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NEET UG CHEMISTRY MCQ: THERMODYNAMICS

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Application +4 | -1
Consider the following cases of standard enthalpy of reaction (ΔHr∘\left(\Delta \mathrm{H}_{\mathrm{r}}^{\circ}\right. in kJ mol−1)\left.\mathrm{kJ} \mathrm{~mol}^{-1}\right)

C2H6(g)+72O2(g)→2CO2(g)+3H2O(ℓ);ΔH1∘=−1550C(graphite)+O2(g)→CO2(g);ΔH2∘=−393.5H2(g)+12O2(g)→H2O(ℓ);ΔH3∘=−286\begin{aligned} & \mathrm{C}_2 \mathrm{H}_6(\mathrm{g})+\frac{7}{2} \mathrm{O}_2(\mathrm{g}) \rightarrow 2 \mathrm{CO}_2(\mathrm{g})+3 \mathrm{H}_2 \mathrm{O}(\ell) ;\Delta \mathrm{H}_1^{\circ}=-1550 \\ & \mathrm{C}(\text {graphite})+\mathrm{O}_2(\mathrm{g}) \rightarrow \mathrm{CO}_2(\mathrm{g}) ;\Delta \mathrm{H}_2^{\circ}=-393.5 \\ & \mathrm{H}_2(\mathrm{g})+\frac{1}{2} \mathrm{O}_2(\mathrm{g}) \rightarrow \mathrm{H}_2 \mathrm{O}(\ell) ;\Delta \mathrm{H}_3^{\circ}=-286 \end{aligned}

The magnitude of ΔHf C2H6(g)∘\Delta \mathrm{H}_{\text{f }\mathrm{C}_2 \mathrm{H}_6(\mathrm{g})}^{\circ} is ____ kJ mol−1\mathrm{kJ} \mathrm{~mol}^{-1} (Nearest integer).
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Correct option: A
95
The correct answer is 95 because it involves using Hess's Law to calculate the enthalpy of formation of C2H6 from the given combustion and formation enthalpies.

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The internal energy of one mole of a gas is: An ideal gas is allowed to expand both reversibly and irreversibly in an isolated system. If Ti{T_i} is the initial temperature and Tf{T_f} is the final temperature, which of the following statements is correct? For which of the process, ∆S is negative? The work done by a weightless piston in causing an expansion ∆V (at constant temperature), when the opposing pressure, P is variable, is given by: Assertion :- At constant pressure of the change H2O(s)→H2O(g)\mathrm{H}_2 \mathrm{O}(\mathrm{s}) \rightarrow \mathrm{H}_2 \mathrm{O}(\mathrm{g}) work done is negative.

Reason :- During phase transition work done is always negative. For the reaction, X2O4(l)⟶2XO2(g),ΔU=2.1 kcal,ΔS=20 cal K−1\mathrm{X}_2 \mathrm{O_4(l}) \longrightarrow 2 \mathrm{XO}_2(\mathrm{g}), \Delta U=2.1 \mathrm{~kcal}, \Delta S=20 \mathrm{~cal} \mathrm{~K}^{-1} at 300 K300 \mathrm{~K}. Hence, ΔG\Delta G is
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