Chapter 20, Problem 20.140MP

Formation constants for the ammonia and ethylenediamine complexes of nickel(II) indicate that Ni(en)₃²⁺ is much more stable than Ni(NH₃)₆²⁺:

(1)     $\begin{array}{l}\text{Ni}{\left({\text{H}}_2\text{O}\right)}_6{}^{2+}\left(aq\right)+6{\text{ NH}}_3\left(aq\right)\rightleftharpoons \\ \text{ Ni}{\left(\text{NH}\right)}_6{}^{2+}\left(aq\right)+6{\text{ H}}_2\text{O}\left(l\right)K_{\text{f}}=2.0\times {10}^8\\ \end{array}$

(2)     $\begin{array}{l}\text{Ni}{\left({\text{H}}_2\text{O}\right)}_6{}^{2+}\left(aq\right)+3{\text{ en}}_3\left(aq\right)\rightleftharpoons \\ \text{ Ni}{\left(\text{en}\right)}_6{}^{2+}\left(aq\right)+6{\text{ H}}_2\text{O}\left(l\right)K_{\text{f}}=4\times {10}^{17}\\ \end{array}$

The enthalpy changes for the two reactions, ΔH°₁ and Δ H°₂, should be about the same because both complexes have six Ni – N bonds.

1. (a) Which of the two reactions should have the larger entropy change, ΔS°? Explain.
2. (b) Account for the greater stability of Ni(en)₃²⁺ in terms of the relative values of ΔS° for the two reactions.
3. (c) Assuming that ΔH°₂ – ΔH°₁ is zero, calculate the value of ΔS°₂ – ΔS°₁