Question

Consider the formation of [Ni(en)3]2+ from [Ni(H2O)6]2+.

The stepwise ΔG∘ values at 298 K are

ΔG∘1 for first step=−42.9 kJ⋅mol−1

ΔG∘2 for second step=−35.8 kJ⋅mol−1

ΔG∘3 for third step=−24.3 kJ⋅mol−1

Calculate the overall formation constant (Kf) for the complex [Ni(en)3]2+.

Answer 1

[Ni(H2O6]^2+ +3en ? [Nien3]^2+ + 6H2O
Kform = [[Nien3]^2+]/[Ni(H2O6]^2+][en]^3
Can broken down into stepwise equilibria [3]; I won’t draw them all out but you should.
Example K1 = [[Nien(H2O)4]]/Ni(H2O)6][en]
Kform = K1×K2×K3
-RTlnKeq = ?G
lnK1 = ?G/-RT = 42900/298×8.314 = 17.32
K1 = 3.31×10^7
lnK2 = ?35800/298×8.314 = 14.45
K2 = 1.89×10^6
lnK3 = ?24300/298×8.314 = 9.81
K3 = 1.82×10^4
ln Kform = ln(K1+K2+K3) = 17.32 + 14.45 + 9.81 = 41.58
Kform = 1.14×10^18
Lit. [1] 2.1×10^18 [2] 4.1×10^17