Problem 13 - Chemistry: The Central Science in SI Units, Global Edition

Question

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.

Step-by-Step Solution

Answer

(a) True (b) False (c) False (d) False (e) False

Step 1: (a) Spontaneous Reaction in One Direction and Nonspontaneous in the Reverse Direction

For a reaction to be spontaneous, it should have a negative Gibbs free energy (ΔG). When the reaction reverses, the sign of ΔG also reverses. So, if the forward reaction is spontaneous (ΔG < 0), the reverse reaction will have ΔG > 0, which means the reverse reaction is nonspontaneous. Therefore, the statement is \(True\).

Step 2: (b) Spontaneity and Speed of Processes

Spontaneity of a process just depends on the initial and final states, and does not account for the speed of the process. It is determined by the Gibbs free energy (ΔG). Processes with negative ΔG are spontaneous, but it does not necessarily mean they are fast. Some spontaneous processes can be quite slow. Therefore, the statement is \(False\).

Step 3: (c) Spontaneity and Reversibility of Processes

Spontaneity does not directly relate to the reversibility of a process. Spontaneity is defined by the Gibbs free energy (ΔG), and for a process to be reversible, it should be in equilibrium, meaning there should be no net change in free energy. The statement that most spontaneous processes are reversible is not correct. Therefore, the statement is \(False\).

Step 4: (d) Isothermal Process and Heat Loss

An isothermal process is a process that occurs at a constant temperature. It does not necessarily mean that the system loses no heat. During an isothermal process, heat can be exchanged between the system and its surroundings, as it should maintain a constant temperature. Therefore, the statement is \(False\).

Step 5: (e) Maximum Work Accomplished by Irreversible or Reversible Processes

The maximum amount of work that can be accomplished by a system is when the process is reversible. This is because, during a reversible process, the system is always near equilibrium, and the work done by the system is maximized in small increments. In contrast, an irreversible process deviates from equilibrium and results in less work being done by the system due to losses caused by factors such as friction and turbulence. Therefore, the statement is \(False\).

Key Concepts

Spontaneous Reactions

In thermodynamics, a spontaneous reaction refers to a process that proceeds on its own without needing energy from an external source. The spontaneity of a reaction is determined by the Gibbs free energy change (ΔG). If ΔG is negative, the process is spontaneous. Conversely, if a reaction is spontaneous in one direction, the reverse reaction will have a positive ΔG, indicating non-spontaneity. This concept helps in predicting the directionality of chemical reactions. However, it's important to remember that spontaneity doesn't imply the speed of the process—some spontaneous reactions occur slowly.

Gibbs Free Energy

Gibbs free energy is an important thermodynamic quantity that predicts whether a reaction will occur spontaneously. It is represented by the symbol ΔG, which is calculated using the equation: \( ΔG = ΔH - TΔS \).
Where:

• ΔH is the change in enthalpy (heat content).
• T is the absolute temperature (in Kelvin).
• ΔS is the change in entropy (disorder).

A negative ΔG suggests a spontaneous reaction, while a positive ΔG indicates a non-spontaneous reaction. Understanding Gibbs free energy helps chemists and engineers design reactions and processes that are energetically favorable. It's crucial in both natural and industrial systems.

Reversible and Irreversible Processes

In thermodynamics, processes can be classified as reversible or irreversible. A reversible process is ideally one where the system changes state in such a manner that the system and its surroundings can be returned to their original states without any net change. This occurs when the process happens infinitely slowly, maintaining equilibrium throughout.
Irreversible processes, however, proceed at a finite rate and cause the system to deviate from equilibrium temporarily. Factors such as friction and turbulence often make a process irreversible. As a rule, reversible processes perform the maximum amount of work. Whereas, the irreversible processes perform less work due to energy losses. This distinction is critical in understanding how to maximize efficiency in engines and other thermodynamic systems.

Isothermal Process

An isothermal process is a thermodynamic process in which the temperature of the system remains constant. The term 'isothermal' is derived from 'iso-' meaning "same," and 'thermal' relating to temperature. During an isothermal process, a system can exchange heat with its surroundings to maintain a steady temperature. This is in contrast to processes where temperature changes occur, such as adiabatic processes where no heat is transferred.
Isothermal processes are common in various applications, such as in refrigerators and heat engines. Understanding these processes is crucial in designing systems that must maintain constant temperatures while performing work or transferring energy.