(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.