Chemical Thermodynamics quiz-8

CHEMICAL THERMODYNAMICS QUIZ-8

Dear Readers,

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Q1. The enthalpy of vaporisation of a substance is 840 J/mol and its boiling point is -173℃.
Its entropy of vaporisation is

•  4.8 J/mol/K
•  12 J/mol/K
•  210 J/mol/K
•  49 J/mol/K

Solution
a) ∆S^vap=(∆H_vap)/T_b =840/173=4.8 J/mol/K

Q2.When ammonium chloride is dissolved in water, the solution becomes cold. The change is:

•  Endothermic
•  Exothermic
•  Super cooling
•  None of these

Solution
a) The heat is provided by solvent and thus, feels cooler.

Q3.  Heat of combustion ∆H for C(s),H₂(g) and CH₄(g) are -94,-68 and -213 kcal/mole
then ∆H for C(s)+2H₂(g)→CH₄(g)is∶

•  -17 kcal
•  -111 kcal
•  -170 kcal
•  -85 kcal

Solution
a) Solve using Hess’s law

Q4. For the gas phase reaction,
PCl₅(g)⇌PCl₃(g)+Cl₂(g)
Which of the following conditions are correct?

•  ∆H>0 and ∆S>0
•  ∆H<0 and ∆S<0
•  ∆H>0 and ∆S<0
•  ∆H=0 and ∆S<0

Solution
a)
∆H=+ve and ∆S=+ve; the disorder increases with increase in moles.
PCl₅⇌PCl₃+Cl₂;∆H=+ve (dissociation)

Q5.Given that standard heat enthalpy of CH₄,C₂H₄ and C₃H₈ are -17.9,12.5,-24.8 kcal/mol.
The ∆H for CH₄+C₂H₄→C₃H₈ is:

•  - 55.2 kcal mol^-1
•  - 30.2 kcal mol^-1
•  55.2 kcal mol^-1
•  - 19.4 kcal mol^-1

Solution
  (d) ∆H=H_C3H8-H_CH4-H_C2H4
= -24.8-(-17.9)-12.5
= 19.4 kcal/mol

Q6. Bond energies of (H─H), (O〓 O) and (O─H) are 105, 120 and 220 kcal/mol respectively,
then ∆H in the reaction , 2H₂(g)+O₂(g)→2H₂O(l):

•  - 115
•  - 130
•   - 118
•  - 550

Solution
(d)
∆H= -2×[2× e_O─H]+2 × e_H─H+e_O─O
= -4 × 220+2 × 105+120= -550 kJ

Q7.The temperature of the system increases during an:

•  Isothermal expansion
•  Adiabatic compression
•  Adiabatic expansion
•  Isothermal compression

Solution
(b) q=∆U-W, if q=0 for adiabatic process, than -∆U=-W or ∆U=W,i.e., work done on the system or work of compression brings in an increase in temperature

Q8.If CH₃COOH+OH^-=CH₃COO^-+H₂O+q₁ and H⁺ +OH^-=H₂O+q₂,then the enthalpy change
for the reaction, CH₃COOH=CH₃COO^- + H⁺ is equal to:

•  q₁ + q₂
•  q₁ - q₂
•  q₂ - q₁
•  -q₁ - q₂

Solution
(b)

Q9. Work done by the system on surroundings is:

•  Positive
•  Negative
•  Zero
•  None of these

Solution
b) Work done by the system or work of expansion is negative. Work done on the system or work of compression positive.

Q10. The maximum work done in expanding 16 g oxygen at 300 K and occupying a volume
of 5 dm³ isothermally until the volume become 25dm³ is:

•   2.01×10³ J
•  +2.81×10³ J
•  2.01×10^-3 J
•  +2.01×10^-6 J

Solution
(a)
-W=+2.303 nRT log⁡V₂/V₁
-W=2.303×16/32×300×8.314 log⁡ 25/5
-W=2.01×10³ J