question_answer

E° value for the \[M{{n}^{3+}}/M{{n}^{2+}}\] couple is much more positive than for \[C{{r}^{3+}}/C{{r}^{2+}}\] couple. Assign reason.

Answer:

                  \[Mn\,(Z=25)\,\,\,:\,\,{{[Ar]}^{18}}\,4{{s}^{2}}3{{d}^{5}}\] \[Cr(Z=24):{{[Ar]}^{18}}\,4{{s}^{1}}3{{d}^{5}}\] \[M{{n}^{2+}}\,:{{[Ar]}^{18}}\,3{{d}^{5}}\]           \[C{{r}^{2+}}:\,{{[Ar]}^{18}}3{{d}^{4}}\] \[M{{n}^{3+}}:\,{{[Ar]}^{18}}\,3{{d}^{4}}\]           \[C{{r}^{3+}}:{{[Ar]}^{18}}3{{d}^{3}}\].   The electronic configuration of \[M{{n}^{2+}}\] ion is more symmetrical as compared to that of \[C{{r}^{2+}}\] ion. Therefore, third ionisation potential of\[M{{n}^{2+}}\]is much higher. Consequently E° value for the \[M{{n}^{3+}}/M{{n}^{2+}}\] couple is much more positive than for \[C{{r}^{3+}}/C{{r}^{2+}}\] couple.