Chemical Thermodynamics Quiz-6

CHEMICAL THERMODYNAMICS QUIZ-6

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Q1.  The entropy change for the reaction given below, 2H₂(g)+O₂ (g)→2H₂O(l)
Is…at 300 K. Standard entropies of H₂(g),O₂(g) and H₂O(l) are 126.6,201.20 and 68.0 JK^-1 mol^-1 respectively.

•  -318.4 JK^-1mol^-1
•  318.4 JK^-1mol^-1
•  31.84 JK^-1mol^-1
•  None of these

Solution
a) ∆S_reaction=∑▒S_product -∑▒S_reactant =2×S_(H2O)-[2×S_{(H2 )}+S_(O2) ]
=2×68-[2×126.6+201.20]
=-318.4 JK^-1 mol^-1

Q2.A mixture of hydrogen and chlorine on exposure to ultra violet sunlight reacts with explosion.
The step involved in the initiation of the reaction is:

•  H₂⟶H^⦁+ H^⦁
•  Cl^⦁+Cl^⦁ ⟶Cl²
•  H₂+Cl₂⟶2HCl
•  Cl₂⟶Cl_⦁+Cl_⦁

Solution
d) It is Cl_2 molecule which dissociates to give free radicals on exposure to light.

Q3.  The internal energy of a substance

•  Increases with increase in temperature
•  Decreases with increases in temperature
•  Can be calculated by the relation E=mc²
•  Remains unaffected with change in temperature

Solution
a)

Q4. In which case of mixing of a strong acid and a base each of 1N concentration, temperature increase is highest?

•  20 mL acid-20mL alkali
•  10 mL acid-40mL alkali
•  25 mL acid-25mL alkali
•  35 mL acid-15mL alkali

Solution
(c) 25 Meq. Of acid and 25 Meq. of base (the maximum value of Meq.) react.

Q5.Heat energy change during the chemical reaction, CO+1/2 O₂ ⟶ CO₂ is known as :

•  Heat of combustion of CO
•  Latent heat of CO₂
•  Latent heat of vaporisation
•  Heat of formation of CO₂

Solution
  (a) Heat of combustion is defined as the amount of the energy released during complete combustion of 1 mole of a substance in oxygen or air.

Q6. Select the correct limitations of III law of thermodynamics

•  Glassy solids at zero kelvin has entropy greater than zero
•  Solids having mixture of isotopes do not have entropy zero at zero kelvin
•  Crystals of CO,N₂O,NO,H₂O,etc.,do not have zero entropy at zero kelvin
•  All of the above

Solution
(d) These all are limitations of third law.

Q7.Heat of solution is defined as :

•  Heat required to dissolved one mole in excess of water
•  Heat evolved, when one mole is dissolved in excess of water
•  Change in heat content of the system when one mole of the solute is dissolved un excess of water, so that further dilution of solution does not bring any heat change
•  None of the above

Solution
(c) It is defined of heat of solution.

Q8.For the following two reactions, (i)CH₄(g)+2O₂(g)→CO₂(g)+2H₂O
∆H=-890.4 kJ
(ii)2HgO(s)→2Hg(l)+O₂(g)-181.6 kJ
Which one of the following statements is correct?

•  Both of them are exothermic
•  Both of them are endothermic
•  (i) is exothermic and (ii) is endothermic
•  (i) is endothermic and (ii) is exothermic

Solution
c)For exothermic reaction, ∆H=(-) for endothermic reaction, ∆H=(+).

Q9. Which of the following conditions will always lead to a non spontaneous change?

•  Positive ∆H and positive ∆S
•  Negative ∆H and negative ∆S
•  Positive ∆H and negative ∆S>
•  Negative ∆S and positive ∆S

Solution
(c) For exothermic reaction, ∆H=(-) for endothermic reaction, ∆H=(+).

Q10. For which one of the following equations is ∆H_reaction^o equal to ∆H_f^o for the product?

•   N₂(g)+O₃(g)⟶N₂O₃(g)
•  CH₄(g)+2Cl₂(g)⟶CH₂Cl₂(l)+2HCl(g)
•  Xe(g)+2F₂(g)⟶XeF₄(g)
•  2CO(g)+O₂(g)⟶2CO₂ (g)

Solution
(c)