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📝 Second law of thermodynamics entropy (13 MCQs)

📖 From Principles of Biochemistry • 1. The Foundations of Biochemistry • 13 questions available

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What is Second law of thermodynamics entropy?

Definition:
The second law of thermodynamics states that in any energy transfer or transformation, the total entropy (disorder) of an isolated system always increases, meaning that natural processes are irreversible and tend toward equilibrium, and in biological systems, although local order can increase (e.g., forming complex molecules), this must be balanced by a greater increase in entropy in the surroundings, usually as heat release.

Working:
This law works by dictating that energy transformations are not 100% efficient, and some energy is always lost as heat, increasing the entropy of the surroundings; the change in entropy is given by $\Delta S = \frac{Q_{\text{rev}}}{T}$ , and the Gibbs free energy equation $\Delta G = \Delta H - T\Delta S$ incorporates entropy, determining the spontaneity of reactions; organisms must overcome entropy by coupling unfavorable reactions to favorable ones (e.g., ATP hydrolysis).

Example:
A simple example is the breakdown of glucose, which releases energy and creates heat, increasing entropy in the surroundings, while the cell uses the released energy to synthesize proteins, increasing order locally, but overall entropy increases, and the body maintains a low-entropy state only by constantly expelling heat to the environment.

Reason:
The second law is fundamental to understanding why life requires continuous energy input, why metabolic processes are irreversible, and the limits of energy conversion, making it essential for bioenergetics, physiology, and environmental science.

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📝 All Second law of thermodynamics entropy MCQs

Q1. Which statement best captures the role of entropy in predicting the spontaneous direction of an isolated process?

A.Entropy must always decrease because systems seek greater stability

B.Entropy tends to increase because energy becomes more dispersed among accessible states ✅

C.Entropy remains constant whenever no chemical bonds are broken

D.Entropy increases only when temperature decreases

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Entropy measures the extent of energy dispersal and the number of accessible microscopic arrangements. For an isolated system undergoing a spontaneous process, the overall entropy tends to increase, reflecting the statistically favored direction.

Q2. Two equal-temperature compartments contain different concentrations of a solute and are suddenly connected. The solute spreads throughout both compartments without external work. What best explains the process?

A.The solute spreads because its entropy decreases

B.The solute spreads because more spatial arrangements become accessible ✅

C.The solute spreads because the total energy is destroyed

D.The solute spreads only if the final concentration is higher than the initial concentration

💡 Difficulty: medium | ✅ Correct: B

📖 Explanation: Initially, solute molecules are restricted mainly to one compartment, limiting their possible arrangements. After mixing, they can occupy positions throughout the combined volume, greatly increasing the number of accessible arrangements and therefore increasing entropy.

Q3. A protein folds from a flexible unfolded state into a compact structure. A student concludes that folding must violate the second law because the protein becomes more ordered. Which response is most accurate?

A.Yes, because entropy of every component must increase separately

B.No, because the surrounding solvent can gain enough entropy to offset the protein's entropy decrease ✅

C.Yes, because biological molecules cannot undergo spontaneous ordering

D.No, because entropy is irrelevant to molecular folding

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: Protein folding can decrease the conformational entropy of the protein itself, yet the total entropy can still increase. Changes in solvent organization, heat exchange, and molecular interactions can compensate for the local ordering.

Q4. A cell maintains highly organized internal structures while continuously exchanging matter and energy with its surroundings. Which model best explains why this does not contradict entropy increase?

A.The cell prevents entropy from existing inside its boundaries

B.The cell decreases total entropy by converting disorder directly into information

C.The cell can maintain local organization while increasing entropy in its surroundings ✅

D.The cell reaches thermodynamic equilibrium and therefore stops producing entropy

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: A living cell is not an isolated system. It can use energy and matter from its surroundings to maintain organized structures while releasing heat and waste products, allowing the entropy of the larger system to increase.

Q5. A reaction mixture is placed in a perfectly insulated container. During the reaction, molecular energy becomes distributed among many more possible molecular motions. Which conclusion is most justified?

A.The process is consistent with an increase in total entropy ✅

B.The process must decrease entropy because the container is insulated

C.The process violates the second law because no heat leaves

D.The process has zero entropy change because temperature is constant

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Insulation prevents energy exchange with the surroundings but does not prevent internal redistribution of energy. If energy becomes accessible through more microscopic arrangements, the total entropy of the isolated system can increase.

Q6. A biochemical process converts one relatively constrained molecular arrangement into several smaller molecules that can move independently. Assuming temperature and other relevant conditions remain comparable, which prediction is most reasonable?

A.Entropy is likely to increase because more independent arrangements become available ✅

B.Entropy must decrease because chemical bonds are broken

C.Entropy remains unchanged because atoms are conserved

D.Entropy increases only if the products contain more total atoms

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Breaking a constrained molecular structure into several independently moving molecules generally increases the number of accessible translational and configurational arrangements. This greater freedom makes an entropy increase plausible under comparable conditions.

Q7. A membrane separates two regions containing unequal concentrations of a dissolved substance. When the membrane becomes permeable, the substance redistributes until concentrations become similar. Why is the final state statistically favored?

A.There are fewer possible molecular arrangements after redistribution

B.More microscopic distributions correspond to the mixed state than to the strongly separated state ✅

C.The molecules lose all kinetic energy after redistribution

D.The membrane forces the molecules into one arrangement

💡 Difficulty: hard | ✅ Correct: B

📖 Explanation: A strongly separated concentration pattern represents relatively few possible molecular distributions. When molecules spread across the available space, vastly more microscopic arrangements become possible, making the mixed state statistically favored.

Q8. A student argues: 'If a process produces a more organized structure, it cannot be spontaneous because spontaneous processes always increase disorder everywhere.' What is the key flaw?

A.Spontaneous processes never involve organization

B.Entropy applies only to chemical reactions

C.A subsystem can become more ordered if the entropy increase elsewhere is sufficiently large ✅

D.Order and entropy are completely unrelated

💡 Difficulty: hard | ✅ Correct: C

📖 Explanation: The second law concerns the entropy change of the relevant total system rather than requiring every component to become more disordered. Local ordering can occur when it is accompanied by a sufficiently large entropy increase elsewhere.

Q9. A researcher compares two processes. Process A changes a molecule into products with many accessible conformations, while Process B restricts the products to one dominant conformation. Which process would generally be associated with the larger configurational entropy increase?

A.Process A, because it provides more accessible molecular arrangements ✅

B.Process B, because restriction always increases entropy

C.Both must have identical entropy changes

D.Neither can have an entropy change because conformations do not affect entropy

💡 Difficulty: medium | ✅ Correct: A

📖 Explanation: Configurational entropy depends on the number of accessible molecular arrangements. A product population capable of occupying many conformations has more possible microscopic states, generally giving it greater configurational entropy than a strongly restricted population.

Q10. A graph shows entropy $S$ on the vertical axis and the progress of a spontaneous process on the horizontal axis. The curve rises rapidly at first and then approaches a plateau. Which interpretation is most consistent with the graph?

A.Entropy initially decreases and later becomes constant

B.Entropy increases throughout the process, with the rate of increase becoming smaller ✅

C.Entropy increases linearly without any change in rate

D.Entropy oscillates because spontaneous processes alternate between order and disorder

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The upward slope indicates increasing entropy as the process proceeds. The flattening curve indicates that the entropy is approaching a limiting state, so the rate of entropy increase becomes smaller rather than remaining constant.

Q11. A reaction causes the reacting molecules to become more ordered, but it releases heat into the surroundings. Which combination could still make the overall process thermodynamically favorable?

A.A decrease in system entropy with no compensating environmental change

B.A sufficiently large entropy increase in the surroundings that outweighs the system's entropy decrease ✅

C.An entropy decrease in both system and surroundings

D.No entropy change anywhere

💡 Difficulty: easy | ✅ Correct: B

📖 Explanation: The system can experience an entropy decrease while the surroundings gain entropy through energy transfer. If the surroundings' entropy increase is sufficiently large, the total entropy change can remain positive, making the overall process compatible with spontaneity.

Q12. Two possible molecular transformations have the following qualitative effects. Transformation X decreases the number of accessible arrangements in the system but substantially increases energy dispersal in the surroundings. Transformation Y increases system arrangements slightly but decreases environmental energy dispersal strongly. Which is more consistent with spontaneous behavior?

A.X, if its total entropy change is positive ✅

B.Y, regardless of environmental entropy

C.Both are equally spontaneous because system entropy alone determines direction

D.Neither can occur because system and surroundings must have equal entropy changes

💡 Difficulty: hard | ✅ Correct: A

📖 Explanation: The relevant criterion is the entropy change of the combined system and surroundings. Transformation X can be spontaneous when its environmental entropy increase exceeds the system's decrease, whereas Y may oppose spontaneity despite local disorder.

Q13. Consider a hypothetical isolated biochemical system containing molecules that can occupy $10^3$ accessible microscopic states initially and $10^6$ states after a spontaneous rearrangement. Which conclusion follows from this change?

A.The final state has lower entropy because fewer states are occupied

B.The entropy decreases by a factor of

10^3

C.The final state has greater entropy because many more microscopic arrangements are accessible ✅

D.Entropy cannot change in an isolated system

💡 Difficulty: easy | ✅ Correct: C

📖 Explanation: Entropy is related to the number of accessible microscopic states through $S=k\ln\Omega$ . Increasing $\Omega$ from $10^3$ to $10^6$ therefore increases entropy, although the increase is logarithmic rather than proportional to the number of states.

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