(i) In general, the E(node) value (M^3+ / M^2+) becomes larger and more positive with increasing atomic number. However, a dip in the E(node) value is observed from Mn to Fe. Explain why there is a dip in the E(node) value.
(ii) Whist aqueous solutions of Cr^3+ salts and Fe^3+ salts are stable, aqueous solutions of Mn^3+ salts and Co^3+ salts are unstable. Briefly explain why concentrated solutions of Mn^3+ and Co^3+ cannot be produced as stable solutions?
(iii) When acidified dichromate (VI) ions are treated with an excess of zinc, a blue solution is obtained. After filtering to remove the unreacted zinc and leaving the solution to stand in air, the colour changed to green. Even when the blue solution was allowed to stand for several hours in absence of air, it slowly turned green with the evolution of a colourless gas.
Explain this observations, quoting relevating E(node) values from the data booklet and writing eqns wherever possible.