ANDChemistry/Chemistry Assignments/Introduction to chemical thermodynamics
Chemical Thermodynamics is that branch of thermodynamics that deals with energy changes accompanying chemical transformations.
The word thermodynamics is a combination of two Greek words thermos (heat) and dynamics (power or motion)
Important Thermodynamic Terms
System: It is that part of the universe on which theoretical or experimental investigations are carried out. A system is said to be a part of the universe to which we direct our attention for theoretical or experimental investigations. It is separated from the rest of the universe by a real or imaginary well defined boundary. For example, if the effect of pressure on the volume of the gas has to be studied, then the system is the gas taken in a container. Similarly, any solution taken in a beaker is known as the system when its properties are being studied.
Surroundings: The rest of the universe in the neighbourhood of the system is known as its surroundings. It lies outside the system and is also called the environment of the system. The system can exchange matter or energy or both with its surroundings. A system and its surrounding are together known as the Universe.
Boundary: Anything that separates the system from the surroundings is known as a boundary. It is also known as the wall of the system. The system interacts with its surroundings across the boundary. There are various types of boundaries depending upon their nature.
- Real or imaginary
- Rigid or nonrigid
- Conducting (diathermic) or nonconducting (adiabatic)
- Permeable, impermeable or semi permeable
- Electrically conducting or non-conducting
- Transparent, translucent or opaque.
A given boundary can belong to various categories. For example, the walls of a beaker constitute a real, rigid, impermeable and diathermic boundary.
Types of systems based on the type of boundary
The system exchanges matter and energy with the surroundings across the boundary. Therefore, the boundary decides the relationship of the system with its surroundings and hence it also decides the category to which the system belongs.
- Open system: The system has a permeable boundary. The exchange of matter (mass, m) and energy(U) can take place between the system and the surroundings. For the open system Δm ≠ 0 and ΔU ≠0.
- Closed system: The system has an impermeable but diathermic boundary. The matter cannot flow across the boundary but energy exchange can take place between the system and the surroundings. For this system Δm= 0 and ΔU≠0.
- Isolated system: The system has an adiabatic boundary. The matter and energy exchange cannot take place between the system and the surroundings. For this system Δm = 0 and ΔU =0.
State of the system
These four variables namely, pressure (p), temperature (T), volume (V) and the amount (number of moles, n) define the state of the system.The physical state and other important properties of the system are decided by these variables. Therefore, these variables are known as state variables.
It is any operation or process that brings about a change in the state of the system. Expansion, compression, heating and cooling are some of the examples of a thermodynamic process. The initial state of the system is defined by the pressure, temperature, volume and amount of the system before the change takes place. The final state of the system is defined by the values of pressure, temperature, volume and amount of the system after the change has taken place. During a process there is a change in any one or more of the state variables.
For any state variable Y, if the change in the state variable is large it is represented by ΔY (delta Y). If the change is infinitesimal (very small) it is represented by dY.
Classification of processes based upon the change in thermodynamic variables
Thermodynamic processes are classified into various categories depending upon the thermodynamic properties that are undergoing a change during the process. These categories are as follows:
- Isothermal process: It is a process during which the temperature of the system remains constant, that is, ∆T = 0.
- Adiabatic process: It is a process during which the system does not exchange heat with the surroundings, that is, q = 0.
- Isobaric process: It is a process during which the pressure of the system remains constant, that is, there is no change in the pressure, ∆P = 0.
- Isochoric process: It is the process during which the volume of the system remains constant, that is, ∆V = 0.
- Cyclic process: It is the process during which the system after undergoing a series of changes comes back to the initial state. The series of changes taking place is known as a cycle.