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UPSC Civil Services (Mains)

Chemistry optional

  • 2 papers
  • 250 marks each
  • 3 h per paper
  • 5 of 8 questions, Q1 and Q5 compulsory

Paper I covers physical and inorganic chemistry.

UPSC Civil Services (Mains), Chemistry optional · 2022 · Paper I

Q2(a). Adiabatic expansion of tyre air against a constant external pressure

Question

An automobile tyre contains air at 320 × 10³ Pa at 20 °C. The stem valve is removed and the air is allowed to expand adiabatically against a constant external pressure of 100 × 10³ Pa until P = Pexternal. For air, Cv,m = (5/2)R. Calculate the final temperature of the gas in the tyre. Assume ideal gas behaviour.

Answer in brief

T₂ = T₁(Cv,m + R·Pext/P₁)/(Cv,m + R) = 0.8036 T₁ = 235.6 K (−37.6 °C); the air cools by 57.6 K.

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Topic notes: the first law of thermodynamics, adiabatic changes, the Joule–Thomson effect and thermochemistry, with every Civil Services, IFoS and Geo-Scientist PYQ →

Coversirreversible adiabatic expansionfirst law of thermodynamicsconstant external pressureCvadiabatic cooling

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