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MHT CET202519 Apr 2025Morning ShiftChemistryThermodynamics (C)Actual

Which from following reactions performs zero work?

Options

  1. ACH _ 4( ~g ) + Cl _ 2( ~g ) CH ₃ Cl _ ( g ) + HCl _ ( g )
  2. B3 H _ 2( ~g ) + N _ 2( ~g ) 2 NH _ 3( ~g )
  3. CC ₂ H _ 2( ~g ) + 5 2 O _ 2( ~g ) 2 CO _ 2( ~g ) + H ₂ O _ (l)
  4. D2 C ₂ H _ 6( ~g ) +7 O _ 2( ~g ) 4 CO _ 2( ~g ) +6 H ₂ O _ (l)

Correct answer

A. CH _ 4( ~g ) + Cl _ 2( ~g ) CH ₃ Cl _ ( g ) + HCl _ ( g )

Step-by-step solution

For a reaction to perform zero work at constant temperature and pressure, the change in moles of gas, n_g , must be zero. This is since the work done by an ideal gas is W = -P V , and at constant T , V = n_g R T P , hence W = - n_g R T . Calculate n_g for each reaction, counting only gaseous species: A: CH _ 4( g ) + Cl _ 2( g ) CH ₃ Cl _ ( g ) + HCl _ ( g ) n_g = (1 + 1) - (1 + 1) = 2 - 2 = 0 . A performs zero work. B: 3 H _ 2( g ) + N _ 2( g ) 2 NH _ 3( g ) n_g = 2 - (3 + 1) = 2 - 4 = -2 . C: C ₂ H _ 2( g ) + 5 2

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