Chemical Thermodynamics Quiz-10

CHEMICAL THERMODYNAMICS QUIZ-10

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Q1. The sublimation energy of I₂(s) is 57.3 kJ/mol and the enthalpy of fusion is 15.5 kJ/mol. The enthalpy of vaporisation of I₂ is

•  41.8 kJ/mol
•  -41.8 kJ/mol
•  72.8 kJ/molspan
•  -72.8 kJ/mol

Solution
(a) According to Hess’s law total heat changes during a chemical reaction are independent of path of reaction. < br> Given, I₂(s)→I₂(g),∆H₁=57.3 kJ/mol …(i)
I₂(s)→I₂(l),∆H₂=+15.5 kJ/mol …(ii)
Required equation I₂(l)→I₂(g),∆H₁=? subtract
Eq. (ii) from Eq. (i)
∴I₂(l)→I₂(g), ∆H=57.3+(-15.5)
=+41.8 kJ/mol

Q2.The heat of formations of CO(g) and CO

•  -67.6 kcal
•  36.5 kcal
•  -36.5 kcal
•  -46.5 kcal

Solution
(a) C(s)+1/2 O₂ (g)→CO₂ (g);∆H₁=-26.4
C(s)+1/2 O₂ (g)→CO₂ (g);∆H_2=-96.0
We have to find for ∆H, for reaction
CO+1/2 O₂→CO₂;∆H₁=?
Eqs.(ii)-(i), we get
∆H=-94-(-26.4)=-67.6 kcal

Q3.  When a bottle of perfume is opened, odourous molecules mix with air and slowly diffuse throughout
the entire room. The incorrect fact about the process is;

•  ∆G=-ve
•  ∆H≃0
•  ∆S=-ve
•  ∆S=+ve

Solution
d) The randomness in gaseous state is more than liquid state.

Q4. In which reaction ∆S is positive?

•  H₂O(l)→H₂O(s)
•  3O₂(g)→2O₃(g)
•  H₂O(l)→H₂O(g)
•  N₂(g)+3H₂(g)→2NH₃(g)

Solution
c) S_V^°-S_L^°=+ve. Conversion of liquid to vapour occurs process.

Q5.In an isothermal process

•  q=0 and ∆E=0
•  q≠0 and ∆E=0
•  q=0 and ∆E≠0
•  q≠0 and ∆E≠0

Solution
 b) For an isothermal process ∆T=0 and ∆E=0 and q≠0

Q6. Work done in reversible isothermal process is given by:

•  2.303 RT log⁡ V₂/V₁
•  nR/((γ-1) )(T₂-T₁)
•  2.303 RT log⁡ V₁/V₂
•  None of these

Solution
a) This is the derived formula for W_rev is isothermal change.

Q7.Which are correct to express work terms?

•  Work = Capacity factor × Intensity factor; where capacity factor is a measure of extent of work done and intensity factor is a measure of force responsible for work
•  Electrical work =E×nF;E is intensity factor; nF is capacity factor
•  Expansion work =P×∆V;P is intensity factor; ∆V is capacity factor
•  All of the above

Solution
(d) These are the expressions to explain work.

Q8.A gas expands isothermally and reversibly. The work done by the gas is:

•  Zero
•  Minimum
•  Maximum
•  Equal to work done

Solution
(c) Maximum work is obtained under reversible conditions (either isothermal or adiabatic).

Q9. In an isobaric process, the ratio of heat supplied to the system (dθ) and work done by the system (dW) for diatomic gas is

•  1:1
•  7:2
•  7:5
•  5:7

Solution
(b) -dQ/dW=dQ/(dQ-dE);[∵dE=dW+dQ:-dW=dQ-dE] =(nC_pdT)/(nC_p dT-nC_vdT)=C_p/((C_p-C_v)) =7/2 for diatomic gas

Q10. Thermodynamics is concerned with:

•  Total energy of a system
•  Energy changes in a system
•  Rate of chemical change
•  Mass changes in nuclear reactions

Solution
b)