PHD

PHDPhysical Chemistry


Thermodynamics


Thermodynamics is a branch of physical chemistry that deals with the study of energy, work, heat, and how these quantities interact with each other and affect matter. It plays a vital role in understanding various chemical processes and reactions. Thermodynamics provides the principles and framework necessary for understanding how energy exchange occurs in both natural phenomena and engineered systems.

Basic concepts

The foundation of thermodynamics lies in several fundamental concepts: system, surroundings, and boundaries. A system is the part of the universe we are interested in, while the surroundings encompass everything else. The boundary separates the system from its surroundings and can be real or imaginary. Thermodynamic systems can be open, closed, or isolated depending on their interactions with the surroundings.

Types of systems

  • Open system: exchanges both energy and matter with its surrounding environment.
  • Closed system: exchanges only energy, not matter, with its surroundings.
  • Isolated system: does not exchange energy or matter with its surroundings.

State functions and variables

State functions are properties that depend only on the current state of the system, not on how that state was reached. Common state functions include internal energy (U), enthalpy (H), entropy (S), and free energy (G). In contrast, path functions such as work and heat depend on the path taken to get from one state to another.

The state of a thermodynamic system can be described using state variables such as pressure (P), volume (V), temperature (T) and concentration. These variables are essential to understanding the behavior of the system and are interrelated through state equations.

Laws of thermodynamics

Thermodynamics is governed by four fundamental laws, known as the zeroth, first, second, and third laws of thermodynamics. Each law introduces fundamental principles about energy and entropy.

Zeroth law of thermodynamics

The zeroth law establishes the concept of temperature and thermal equilibrium. It states that if two systems are in thermal equilibrium with a third system, then they are in thermal equilibrium with each other. Mathematically, if A is in equilibrium with B, and B is in equilibrium with C, then A is in equilibrium with C

First law of thermodynamics

The first law of thermodynamics, often called the law of conservation of energy, states that energy cannot be created or destroyed; it can only change form. The change in the internal energy of a system (ΔU) is equal to the value obtained by subtracting the work done by the system (w) from the heat (q) added to the system.

ΔU = q - w

For example, when a gas in a piston is heated, it expands, doing work on the piston and gaining internal energy from the added heat.

Second law of thermodynamics

The second law introduces the concept of entropy, which is a measure of disorder or randomness in a system. It states that in any thermodynamic process, the total entropy of a system and its surroundings always increases for irreversible processes. In reversible processes, the entropy change remains constant.

Mathematically it is expressed as:

ΔS_universe = ΔS_system + ΔS_surroundings ≥ 0

This theory explains why some processes are spontaneous. For example, a cup of hot tea left in a cold room loses heat until it reaches thermal equilibrium with the room. In this spontaneous process, the entropy of the universe increases.

Third law of thermodynamics

The third law states that the entropy of an ideal crystal at absolute zero (0 K) is exactly zero. This provides a reference point for calculating entropy. As the temperature approaches absolute zero, the entropy of the system becomes minimum.

Visualization of thermodynamic processes

Thermodynamic processes often involve changes in state variables such as pressure, volume, and temperature. We can represent these processes graphically using various types of diagrams, such as pressure-volume (PV), temperature-entropy (TS), and enthalpy-entropy (HS) diagrams.

Pressure-volume diagram (PV diagram)

The PV diagram shows the relationship between the pressure and volume of a system. In these diagrams, different thermodynamic paths can be observed, such as isothermal (constant temperature), isobaric (constant pressure), isochoric (constant volume), and adiabatic (no heat exchange) processes.

Volume Pressure Adiabatic

Example: Understanding isothermal expansion

Isothermal processes occur at constant temperature. Imagine that we have an ideal gas enclosed in a piston that is in thermal contact with a heat reservoir. As the gas expands isothermally, it does work on the piston while absorbing an equal amount of heat from the reservoir to keep the temperature constant.

According to the ideal gas law:

PV = nRT

Here, P is the pressure, V is the volume, n is the number of moles, R is the gas constant, and T is the temperature. Since the temperature remains constant, the equation can be rearranged to show that pressure and volume are inversely proportional during an isothermal process:

P ∝ 1/V

Entropy change in systems

Entropy changes can provide information about the spontaneity and feasibility of chemical reactions. For example, when ice melts at 0 °C to form water, the entropy of the system increases because the structure of liquid water is more disordered than that of solid ice.

Conclusion

Thermodynamics is an important field in physical chemistry, providing the theory necessary to understand energy transformations and the direction of processes. By mastering the laws of thermodynamics and their implications, chemists can predict how systems behave under different conditions and design experiments or industrial processes that use chemical energy efficiently.

This discovery of thermodynamics opens up avenues for further studies in statistical mechanics, quantum chemistry and kinetic theory, where deeper insights into molecular interactions and energy distributions are gained. As you delve deeper into this subject, you will unveil more complex scenarios and a subtle understanding of the microscopic and macroscopic behaviours of matter.


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