting the sharp discontinuity between the negative Ti/V/Cr cluster and the positive Mn/Fe/Co cluster. …