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GATE 2025 MT – Question 19

Metallurgical Thermodynamics · Reaction equilibria and Ellingham diagrams · 1 mark · Multiple choice

Consider the gas-phase reaction $2\mathrm{SO}_2+\mathrm O_2\rightleftharpoons2\mathrm{SO}_3$. If its enthalpy of reaction is negative, which condition promotes a higher equilibrium concentration of SO$_3$?

  1. Higher pressure and higher temperature
  2. Higher pressure and lower temperature
  3. Lower pressure and higher temperature
  4. Lower pressure and lower temperature

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Show answer and explanation

Correct answer: (B) Higher pressure and lower temperature

Explanation

The reaction reduces gas molecules from three to two, so increasing pressure favours products. Its negative enthalpy means cooling favours the exothermic forward reaction. Hence **higher pressure and lower temperature, B** jointly increase equilibrium SO3.