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Spontaneity in Thermodynamics MCQ - Practice Questions with Answers

Edited By admin | Updated on Sep 25, 2023 25:23 PM | #NEET

Syllabus

Quick Facts

10 Questions around this concept.

Solve by difficulty

MEDIUM

For reaction, $2Cl(g)\rightarrow Cl_{2}(g)$ the correct option is:

$\Delta_{r}H<0$ and $\Delta_{r}S<0$

$\Delta_{r}H>0$ and $\Delta_{r}S>0$

$\Delta_{r}H>0$ and $\Delta_{r}S<0$

$\Delta_{r}H<0$ and $\Delta_{r}S>0$

View Solution

Concepts Covered - 2

Spontaneity Criteria Through Entropy

Entropy and criteria of the spontaneity of a chemical process:

The entropy change of chemical reaction together with entropy change of surrounding determine spontaneity of a chemical process under a given set of condition.

$\mathrm{\Delta S_{\text {Total }}=\Delta_{r} S-\frac{\Delta_{r} H}{T}}$

1. In nature, all process are Irreversible followed by an increase in entropy. Entropy of universe tends towards a maximum.

$\mathrm{S}_{\text {Universe }}>0$

2. $\Delta \mathrm{S}_{\mathrm{sys}}+\Delta \mathrm{S}_{\mathrm{surr}}=0 \text { for reversible process} \textrm{ and } \mathrm{S}_{\mathrm{sys}}+\Delta \mathrm{S}_{\mathrm{surr}}>0 \text { for irreversible process }$

3. $\Delta$ S for a cyclic process and at the equilibrium state is zero.

4. For a reversible process

$\begin{array}{c}{\mathrm{S}_{\mathrm{Total}} \text { or } \Delta \mathrm{S}_{\mathrm{Universe}}=0} \\ {\text { So } \quad \Delta \mathrm{S}_{\mathrm{system}}=\Delta \mathrm{S}_{\text {surromding }}}\end{array}$

5. For adiabatic reversible process entropy change is zero.

$\begin{aligned} \Delta \mathrm{S}_{\text {Total }} &=0 \text { so } \\ \text { Hence } & \Delta \mathrm{S}_{\text {system }}=\Delta \mathrm{S}_{\text {surrounding }} \end{aligned}$

6. Entropy change associated which change in temperature from T₂ to T₁ at constant pressure P is given as

$\Delta \mathrm{S}=2.303 \times \mathrm{C}_{\mathrm{P}} \log \frac{\mathrm{T}_{2}}{\mathrm{T}_{1}}$

$\mathrm{C}_{\mathrm{p}}=\text { Molar heat capacity at constant pressure. }$

Some Examples Of Entropy Change

1. When a rubber band is stretched, entropy decreases because the macromolecules get uncoiled and hence arranged in a more ordered manner that is randomness decreases.

2. When an egg is boiled, the entropy increases because denaturation occurs resulting into a change of proteins from helical form into random coiled form.

3. Molecule kept in large volume containers will have high entropy.

4. Cases of increase in entropy—(l) dissolution of a solute in water (2) decomposition of compound (3) vaporization and fusion (4) expansion of ideal gas from one container to an evacuated chamber.

4. The decrease of entropy cases are crystallization, combination.

5. Entropy is directly proportional to atomic weight for example—I₂ > Br₂ > Cl₂

6. Entropy is directly proportional to number of bonds example—Ethane > ethylene > ethylene

Spontaneity Criteria Through Enthalpy (H) And Entropy (S)

Criteria for Spontaneous Change

Processes that involve an increase in entropy of the system (ΔS > 0) are very often spontaneous; however, examples to the contrary are plentiful. By expanding consideration of entropy changes to include the surroundings, we may reach a significant conclusion regarding the relation between this property and spontaneity. In thermodynamic models, the system and surroundings comprise everything, that is, the universe, and so the following is true:

$\Delta S_{\mathrm{univ}}=\Delta S_{\mathrm{sys}}+\Delta S_{\mathrm{surr}}$

$\begin{array}{l}{\Delta S_{\text {univ }}>0\text { spontaneous }} \\ {\Delta S_{\text {univ }}<0\text { nonspontaneous (spontaneous in opposite direction) }} \\ {\Delta S_{\text {univ }}=0\text { reversible (system is at equilibrium) }}\end{array}$

$\begin{array}{l}{\text { Entropy change of }} {\text { surroundings, }} \\ \mathrm{{ \Delta S_{\text {surr }}=\frac{\Delta H_{\text {surr }}}{T}=-\frac{\Delta H_{\text {sys }}}{T}}} \\ \mathrm{{\Delta S_{\text {toal }}=\Delta S_{\text {sys }}+\left(-\frac{\Delta H_{\text {sys }}}{T}\right)}}\end{array}$

1. The objects are at different temperatures, and heat flows from the hotter to the cooler object. This is always observed to occur spontaneously. Designating the hotter object as the system and invoking the definition of entropy yields the following:

$\Delta S_{\mathrm{sys}}=\frac{-q_{\mathrm{rev}}}{T_{\mathrm{sys}}} \quad \text { and } \quad \Delta S_{\mathrm{surr}}=\frac{q_{\mathrm{rev}}}{T_{\mathrm{surr}}}$

2. The objects are at different temperatures, and heat flows from the cooler to the hotter object. This is never observed to occur spontaneously. Again designating the hotter object as the system and invoking the definition of entropy yields the following:

$\Delta S_{\mathrm{sys}}=\frac{q_{\mathrm{rev}}}{T_{\mathrm{sys}}} \quad \text { and } \quad \Delta S_{\mathrm{surr}}=\frac{-q_{\mathrm{rev}}}{T_{\mathrm{surr}}}$

3. The temperature difference between the objects is infinitesimally small, $T_{sys} \approx T_{surr}$ , and so the heat flow is thermodynamically reversible. See the previous section’s discussion). In this case, the system and surroundings experience entropy changes that are equal in magnitude and therefore sum to yield a value of zero for $\mathrm{\Delta S_{univ}}$ . This process involves no change in the entropy of the universe.