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GATE 2025 CH – Question 42

Chemical Reaction Engineering · Single and multiple reactions in ideal reactors · 2 marks · Multiple choice

A zero-order gas phase reaction $A \to B$ with rate $(-r_A) = k = 100$ mol/(L min) is carried out in a mixed flow reactor of volume 1 L. Pure $A$ is fed to the reactor at a rate of 1 mol/min. At time $t = 0$, the outlet flow is stopped while the inlet flow rate and reactor temperature remain unchanged. Assume that the reactor was operating under steady state before the flow was stopped ($t < 0$). The rate of consumption of $A$, $-dC_A/dt$, in mol/(L min), at $t = 1$ min is

  1. 63.2
  2. 36.8
  3. 90.6
  4. 99.0

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Correct answer: (D) 99.0

Explanation

With the outlet closed, the mole balance on the 1 L reactor is $\frac{dC_A}{dt} = \frac{F_{A0}}{V} - k = 1 - 100 = -99$ mol/(L min). For a zero-order reaction the rate does not depend on the concentration, so this is constant in time, and at $t = 1$ min the net rate of decrease is 99.0 mol/(L min).