The Mond process was used in the late 19th and early 20th centuries to purify nickel metal away from — Thermodynamics Chemistry Question
Problem Context
The Mond process was used in the late 19th and early 20th centuries to purify nickel metal away from impurities such as iron and cobalt. The process relies on the facile reaction of nickel with carbon monoxide to form volatile nickel tetracarbonyl, which can be separated from the solid impurities:
Ni(s) + 4 CO(g) → Ni(CO)4(g)
Draw or clearly describe the three-dimensional arrangement of the atoms in Ni(CO)4.
Model Answer
The nickel center is tetrahedral and the Ni–C–O bond is linear:
Calculate the average bond dissociation enthalpy for the nickel-carbon bonds in Ni(CO)4(g).
Model Answer
For the gas-phase reaction
Ni(CO)4(g) → Ni(g) + 4 CO(g) ∆H° = 4•(–110.5 kJ mol-1) + 430 kJ mol-1 – (–607 kJ mol-1) = 595 kJ mol-1
Since this reaction involves breaking four nickel-CO bonds, the average BDE is (595 kJ mol-1)/4 = 149 kJ mol-1.
Calculate ∆H°rxn and ∆S°rxn for the production of Ni(CO)4(g) by the Mond process.
Model Answer
∆H°rxn = (–607 kJ mol-1) – 4•(–110.5 kJ mol-1) = –165 kJ mol-1
∆S°rxn = (417 J mol-1 K-1) – 4•(198 J mol K-1) – 30 J mol-1 K-1 = –405 J mol-1 K-1
The boiling point of Ni(CO)4 at 1 bar pressure is 42 °C and its heat of vaporization is 29.0 kJ mol-1. Calculate ∆H°f and S° for Ni(CO)4(l).
Model Answer
For:
Ni(CO)4(l) → Ni(CO)4(g) ∆G°vap = 0 at the boiling point at 1 bar pressure and T = 315 K. ∆H°vap = 29.0 kJ mol-1.
So:
0 = 29000 J mol-1 – (315 K)(∆S°vap)
∆S°vap = 92.1 J mol-1 K-1
∆H°f (Ni(CO)4(g)) – ∆H°f (Ni(CO)4(l)) = ∆H°vap
–607 kJ mol-1 – ∆H°f (Ni(CO)4(l)) = 29.0 kJ mol-1
∆H°f (Ni(CO)4(l)) = –636 kJ mol-1
S° (Ni(CO)4(g)) – S° (Ni(CO)4(l)) = ∆S°vap
417 J mol-1 K-1 – S° (Ni(CO)4(l)) = 92.1 J mol-1 K-1
S° (Ni(CO)4(l)) = 325 J mol-1 K-1
After the Ni(CO)4 vapor has been separated from the solid impurities, Ni metal is recovered by heating the Ni(CO)4 to high temperature, causing it to re-form nickel metal and carbon monoxide. At 230 °C, what is the equilibrium pressure of Ni(CO)4(g) in the presence of nickel metal, assuming that the carbon monoxide pressure is maintained at 0.10 bar?
Model Answer
At 230 ºC = 503 K, ∆G°rxn = –165 kJ mol-1 –(503 K)(–405 J mol-1 K-1) = 39 kJ mol-1
Keq = e–(39000 J mol-1)/(503 K • 8.314 J mol-1 K-1) = 9.5 × 10-5
9.5 × 10-5 = p(Ni(CO)4)/p(CO)4 = p(Ni(CO)4)/(0.1)4
p(Ni(CO)4) = 9.5 × 10-9 bar