Chemical Thermodynamics Quiz-2

CHEMICAL THERMODYNAMICS QUIZ-2

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Q1. Given, NH₃(g)+3Cl₂(g) ⇌NCl₃(g)+3HCl(g); - ∆H₁
N₂(g)+3H₂(g)⇌2NH₃(g); -∆H₂
H₂(g)+Cl₂(g)⇌2HCl(g);∆H₃
The heat of formation of NCl₃(g) in the terms of ∆H₁,∆H₂ and ∆H₃ is:

•  ∆H_f= -∆H₁+(∆H₂)/2 -3/2 ∆H₃
•  ∆H_f= ∆H₁+(∆H₂)/2 -3/2 ∆H₃
•  ∆H_f= ∆H₁-(∆H₂)/2 -3/2 ∆H₃
•  None of the above

Solution
(a) Find 1/2 N₂+3/2 Cl₂ ⟶NCl₃; Multiply Eqs. (ii) by 1/2, (iii) 3/2 and subtract from Eq.(i); we get ∆H_f= -∆H₁- [-((∆H₂)/2 +3/2 ∆H₃)]
= - ∆H₁+(∆H₂)/2 -3/2 ∆H₃.

Q2.Which has the least entropy?

•  Graphite
•  Diamond
•  N₂(g)
•  N₂O(g)

Solution
b) Graphite possesses sp^2-hybridisation and has flat layer structure whereas diamond possesses sp^3-hybridisation and has rigid tetrahedral nature

Q3.  The enthalpy change for the process, C(s) ⟶ C(g) is known as enthalpy of :

•  Fusion
•  Vaporisation
•  Combustion
•  Sublimation

Solution
d) The process involves conversion of 1 mole of C(s) to C(g),i.e., sublimation.

Q4. A boiled egg show a/an …in entropy.

•  Increase
•  Decrease
•  No change
•  None of these

Solution
a) No doubt solidification shows a decrease in entropy but in egg proteins structure are disordered in solid state due to denaturation

Q5.Heat of neutralisation of which acid-base reaction is - 57.32 kJ for?

•  CH₃COOH+NaOH
•  HCl+NH₄OH
•  HCOOH+KOH
•  HNO₃+LiOH

Solution
 d) Strong acid (HNO₃) and strong base (LiOH).

Q6. For a reaction, ∆H=9.08 kJ mol^-1 and ∆S=35.7 JK^-1 mol^-1. Which of the following statement is correct for the reaction?

•  Reversible and isothermal
•  Reversible and exothermic
•  Spontaneous and endothermic
•  Spontaneous and exothermic

Solution
c) ∆H and ∆S both are +ve for spontaneous change and ∆H=+ve for endothermic reaction

Q7. In a chemical reaction ∆H=150 kJ and ∆S=100 JK^-1 at 300 K. The ∆G for the reaction is:

•  Zero
•  300 kJ
•  330 kJ
•  120 kJ

Solution
d) ∆G=∆H-T∆S ∆G=150-(100×300)/10^-3) =120 kJ

Q8.Enthalpy of vaporisation for water is 186.5 kJ mol^-1. The entropy change during vaorisation is …kJ K^-1 mol^-1

•  0.5
•  1.0
•  1.5
•  2.0

Solution
a) ∆S=(∆H_v)/T ∆H=186.5 kJ T=373 K ∴∆S=186.5/373=0.5 kJ K^{-1 mol-1}

Q9. When water is added to quick lime, the reaction is :

•  Explosive
•  Endothermic
•  Exothermic
•  None of these

Solution
c) CaO+H₂O ⟶Ca(OH)₂; ∆H= -ve; the solution of lime heats up.

Q10. The work done by a system is 8 J, when 40 J heat is supplied to it. The change in
internal energy of the system during the process is:

•  32 J
•  40 J
•  36 J
•  44 J

Solution
a) q=∆U-W;-W is work done by the system ∆U=40-8=32 J (∵-W=8)