Question

Give a detailed example of each of the following, with an explanation: (a) a thermodynamically spontaneous process; (b) a process that would violate the first law of thermodynamics; (c) a process that would violate the second law of thermodynamics; (d) an irreversible process; (e) an equilibrium process.

Short answer

Examples: a) A thermodynamically spontaneous process: Melting of ice at room temperature. b) A process violating the first law of thermodynamics: Concept of a 'Perpetual motion machine'. c) A process violating the second law of thermodynamics: Cold coffee reheating itself to room temperature. d) An irreversible process: Burning a piece of paper. e) An equilibrium process: A glass of saturated salt water kept undisturbed at constant temperature and pressure.

Step-by-step solution

Identifying a Thermodynamically Spontaneous Process

A thermodynamically spontaneous process is one that can occur without any external energy input. A natural and common example is the melting of ice at room temperature. This process is spontaneous because given enough time, the ice at a temperature below 0°C will absorb energy from the environment and transform into water, without any outside interference.

Identifying a Process that Violates the First Law of Thermodynamics

The First Law of Thermodynamics states that energy can neither be created nor destroyed, only transferred or changed in form. Thus, any process claiming to create or destroy energy would violate this law. A 'perpetual motion machine' is a hypothetical example, as it claims to work indefinitely without an energy source, implying that it creates its own energy, which is in direct violation of the First Law.

Identifying a Process that Violates the Second Law of Thermodynamics

The Second Law of Thermodynamics states that the entropy of an isolated system can never decrease over time. It is essentially a statement about the natural direction of heat flow from hot to cold. A cold object warming itself without any heat source, such as a cold cup of coffee reheating itself to room temperature, is a process that would violate the Second Law.

Identifying an Irreversible Process

An irreversible process is one that cannot return to its original state without inducing some change in the surroundings. Burning a piece of paper is an example. Once the paper is burnt, it cannot return to its original form.

Identifying an Equilibrium Process

An equilibrium process is one in which the system is at equilibrium at every stage of the process. The process proceeds infinitesimally slow so that the system composition remains uniform throughout the process. A glass of saturated salt water left undisturbed at a constant temperature and pressure is an example. In this state, the rate of salt dissolving is equal to the rate of salt precipitating from the solution, so we can consider the system to be in equilibrium.

Key concepts

First Law of Thermodynamics

The First Law of Thermodynamics is a fundamental principle that tells us energy in a closed system is constant. It can be transferred or converted from one form to another, but it can't be created from nothing or destroyed. This means that every event or process involving energy exchange must account for all the energy within a system. For instance, when you burn fuel in a car engine, the chemical energy is transformed into kinetic energy to move the car, with some energy lost as heat. Every bit of energy can be traced back to its original form.

• This law emphasizes the conservation of energy.
• It helps to validate that energy inputs and outputs in a system are balanced.
• Violations of this law, like perpetual motion machines, are impossible.

Second Law of Thermodynamics

The Second Law of Thermodynamics introduces the concept of entropy, which in simple terms is the measure of disorder or randomness in a system. It tells us that in any energy exchange, the total entropy of a system and its surroundings always increases over time. One of the common consequences of this law is that heat will naturally flow from a hot object to a cold one, and not the other way around without external work. This sets a direction for processes, highlighting the irreversibility of natural events.

• Entropy tends to increase.
• This law explains why certain processes cannot occur naturally.
• Attempts to spontaneously lower a system's entropy would violate this law.

Irreversible Processes

In thermodynamics, irreversibility refers to processes that can't return to their initial state without leaving a change in the environment. A classic example is burning wood. Once burnt, wood turns into ash and gases, and it’s impossible to revert these products back to the original wood state using only natural processes. Irreversible processes increase the entropy of the universe.

• Include most natural processes due to entropy increase.
• They cannot be perfectly reversed.
• Include combustion, mixing of substances, etc.

Equilibrium Processes

Equilibrium processes are those where the system remains in a stable state throughout the process. They are theoretical concepts often used to model real-world situations where very small changes occur slowly enough that the system remains nearly balanced at every point in time. In this state, the forward and reverse processes occur at the same rate. An example is the saturation point of a salt solution, where exactly as much salt dissolves as precipitates at any given time.

• Occurs when the rates of opposing processes are equal.
• Characterized by stable conditions over time.
• Helps understand processes in a controlled, theoretical manner.

Spontaneous Processes

Spontaneous processes are those that occur naturally, without needing an external energy input, because they lead to an increase in entropy or a decrease in free energy. A classic example is ice melting at room temperature. Without any help, heat from the surrounding environment causes ice to melt, illustrating how the process progresses naturally. Spontaneity in these processes does not imply speed, but rather the natural tendency for these changes to occur.

• Occur naturally under given conditions.
• Often associated with an increase in entropy.
• Can occur slowly, but without external driving forces.