GATE 2019 MT – Question 52
A 50 mm diameter nickel sphere oxidizes in 60 vol% Ar and 40 vol% O₂ at 1 atm and 1173 K. Oxygen transfer controls, its surface concentration is zero, and mass-transfer coefficient is 0.03 m/s. With R=8.205×10⁻⁵ m³ atm/(mol K), find oxidation rate in mol Ni/min (two decimal places).
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Correct answer: 0.11 to 0.13
Explanation
Bulk oxygen concentration is $C=0.4/(RT)=4.156$ mol/m³. Sphere area is πd²=π(0.05)². Oxygen rate is kAC, which becomes $0.03\pi(0.05)^2(4.156)60=0.05876$ mol/min. Since 2Ni+O₂→2NiO, nickel consumption is twice this: 0.1175≈0.12 mol/min.