Chemical Thermodynamics Quiz-9

CHEMICAL THERMODYNAMICS QUIZ-9

Dear Readers,

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Q1. Given that ∆H_{(r298 K)}=-54.07 kJ mol^-1 and ∆S_{(r298 K)}^°=10 J mol^-1 and R=8.314 JK^-1 mol^-1. The value of log_10⁡K for a reaction, A⇌B is:

•  5
•  10
•  95
•  100

Solution
b) ∆G°=∆H°-T∆S°
=-54.07-298×10×10^-3
=-57.05 kJ
Also, ∆G°=2.303 RT log_10⁡K
log₁₀⁡K=(-57.05×10³)/(2.303×8.314×298)

Q2.A thermodynamic state function is:

•  One which obeys all the laws of thermodynamics
•  A quantity which is used to measure thermal changes
•  A quantity whose value is independent of the path
•  A quantity which is used to express pressure-volume work

Solution
c) The fact for a quantity referred as state function

Q3.  The work done by a system in an expansion against a constant external pressure is:

•  ∆P∙∆V
•  -P∙∆V
•  Q
•  V∙∆P

Solution
(b) W_irr. For a process at constant pressure =-P∙∆V; note that work is irreversible if expansion is made at constant pressure.

Q4. ∆H for the reaction given below represents,
CO₂(g)+H₂(g)⟶CO(g)+H₂O(g); ∆H=40 kJ:

•  Heat of formation
•  Heat of combustion
•  Heat of neutralisation
•  Heat of reaction

Solution
(d) Heat changes during any chemical reaction are referred as heat of reaction are referred as heat of reaction for that change.

Q5.∆S_o will be highest for the reaction

•  Ca(s)+1/2 O₂(g)→CaO(s)
•  CaCO₃(g)→CaO(s)+CO₂(g)
•  C(g)+O₂(g)→CO₂(g)
•  N₂(g)+O₂(g)→2NO(g)

Solution
  b) Eq.(b) shows largest phase change ie,gas → solid

Q6. The enthalpy of dissolution of BaCl₂(s) and BaCl₂∙2H₂O(s) are -20.6 and 8.8 kJ per mol
respectively.The enthalpy of hydration for, BaCl₂(s)+2H₂O→BaCl₂∙2H₂O(s) is

•  29.4 kJ
•  -29.4 kJ
•  -11.8 kJ
•  38.2 kJ

Solution
b)

Q7.A thermodynamic quantity is that:

•  Which is used in thermochemistry
•  Which obeys all the laws of thermodynamics
•  Quantity which depends only on the state of the system
•  Quantity which is used in measuring thermal change

Solution
c) e.g.,U,H,S, etc.

Q8.The heat evolved during the combustion of 112 litre of water gas (mixture of equal volume of H₂ and CO) is :
H₂(g)+1/2 O₂(g)→H₂O(g); ∆H= -241.8 kJ

CO(g)+1/2 O₂(g)→CO₂(g); ∆H= -283 kJ

•   241.8 kJ
•  283 kJ
•  -1312 kJ
•  1586 kJ

Solution
(c) ∆H for combustion of 56 litre H₂= (- 24.1 × 56)/22.4
∆H for combustion of 56 litre CO=(-263 × 56)/22.4
∴ Total ∆H= -1312 kJ

Q9. The enthalpy of formation of HI is 30.4 kJ. Which statement is false according to this observation?

•  HI is an endothermic compound
•  For the reaction,H₂(g)+I₂(g)⟶2HI(g);∆H=60.8 kJ
•  HI is a stable compound
•  HI is an unstable compound

Solution
(c) Due to positive ∆H, HI is endothermic compound and unstable.

Q10. Select the incorrect statement

•  PV work is usually negligible for solid and liquid
•  For a closed system with P-V work only, an isobaric process that has q=+ve must have ∆T=+ve.
•  For a cyclic process q=0
•  Black phosphorus is most stable form of P but H°_f=0 for white phosphorus.

Solution
(b) Rest all are correct .