Chemistry - Thermodynamics - I Question with Solution | TestHub
The vapour pressure of water at 343 K is 234 mm Hg. The external pressure on H₂O (l) taken in a cylinder fitted with a frictionless movable piston initially containing 1.8 L (= 1.8 kg) of H₂O (l) at 343 K is increased to 1 atm at constant temperature. Then heat is supplied keeping the pressure constant till 0.9 L of H₂O (l) is evaporated to form H₂O (g) at 373 K. Assume the internal energy of the liquid to be dependent only on temperature. Answer the following questions by carefully observing the diagrams and the data provided. Given:
Specific heat of H₂O (l) = 4.2 J/°C-g
ΔHvap at 373 K and 1 atm = +40 kJ/mol
1 L-atm = 100 J
R = 8 J/K-mol
Total change in enthalpy (in kJ) going from State 1 to State 3 is
Options:
Answer:
Solution:
ΔH(1→2) = 124.6 J ≈ 0.125 kJ (Q25), and ΔH(2→3) = qp = 226.8 + 2000 = 2226.8 kJ. Total ΔH = 2226.8 + 0.125 ≈ 2226.9 kJ. The close distractor (d) drops the small State 1 → 2 contribution; (a) is ΔU, not ΔH; (b) subtracts the vapour work from the wrong leg. Matches (c).
