Question

Which is not a spontaneous process? (1) Expansion of a gas into vacuum (2) Water flowing down a hill (3) Heat flowing from a colder body to a hotter body (4) Evaporation of water from clothes during drying

Short answer

Option (3): Heat flowing from a colder body to a hotter body.

Step-by-step solution

Understand the concept of spontaneity

A spontaneous process is a process that occurs without needing to be driven by an external force or energy. These processes typically result in an increase in entropy (disorder) of the system.

Analyze each option

Consider whether each given process happens naturally or requires external intervention: 1. Expansion of a gas into a vacuum: Happens naturally as gas molecules spread out to fill the available space. 2. Water flowing down a hill: Naturally occurs due to gravity. 3. Heat flowing from a colder body to a hotter body: This goes against the natural direction of heat flow (from hot to cold). 4. Evaporation of water from clothes: Happens naturally as water evaporates into the air.

Identify the non-spontaneous process

From the analysis in Step 2, the process listed as option (3) requires external work or energy to occur. Heat naturally flows from hot to cold, not the other way around. Therefore, heat flowing from a colder body to a hotter body is not a spontaneous process.

Key concepts

entropy

Entropy is a measure of the disorder or randomness in a system. In thermodynamics, it plays a crucial role in determining the direction of spontaneous processes. Generally, for a process to be spontaneous, the total entropy of the system and its surroundings must increase.
For example, when a gas expands into a vacuum, its molecules spread out, increasing the system's entropy. This natural tendency to move towards a state of higher disorder is what drives spontaneous processes.
Entropy can be viewed as a driving force in nature, ensuring that systems evolve towards states that are more probable and disordered.

natural processes

Natural processes are phenomena that occur without the need for external energy or intervention. They follow the path dictated by the laws of thermodynamics.
Examples of natural processes include:

• The expansion of gas in a vacuum
• Water flowing downhill due to gravity
• Evaporation of water from surfaces

In each of these scenarios, the processes happen because they lead to an increase in the system's entropy, aligning with the second law of thermodynamics.
Understanding natural processes helps explain why certain reactions and changes occur spontaneously, embracing the concept of entropy increase.

external energy in thermodynamics

In thermodynamics, external energy is often required to drive non-spontaneous processes. These are processes that do not naturally increase the system's entropy.
For example, to make heat flow from a colder body to a hotter body, as seen in refrigerators, you need to input external energy. This process is against the natural flow of heat, which typically moves from hot to cold regions.
It's important to note that while external energy can force non-spontaneous processes to occur, it does so by generally increasing the overall entropy of the universe, thus adhering to the second law of thermodynamics.

heat transfer

Heat transfer deals with the movement of thermal energy from one body to another. It always occurs in a specific direction, from a higher-temperature body to a lower-temperature body.
This behavior aligns with the second law of thermodynamics. For example, if you place a hot object in contact with a cold object, heat will flow naturally from the hot object to the cold one until thermal equilibrium is reached.
This natural direction of heat transfer is easy to observe in daily life, such as when a hot drink loses its heat to the surrounding cooler air.

non-spontaneous processes

Non-spontaneous processes are those that require external work or energy input to occur. They are the opposite of spontaneous processes, which happen without any external intervention.
A common example of a non-spontaneous process is heat flowing from a colder body to a hotter body. This situation requires external energy, like when using a refrigerator to cool its contents against the natural heat flow.
Understanding non-spontaneous processes helps us apply thermodynamic principles practically, especially in designing systems that efficiently manage energy transfer and create desired outcomes against natural tendencies.