It is a combination of two words, thermo ‘heat’ and dynamics ‘flow’, meaning flow of heat. Thermodynamics is a branch of physics that studies the relationships between heat, work, temperature, and energy. It studies the behavior of energy and its interactions with matter at various scales, from microscopic to macroscopic. It can predict whether a given process will occur spontaneously or not under a given set of conditions.
Thermodynamics is based on four laws, which are called:
Zeroth Law: If two systems are in thermal equilibrium with a third system, they are also in thermal equilibrium with each other.
First Law (Energy Conservation): Energy cannot be created or destroyed, only converted from one form to another. Mathematically, it is written as, ΔE = q + w (or ΔE = q – w).
Second Law: The total entropy of an isolated system always increases over time.
Third Law: As the temperature of a system approaches absolute zero, its entropy approaches a minimum value.
THERMOCHEMISTRY
Thermochemistry is the branch of chemistry that deals with the relationship between chemical reactions and energy changes, particularly heat. It focused on the study of heat energy changes that occur during chemical reactions and changes of state (like melting or boiling).
THERMODYNAMIC TERMS
System: any real or imaginary part of the universe that is under study or under investigation is called a system. It may be either a defined amount of matter or a region in space, whose behavior (e.g., energy, temperature, pressure) we are studying. It could be the gas inside a piston, liquid in a flask, or even the interior of a black hole in more abstract contexts. The systems can be open, closed, or isolated.
Surroundings: The surroundings (sometimes called environment) are everything outside the system that interacts with it and can exchange energy and matter with the system.
Boundary: The boundary is the (real or imaginary) surface that separates the system from its surroundings and defines the limits of the system.
For example, a cup of coffee
- System: the liquid coffee inside the cup.
- Boundary: the inner surface of the cup (physical and real).
- Surroundings: the air in the room, hands, the kitchen surfaces, etc.
Open System: An open system is a system that can exchange both matter and energy (heat or work) with its surroundings.
For example, a cup of hot coffee with the lid off, where steam (matter) escapes and heat (energy) is transferred to the surroundings. Here, the boundary is open and uninsulated.
Closed System: A closed system is a system that can exchange energy (heat or work) but not matter with its surroundings.
For example, a sealed cup of hot coffee, where heat (energy) is transferred to the surroundings, but matter (coffee) is not exchanged. Here, the boundary is sealed but not isolated
Isolated System: An isolated system is a system that cannot exchange either matter or energy with its surroundings.
For example, a perfectly insulated thermos, where neither matter nor energy can be exchanged with the surroundings. Here, the boundary is sealed and isolated, and no interactions are possible with the surroundings.
Extensive Properties: Extensive properties are properties of a system that depend on the size or amount of matter in the system. These properties are additive, meaning that the total value of an extensive property is the sum of the values for each part of the system. For example, Mass, volume, energy, entropy, enthalpy, and number of moles.
Intensive Properties: Intensive properties are properties of a system that do not depend on the size or amount of matter in the system. For example, Temperature, pressure, density, and specific heat capacity. Intensive properties are not additive.
Thermodynamic state: A thermodynamic state of a system refers to its current equilibrium condition, defined by measurable properties such as pressure, temperature, volume, and composition. These variables, often called state variables, uniquely identify the state of a system.
For example, for a simple compressible gas (with fixed composition), knowing pressure and temperature fully defines its state. We can then calculate volume, internal energy, entropy, or any other property by equations of interconversions.
State function: A state function (also known as a function of state or point function) is a property whose value depends only on the system’s current state, not on the path or history of how the system reached that state. In essence, the difference between initial and final values of a state function is path independent, and its values are defined at the equilibrium states.
For example, internal energy (E), enthalpy (H), entropy (S), Gibbs free energy (G), pressure (P), temperature (T), and volume (V). The examples of state functions are always written in capital letters.
Equation of state: A mathematical relation connecting thermodynamic variables like P, V, and T, used to describe the system’s state. Examples include the Ideal Gas Law (PV = nRT) and van der Waals equation for real gases.
Thermodynamic Equilibrium: A state of a physical system where all macroscopic properties such as temperature, pressure, and chemical composition are uniform and remain constant over time, and no net flows of mass, energy, or chemical reactions occur. In this state, spontaneous changes do not occur unless the system is disturbed externally.
A system is in full thermodynamic equilibrium only if the following three conditions are simultaneously satisfied:
Thermal Equilibrium
Uniform temperature throughout the system.
No net heat transfer within the system or with surroundings.
If two objects at different temperatures are in contact, heat flows until temperatures equalize.
Mechanical Equilibrium
Uniform pressure across the system.
No unbalanced forces or pressure gradients exist, and no mechanical work flows spontaneously.
Chemical (or Diffusive) Equilibrium
Chemical potentials of all species are identical throughout the system.
No net chemical reactions, phase changes, or diffusion occur.
No net mass or energy flows within or between the system and the surroundings.
For example, imagine a sealed, insulated cylinder with gas and a movable piston, initially disturbed. After waiting:
- Temperature becomes uniform → thermal equilibrium
- Internal and external pressures balance → mechanical equilibrium
- Chemical composition stabilizes (no reactions or diffusion) → chemical equilibrium
Thermodynamical processes: A thermodynamic process is defined as the path or process by which a system changes from one state to another.
Isothermal Process: An isothermal process is a thermodynamic process that occurs at constant temperature, i.e., the temperature remains constant throughout the process. In this process, ΔT = 0, Internal energy change ΔE = 0 (for ideal gas), and the equation used for ideal gas in an isothermal process is PV=constant.
Examples: slow expansion or compression of a gas in a thermally conducting container.
Adiabatic Process: A process in which no heat is exchanged between the system and its surroundings. In this process, heat transfer q = 0, and internal energy change ΔE =W (for an ideal gas). The equation used for ideal gas in an isothermal process is PVγ= constant (Where γ =Cp/Cv).
For example, rapid compression or expansion of gas in an insulated cylinder.
Isobaric Process: A process that takes place at constant pressure is called an isobaric process. In this process ΔP = 0. The equation used for ideal gas in an isobaric process is q=ΔE+ PΔV.
Example: Heating water in an open vessel or chemical reactions occurring in an open container.
Isochoric Process: A process that occurs at constant volume. In this process ΔV = 0. No work is done (W = 0), and heat added = Change in internal energy (q = ΔE).
For example, heating gas in a sealed rigid container.