## Thermodynamics Questions and Answers Part-8

1. Bond dissociation enthalpy of $H_{2}, Cl_{2} and HCl$    are 434 , 242 and 431 kJ $mol^{-1}$  respectively. Enthalpy of formation of HCl is :
a) $93 kJ mol^{-1}$
b) $– 245 kJ mol^{-1}$
c) $– 93 kJ mol^{-1}$
d) $245 kJ mol^{-1}$

Explanation:

2. The values of $\triangle H and \triangle S$   for the reaction, $C\left(graphite\right)+CO_{2}\left(g\right)\rightarrow 2CO\left(g\right)$        are 170 kJ and $170 JK^{-1}$  , respectively. This reaction will be spontaneous at
a) 910 K
b) 1110 K
c) 510 K
d) 710 K

Explanation:

3. Standard entropies of $X_{2},Y_{2}and XY_{3}$   are 60, 40 and $50 JK^{-1}mol^{-1}$   respectively. For the reaction
$\frac{1}{2}X_{2}+\frac{3}{2}Y_{2}\rightleftharpoons XY_{3},\triangle H=– 30 kJ$
to be at equilibrium, the temperature should be:
a) 750 K
b) 1000 K
c) 1250 K
d) 500 K

Explanation:

4. For vaporization of water at 1 atmospheric pressure, the values of $\triangle H and \triangle S$   are $40.63 k J mol^{-1} and 108.8 JK^{-1}mol^{-1}$       respectively. The temperature when Gibbs energy change $\left(\triangle G \right)$ for this transformation will be zero, is:
a) 293.4 K
b) 273.4 K
c) 393.4 K
d) 373.4 K.

Explanation:

5. The following two reactions are known :
$Fe_{2}O_{3}\left(s\right)+3CO\left(g\right)\rightarrow 2Fe\left(s\right)+3CO_{2}\left(g\right);\triangle H= –26.8 kJ$
$FeO\left(s\right)+CO\left(g\right)\rightarrow Fe\left(s\right)+CO_{2}\left(g\right);\triangle H= –16.5 kJ$
The value of $\triangle H$   for the following reaction
$Fe_{2}O_{3}\left(s\right)+CO\left(g\right)\rightarrow 2FeO\left(s\right)+CO_{2}\left(g\right)$
a) + 6.2 kJ
b) + 10.3 kJ
c) – 43.3 kJ
d) – 10.3 kJ

Explanation:

6. Three moles of an ideal gas expanded spontaneously into vacuum. The work done will be :
a) Zero
b) Infinite
c) 3 Joules
d) 9 Joules

Explanation: Ideal gas during spontaneous expansion into vacuum does not do any external work.

7. If the enthalpy change for the transition of liquid water to steam is $30 kJ mol^{-1}$   at 27ºC, the entropy change for the process would be :
a) $10 J mol^{-1}K^{-1}$
b) $1.0 J mol^{-1}K^{-1}$
c) $0.1 J mol^{-1}K^{-1}$
d) $100 J mol^{-1}K^{-1}$

Explanation:

8. Enthalpy change for the reaction,$4H\left(g\right)\rightarrow2H_{2}\left(g\right)$     is – 869.6 kJ. The dissociation energy of H–H bond is :
a) – 434.8 kJ
b) – 869.6 kJ
c) + 434.8 kJ
d) + 217.4 kJ

Explanation:

9. Consider the following processes :
$1\diagup2 A\rightarrow B$         +150 kJ/mol
$3B\rightarrow 2C+D$         –125 kJ/mol
$E+A\rightarrow2D$         +350 kJ/mol
For $B+D\rightarrow E+2C,\triangle H$     will be :
a) 525 kJ/mol
b) – 175 kJ/mol
c) – 325 kJ/mol
d) 325 kJ/mol

10. In which of the following reactions, standard entropy change ($\triangle S^{\circ}$ is positive and standard Gibb’s energy change $\triangle G^{\circ}$ decreases sharply with increase in temperature ?
a) $C\left(graphite\right)+\frac{1}{2}O_{2}\left(g\right)\rightarrow CO\left(g\right)$
b) $CO\left(g\right)+\frac{1}{2}O_{2}\left(g\right)\rightarrow CO_{2}\left(g\right)$
c) $Mg\left(s\right)+\frac{1}{2}O_{2}\left(g\right)\rightarrow MgO\left(s\right)$
d) $\frac{1}{2}C\left(graphite\right)+\frac{1}{2}O_{2}\left(g\right)\rightarrow \frac{1}{2}CO_{2}\left(g\right)$