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.

Short answer

(a) True - A spontaneous reaction in one direction will be nonspontaneous in the reverse direction under the same conditions, as it moves away from equilibrium. (b) False - Spontaneity refers to the tendency of a process, not its speed, so spontaneous processes can be slow or fast. (c) True - Most spontaneous processes are reversible, although some, like combustion, are irreversible. (d) False - An isothermal process involves constant temperature, not no heat exchange; heat can be lost or gained to maintain constant temperature. (e) False - The maximum amount of work is accomplished by reversible processes, as they are more efficient with minimal energy loss compared to irreversible processes.

Step-by-step solution

Statement (a)

A reaction that is spontaneous in one direction will be nonspontaneous in the reverse direction under the same reaction conditions. This statement is true. A spontaneous reaction occurs naturally and tends to move towards equilibrium. Under the same conditions, going in the reverse direction will mean moving away from equilibrium, making the reaction nonspontaneous.

Statement (b)

All spontaneous processes are fast. This statement is false. Spontaneity refers to the natural tendency of a process, not its speed. There can be slow spontaneous processes and fast spontaneous processes. The rate of a process depends on the reaction rate, activation energy, and other factors.

Statement (c)

Most spontaneous processes are reversible. This statement is true. A reversible process is one where the system and its surroundings can be returned to their initial states, and most spontaneous processes are reversible. However, some spontaneous processes, such as combustion, are irreversible because they release a large amount of energy and cannot be easily reversed. But in general, spontaneous processes can be reversed under certain conditions, making them reversible.

Statement (d)

An isothermal process is one in which the system loses no heat. This statement is false. An isothermal process is one in which the temperature of the system remains constant. This does not imply that there is no heat exchange between the system and its surroundings. In fact, during an isothermal process, the system may lose or gain heat to maintain a constant temperature.

Statement (e)

The maximum amount of work can be accomplished by an irreversible process rather than a reversible one. This statement is false. The maximum amount of work can be accomplished by a reversible process, not an irreversible one. During a reversible process, the system and its surroundings are always in equilibrium, allowing the process to have maximum efficiency and accomplish the greatest amount of work. Irreversible processes are less efficient due to the presence of dissipative forces, such as friction and turbulence, which cause energy to be lost as heat.

Key concepts

Spontaneous Reactions

Spontaneous reactions are processes that occur naturally under given conditions without needing any external energy input. Imagine you have a ball at the top of a hill: spontaneous reactions are like when the ball rolls down without anyone pushing it. This natural tendency for reactions to occur is due to their movement towards equilibrium. Importantly, if a reaction is spontaneous in one direction, it will be nonspontaneous in the reverse direction under the same conditions. It’s like that ball couldn’t roll back up unless someone pushes it. The spontaneity of a reaction does not tell us the speed—it might happen quickly or slowly. Understanding these concepts helps us predict whether reactions will occur naturally or require assistance.

Reversible Processes

Reversible processes are special because they can go forwards or backwards between states in a way that the system and surroundings can be restored to their original conditions. Imagine a pendulum swinging back and forth: given no losses, it can stay in motion indefinitely. Most spontaneous processes can be made reversible under certain conditions, which means they can fully return to the starting state. This is important because reversible processes can reach a point where they extract maximum work or energy. However, many real-world processes, like combustion, release large amounts of energy, making true reversibility impossible because they can’t easily return to their starting state.

Isothermal Processes

Isothermal processes keep the temperature constant. Think of keeping a cup of your favorite tea at the exact same temperature while you sip it. During these processes, heat can either enter or leave the system, as long as the temperature stays unchanged. This means an isothermal process is not about an absence of heat change, but rather about balancing heat exchanges to maintain constant temperature. Understanding isothermal processes is key in areas such as thermodynamics and engineering, especially when dealing with gas expansions and compressions.

Irreversible Processes

Irreversible processes are like one-way roads. Once they proceed in a direction, they cannot naturally revert to their initial state. For example, breaking an egg or stretching silly putty cannot return to their original forms on their own. These processes are less efficient and often involve dissipative factors like friction or turbulence, which lead to energy loss typically as heat. Because they don't maintain equilibrium, they accomplish less work compared to reversible processes. Learning about irreversible processes helps us understand the limitations and inefficiencies encountered in practical applications, such as engines and refrigerators.

Reaction Kinetics

Reaction kinetics studies how fast reactions occur and the factors affecting these rates. If we think of a reaction as a race, kinetics tells us if it’s a sprint or a marathon. Key elements influencing kinetics include reactant concentrations, temperature, and presence of catalysts. Even when a process is spontaneous, it can vary in speed; thus kinetics helps us predict time frames. Catalysts are like coaches helping the reaction proceed faster without being consumed. Understanding kinetics allows us to optimize conditions for desired reaction rates, which is crucial in chemical manufacturing and various scientific applications.