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Chapter 19: Problem 13

Indicate whether each statement is true or false. (a) A reaction that is spontaneous in one direction will be nonspontaneous in the reverse direction under the same reaction conditions. (b) All spontaneous processes are fast. (c) Most spontaneous processes are reversible. (d) An isothermal process is one in which the system loses no heat. (e) The maximum amount of work can be accomplished by an irreversible process rather than a reversible one.

Short Answer

Expert verified
(a) True; (b) False; (c) False; (d) False; (e) False.

Step by step solution

01

Understanding Spontaneity

A spontaneous reaction in one direction means it naturally occurs without any external input. This implies that the reverse reaction, under the same conditions, cannot occur naturally and is nonspontaneous. Therefore, statement (a) is true.
02

Speed of Spontaneous Processes

Spontaneity and speed are different concepts. A reaction can be spontaneous (favorable to occur) but may happen slowly over time. Hence, statement (b) that all spontaneous processes are fast is false.
03

Reversibility of Spontaneous Processes

Spontaneity often leads to irreversible changes due to the increase in total entropy. Most natural spontaneous processes are not easily reversed without external intervention. Thus, statement (c) is false.
04

Isothermal Process Characteristics

An isothermal process occurs at constant temperature, allowing heat exchange with surroundings to maintain this balance. Therefore, it doesn't mean no heat is exchanged, making statement (d) false.
05

Work in Reversible vs Irreversible Processes

According to thermodynamics, reversible processes perform the maximum amount of work because they occur infinitely slowly, allowing the system to be in equilibrium. Therefore, statement (e) is false.

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Key Concepts

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

Spontaneous Reactions
A spontaneous reaction is one that occurs naturally without any external force or energy to provoke it. For instance, rust forming on iron when exposed to air is a spontaneous reaction. The key aspect of spontaneity is that it indicates a direction of natural change.
  • A spontaneous reaction in one direction means the reverse reaction under the same conditions is nonspontaneous.
  • This is because a spontaneous reaction releases free energy, making it less likely for the reverse to naturally occur under the same conditions.
Thus, if you observe a reaction occurring spontaneously in one direction, know that the opposite direction would require external effort or changes in conditions to happen.
Reversible Processes
The concept of reversibility in thermodynamics is quite fascinating and relates to processes that can be reversed without any net change in both the system and its surroundings. In reality, truly reversible processes are idealizations and don鈥檛 occur in nature.
  • These processes are characterized by infinitesimal steps and equilibrium conditions, allowing restoration to the initial state without leaving any alterations in the universe.
  • In contrast, most spontaneous processes in nature are not reversible because they often involve an increase in the universe's entropy.
Therefore, the statement that most spontaneous processes are reversible does not hold true since practical reversibility is rare.
Entropy
Entropy is a measure of disorder or randomness in a system. It's a central concept in thermodynamics, indicating the direction and feasibility of processes. The second law of thermodynamics states that the total entropy of an isolated system can never decrease over time.
  • This is why spontaneous processes tend to increase the universe鈥檚 total entropy.
  • When a process increases entropy significantly, it's usually irreversible because it鈥檚 rare to spontaneously restore order without additional work or energy.
Entropy thus plays a critical role in determining whether a reaction or process can happen spontaneously.
Isothermal Process
An isothermal process is defined as one where the temperature remains constant. This means any heat exchange between the system and its surroundings is precisely balanced to maintain this constant temperature.
  • This does not imply that no heat is exchanged. Instead, heat must be transferred to or from the system to keep temperature steady.
  • Such processes are exemplified by slow, controlled expansions or compressions of gases.
So, while the temperature is constant, heat flow is integral to ensuring that balance, contradicting any notion that isothermal means no heat is exchanged.

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Most popular questions from this chapter

(a) Is the standard free-energy change, \(\Delta G^{\circ}\), always larger than \(\Delta G ?\) (b) For any process that occurs at constant temperature and pressure, what is the significance of \(\Delta G=0 ?\) (c) For a certain process, \(\Delta G\) is large and negative. Does this mean that the process necessarily has a low activation barrier?

Indicate whether each statement is true or false. (a) The second law of thermodynamics says that entropy can only be produced but cannot not be destroyed. (b) In a certain process the entropy of the system changes by \(1.2 \mathrm{~J} / \mathrm{K}\) (increase) and the entropy of the surroundings changes by \(-1.2 \mathrm{~J} / \mathrm{K}\) (decrease). Thus, this process must be spontaneous. (c) In a certain process the entropy of the system changes by \(1.3 \mathrm{~J} / \mathrm{K}\) (increase) and the entropy of the surroundings changes by \(-1.2 \mathrm{~J} / \mathrm{K}\) (decrease). Thus, this process must be reversible.

A certain constant-pressure reaction is barely nonspontaneous at \(45^{\circ} \mathrm{C}\). The entropy change for the reaction is \(72 \mathrm{~J} / \mathrm{K} .\) Estimate \(\Delta \mathrm{H}\).

For a particular reaction, \(\Delta H=30.0 \mathrm{~kJ}\) and \(\Delta S=90.0 \mathrm{~J} / \mathrm{K}\). Assume that \(\Delta H\) and \(\Delta S\) do not vary with temperature. (a) At what temperature will the reaction have \(\Delta G=0 ?\) (b) If T is increased from that in part (a), will the reaction be spontaneous or nonspontaneous?

For the isothermal expansion of a gas into a vacuum, \(\Delta E=0, q=0,\) and \(w=0 .(\mathbf{a})\) Is this a spontaneous process? (b) Explain why no work is done by the system during this process. (c) What is the "driving force" for the expansion of the gas: enthalpy or entropy?
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