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Chapter 19: Problem 35

Does the entropy of the system increase, decrease, or stay the same when (a) a solid melts, (b) a gas liquefies, (c) a solid sublimes?

Short Answer

Expert verified
(a) increase, (b) decrease, (c) increase.

Step by step solution

01

Analyze Melting (Solid to Liquid)

When a solid melts into a liquid, its molecules move from a more ordered, rigid state to a less ordered, more fluid state. This transition increases randomness and disorder in the system, leading to an increase in entropy.
02

Analyze Liquefaction (Gas to Liquid)

When a gas condenses into a liquid, its particles move from a highly disordered state to a more ordered one, as they are much closer in the liquid state. This transition decreases randomness and disorder, causing a decrease in entropy.
03

Analyze Sublimation (Solid to Gas)

During sublimation, a solid transitions directly into a gas. The molecules move from a highly ordered state to a highly disordered state. This dramatic increase in disorder leads to an increase in entropy.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Phase Transitions
Phase transitions describe the process of changing a substance from one state of matter to another. Common transitions include melting (solid to liquid), vaporization (liquid to gas), and sublimation (solid to gas). These changes occur when energy in the form of heat is added or removed from a system, altering the intermolecular forces within a substance.
  • Melting: The transition from a solid to a liquid involves particles moving from a fixed, ordered structure to a less ordered, fluid arrangement. This process leads to an increase in entropy, as the molecules gain energy and freedom to move more chaotically.
  • Vaporization and Sublimation: When a liquid transitions to a gas (vaporization) or a solid to a gas (sublimation), it results in a significant increase in randomness and entropy. Both processes involve molecules spreading out into much less orderly gaseous phases.
  • Liquefaction: Conversely, the transition from gas to liquid, called liquefaction, reduces entropy. The particles move from a dispersed, highly chaotic state to a more ordered liquid state as they lose energy and come closer together.
Thermodynamics
Thermodynamics is the branch of physics that deals with heat, work, and the energy transformations in physical systems. It provides the principles governing how and why phase transitions occur, heavily relying on the concept of entropy. Entropy is a measure of disorder or randomness in a system. The Second Law of Thermodynamics states that the total entropy of an isolated system can never decrease over time. In idealized processes, while the system undergoes some phase transitions, the entropy change can be zero. However, in real-world processes, like melting and sublimation, the entropy tends to increase due to the increased disorder.
  • Energy Input and Output: During a phase transition, energy input or output can shift equilibrium between states. Melting and sublimation typically require energy input, while liquefaction usually releases energy.
  • Spontaneity: A core principle of thermodynamics is that processes move towards maximum entropy. Thus, spontaneous processes, like melting ice in a warm room, occur because they increase the system's total entropy.
  • System Equilibrium: Phase transitions signal changes towards equilibrium where the entropy of the system and surroundings find balance. This concept ties directly to broader thermodynamic principles that govern physical and chemical changes.
Disorder in Systems
In thermodynamics, disorder or randomness within a system is quantified by entropy. The more disordered a system is, the higher its entropy. Changes in phase often signify a shift in entropy due to alterations in molecular arrangement.
  • Order vs. Disorder: A solid's particles are closely packed in an orderly lattice, representing low entropy. As it melts or sublimes, the increasing freedom of movement and less structured arrangement cause higher entropy.
  • Gas State: A gas's molecules move randomly and freely, covering more volume and hence possessing higher entropy compared to liquids or solids. This is why transitions like sublimation involve such an entropy increase; solids go directly into the gas phase, creating massive disorder.
  • Entropy Changes: Recognizing entropy changes helps predict the feasibility of phase transitions. Processes leading to higher entropy are typically more favorable in nature. Thus, understanding the behavior of systems in terms of disorder helps explain why processes like sublimation and melting spontaneously occur under the right conditions.

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Most popular questions from this chapter

Consider the melting of ice (solid water) to liquid water at a pressure of \(101.3 \mathrm{kPa}\). (a) Is this process endothermic or exothermic? (b) In what temperature range is it a spontaneous process? (c) In what temperature range is it a nonspontaneous process? (d) At what temperature are the two phases in equilibrium?

(a) What sign for \(\Delta S\) do you expect when the volume of \(0.200 \mathrm{~mol}\) of an ideal gas at \(27^{\circ} \mathrm{C}\) is increased isothermally from an initial volume of \(10.0 \mathrm{~L} ?(\mathbf{b})\) If the final volume is \(18.5 \mathrm{~L},\) calculate the entropy change for the process. (c) Do you need to specify the temperature to calculate the entropy change?

The following processes were all discussed in Chapter \(18,\) "Chemistry of the Environment." Estimate whether the entropy of the system increases or decreases during each process: (a) photodissociation of \(\mathrm{O}_{2}(g),(\mathbf{b})\) formation of ozone from oxygen molecules and oxygen atoms, \((\mathbf{c})\) diffusion of CFCs into the stratosphere, (d) desalination of water by reverse osmosis.

As shown here, one type of computer keyboard cleaner contains liquefied 1,1-difluoroethane \(\left(\mathrm{C}_{2} \mathrm{H}_{4} \mathrm{~F}_{2}\right),\) which is a gas at atmospheric pressure. When the nozzle is squeezed, the 1,1 -difluoroethane vaporizes out of the nozzle at high pressure, blowing dust out of objects. (a) Based on your experience, is the vaporization a spontaneous process at room temperature? (b) Defining the 1,1 -difluoroethane as the system, do you expect \(q_{\mathrm{sys}}\) for the process to be positive or negative? (c) Predict whether \(\Delta S\) is positive or negative for this process. (d) Given your answers to (a), (b), and (c), do you think the operation of this product depends more on enthalpy or entropy? [Sections 19.1 and 19.2 ]

For a certain chemical reaction, \(\Delta H^{\circ}=-40.0 \mathrm{~kJ}\) and \(\Delta S^{\circ}=-150.0 \mathrm{~J} / \mathrm{K} .(\mathbf{a})\) Does the reaction lead to an increase or decrease in the randomness or disorder of the system? (b) Does the reaction lead to an increase or decrease in the randomness or disorder of the surroundings? (c) Calculate \(\Delta G^{\circ}\) for the reaction at \(298 \mathrm{~K}\). (d) Is the reaction spontaneous at \(298 \mathrm{~K}\) under standard conditions?
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