# Important Concepts in Thermodynamics

## Important Concepts in Thermodynamics:

• Thermodynamics- That branch of chemistry that deals with the different forms of energy and the quantitative relationship between them are called thermodynamics.
• System- The part of the universe that is under thermodynamic investigation is called system. For example- a cup of hot tea under observation, a satellite under observation.
• Surroundings- The part of the universe that is around a system is called surroundings i.e. system + surroundings = universe.
• Boundary- The surface that separates the system from the surroundings is called boundary.
• Open System- A system that can exchange matter as well as energy with its surroundings is called an open system. Example- a cup of hot tea without a lid.
• Closed System- A system that can exchange energy but not the matter with its surroundings is called a closed system. Example- a cup of hot tea with a lid.
• Isolated System- A system that can exchange neither energy nor matter with its surroundings is called an isolated system. Example- tea in an insulated / thermos flask.
• Homogenous System- A system that is completely uniform throughout is called a homogenous system. Such a system consists of only one phase. Example- a pure single solid or a liquid or a gas, NaCl dissolved in water, a mixture of C2H5OH and water.
• Heterogeneous System- A system which consists of two or more phases (i.e. which is not uniform throughout) is called a heterogeneous system. Example- ice in contact with water, petrol and water mixture etc.
• Phase- A phase is a homogeneous physically distinct and mechanically separable identity in a system.
• Macroscopic System- A system which consists of a large number of atoms, molecules or ions (matter) is called a macroscopic system. In thermodynamics, only macroscopic systems are studied.
• Macroscopic Properties- These are the properties which are associated with the macroscopic system. Example- pressure, volume, temperature etc.
• Intensive Properties- A property which does not depend upon the amount of the system is called an intensive property. Example- pressure, temperature, density, refractive index etc.
• Extensive Properties- A property that depends upon the amount of the system is called an extensive property. Example- volume, mass, internal energy, enthalpy, entropy, free energy etc.
• State Variables- The properties which determine the state of a system are called state variables. Pressure (P), temperature (T), volume (V) and amount [no. of moles present (n)] are important state variables. If the state variables are fixed, the other properties of the system are also fixed. Any change in state variables produces a change in other properties of the system.
• State Functions or State Properties- The properties of the system, which depend only upon the state variables, are called state functions or state properties. The change in a state function depends upon only the initial and final states of the system and it is independent of the manner in which the change is brought about. In other words, state functions do not depend upon the path that the process follows. Some state functions are volume, energy, enthalpy, entropy, free energy etc.

#### Thermodynamic Process:

• Isothermal Process- A process is said to be isothermal if the temperature of the system remains constant during the process. Such a process takes place when there is an exchange of energy between the system and surroundings. For an isothermal process ΔT = 0.
• Adiabatic Process- A process is said to be adiabatic if the temperature of the system does not remain constant during the process. Such a process takes place when there is no exchange of heat between the system and the surroundings. For an adiabatic process, heat exchange between system and surrounding i.e. q = 0.
• Isobaric Process- A process which occurs at constant pressure is called an isobaric process. For an isobaric process ΔP = 0.
• Isochoric Process- A process, which takes place at constant volume, is called an isochoric process. For an isochoric process ΔV = 0.
• Cyclic Process- When a system after undergoing a series of changes finally returns to its original state, it is said to have completed a cycle and the entire process is called a cyclic process. Fort a cyclic process ΔU = 0.