UPSC Civil Services (Mains), Chemistry optional · 2024 · Paper I
Q1(d)(i). Will CCl₂F₂ condense at 18 atm and 20 °C?
Question
The compound dichlorodifluoromethane (CCl₂F₂) has a normal boiling point of −30 °C, a critical temperature of 112 °C, and a corresponding critical pressure of 40 atm. If the gas is compressed to 18 atm at 20 °C, will the gas condense? Give your answer on the basis of graphical presentation.
Answer in brief
Yes. At 20 °C the vapour pressure of CCl₂F₂ is only about 5.5 atm, so at 18 atm the stable phase is the liquid.
Key points
- Two points of the liquid–vapour curve are given: the normal boiling point (243 K, 1 atm) and the critical point (385 K, 40 atm).
- Between them ln p is linear in 1/T (Clausius–Clapeyron); the two points fix ΔvapH/R = 2.43 × 10³ K.
- At 293 K, ln p = 1.71, so p = 5.5 atm.
- 20 °C is below the critical temperature, so pressure alone can liquefy the gas; (20 °C, 18 atm) lies above the curve, in the liquid region.
- On compression, liquid appears at 5.5 atm and the pressure stays constant until all the vapour has condensed.
The full model answer
As written for the answer booklet: the diagram carries the working, the lines stay short. Read here only; it is not offered as a download.
Coverscritical temperaturevapour pressure curveClausius–Clapeyron equationliquefaction of gasesphase diagramCCl₂F₂
All parts of Question 1
- 1(a)(i) Most probable distance of the 1s electron5 marks
- 1(a)(ii) Lattice energies of NaBr and NaI above the calculated values5 marks
- 1(b)(i) Separation factor for ²³⁵UF₆ and ²³⁸UF₆ in one effusion stage5 marks
- 1(b)(ii) Unit cell and the cubic Bravais lattices5 marks
- 1(c) Normal boiling point of mercury10 marks
- 1(d)(i) Will CCl₂F₂ condense at 18 atm and 20 °C?5 marks
- 1(d)(ii) Overvoltage and its applications5 marks
- 1(e) Temperature at which the uncatalysed reaction matches catalase at 20 °C10 marks