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Derivation of Kp and Kc

As negative E...

Question

As negative E value decreases , stability decreases. For example E(Cr+3/Cr) is more negative than E(Fe+2/Fe).
So Cr +3 is more stable. Is this statement always true?

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Solution

Yes, it is true. This is because, the greater the negative value, the greater energy is needed to reduce the corresponding cation. Similarly, for negative ions, the greater the postive value, the greater energy is needed to oxidise the corresponding anion.

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Q. E⁰ (Fe³⁺/Fe²⁺) is positive . This means Fe can be reduced to +2 from +3 but then enthalpy of hydration is also negative ( when oxidizing Fe+2 to Fe+3 because Fe+3 has more charge density so it will make system stable nd release energy). Then why is E⁰(Fe3+/Fe2+) positive?

E^{0}

{M n}^{+ 3} / {M n}^{+ 2}

couple much more positive than

{C r}^{+ 3} / {C r}^{+ 2}

.

M^{2 +} / M

M^{3 +} / M^{2 +}

E^{⊖}

values for some metals are as follows:

$C r^{2 +} / C r$	$- 0.9 V$ $	$C r^{3} / C r^{2 +}$	$- 0.4 V$
$M n^{2 +} / M n$	$- 1.2 V$	$M n^{3 +} / M n^{2 +}$	$+ 1.5 V$
$F e^{2 +} / F e$	$- 0.4 V$	$F e^{3 +} / F e^{2 +}$	$+ 0.8 V$

Use this data to comment upon:
(i) the stability of

F e^{3 +}

in acid solution as compared to that of

C r^{3 +}

M n^{3 +}

and
(ii) the ease with which iron can be oxidised as compared to a similar process for either chromium or manganese metal

Q. The correct order of

E^{0} M^{2 +} / M

values with negative sign for the four successive elements Cr, Mn, Fe and Co is

Which is more stable in the below given pair?

C r^{2 +}

C r^{3 +}

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