Thermodynamics Chapter-Wise Test 2

Correct answer Carries: 4.

Wrong Answer Carries: -1.

For the reaction \( 2A(g) + B(g) \rightarrow 2D(g) \), \( \Delta U = -10.5 \, \text{kJ} \), \( \Delta S = -44.1 \, \text{J/K} \) at 298 K. What is \( \Delta G \)?

Calculate \( \Delta H = \Delta U + \Delta n_g RT \), where \( \Delta n_g = 2 - 3 = -1 \), \( RT = 8.314 \times 298 \times 10^{-3} = 2.4776 \, \text{kJ} \). Thus, \( \Delta H = -10.5 - 2.4776 = -12.9776 \, \text{kJ} \). Then, \( \Delta G = \Delta H - T\Delta S = -12.9776 - 298 \times (-0.0441) = -12.9776 + 13.1418 = 0.15 \, \text{kJ} \).

-10.5 kJ
10.5 kJ
0.15 kJ
-0.15 kJ
3

The bond enthalpy of C-Cl in \( \ce{CCl4(g)} \) is to be calculated given: \( \Delta H_f^\circ(\ce{CCl4,g}) = -95.7 \, \text{kJ/mol} \), \( \Delta H_a(\ce{C,g}) = 715 \, \text{kJ/mol} \), \( \Delta H_a(\ce{Cl2,g}) = 242 \, \text{kJ/mol} \). What is the bond enthalpy?

For \( \ce{CCl4(g)} \rightarrow \ce{C(g)} + 4\ce{Cl(g)} \), \( \Delta H = \Delta H_a(\ce{C}) + 2 \times \Delta H_a(\ce{Cl2}) - \Delta H_f^\circ(\ce{CCl4,g}) = 715 + 2 \times 242 - (-95.7) = 715 + 484 + 95.7 = 1294.7 \, \text{kJ/mol} \). Bond enthalpy = \( 1294.7 / 4 = 323.7 \, \text{kJ/mol} \).

242.0 kJ/mol
715.0 kJ/mol
323.7 kJ/mol
95.7 kJ/mol
3

Which thermodynamic quantity is a measure of disorder in a system?

Entropy (\(S\)) is a measure of the disorder or randomness in a system, as defined in thermodynamics.

Entropy
Enthalpy
Internal energy
Gibbs free energy
1

For a reaction with \(\Delta H = 120 \, \text{kJ/mol}\) and \(\Delta S = 300 \, \text{J/K}\), what is \(\Delta G\) at 400 K?

\(\Delta G = \Delta H - T\Delta S = 120 - 400 \times 0.3 = 120 - 120 = 0 \, \text{kJ/mol}\).

120 kJ/mol
-120 kJ/mol
300 kJ/mol
0 kJ/mol
4

For an exothermic reaction at constant pressure, the heat evolved is equal to:

For an exothermic reaction at constant pressure, heat evolved \(q_p = \Delta H\), and since it’s exothermic, \(\Delta H < 0\), so \(q_p = \Delta H\).

\(\Delta U\)
\(\Delta H\)
\(\Delta G\)
\(-\Delta U\)
2

For an ideal gas (\(\gamma = 1.4\)) expanding adiabatically from 15 L to 30 L at 450 K, what is the final temperature?

\(T_2 = T_1 (V_1/V_2)^{\gamma-1} = 450 \times (15/30)^{0.4} = 450 \times (0.5)^{0.4} = 450 \times 0.7579 = 341 \, \text{K}\).

225 K
341 K
595 K
450 K
2

For a process with \(\Delta H = 50 \, \text{kJ}\) and \(\Delta G = 20 \, \text{kJ}\) at 300 K, what is \(\Delta S\)?

\(\Delta G = \Delta H - T\Delta S\), so \(20 = 50 - 300 \Delta S\), \(300 \Delta S = 30\), \(\Delta S = 0.1 \, \text{kJ/K} = 100 \, \text{J/K}\).

150 J/K
50 J/K
-100 J/K
100 J/K
4

What is the relationship between \(\Delta H\) and \(\Delta U\) for the reaction \(2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)\) at 298 K? (\(R = 8.314 \, \text{J/mol·K}\))

\(\Delta H = \Delta U + \Delta n_g RT\). Here, \(\Delta n_g = 2 - 3 = -1\), \(RT = 8.314 \times 298 \times 10^{-3} = 2.48 \, \text{kJ}\), so \(\Delta H = \Delta U - 2.48 \, \text{kJ}\).

\(\Delta H = \Delta U + 2.48 \, \text{kJ}\)
\(\Delta H = \Delta U - 2.48 \, \text{kJ}\)
\(\Delta H = \Delta U\)
\(\Delta H = \Delta U + 4.96 \, \text{kJ}\)
2

Which of the following processes has a negative \(\Delta S\)?

Entropy decreases when disorder decreases, e.g., in condensation (\(H_2O(g) \rightarrow H_2O(l)\)), as gas to liquid reduces randomness.

\(H_2O(s) \rightarrow H_2O(l)\)
\(H_2O(g) \rightarrow H_2O(l)\)
\(2H_2(g) + O_2(g) \rightarrow 2H_2O(g)\)
\(NH_4Cl(s) \rightarrow NH_3(g) + HCl(g)\)
2

Using Hess’s law, calculate \(\Delta H\) for \(2S(s) + 3O_2(g) \rightarrow 2SO_3(g)\) given: \(S(s) + O_2(g) \rightarrow SO_2(g)\), \(\Delta H = -296.8 \, \text{kJ/mol}\); \(2SO_2(g) + O_2(g) \rightarrow 2SO_3(g)\), \(\Delta H = -197.8 \, \text{kJ/mol}\).

Multiply first by 2: \(2S(s) + 2O_2(g) \rightarrow 2SO_2(g)\), \(\Delta H = -593.6 \, \text{kJ}\). Add second: \(\Delta H = -593.6 + (-197.8) = -791.4 \, \text{kJ}\).

-494.6 kJ
791.4 kJ
-791.4 kJ
-98.9 kJ
3

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