## State Function:

• In thermodynamics, a state function, state quantity, or a function of the state, is a property of a system that depends only on the initial and final state the system, not on the way in which the system acquired that state.
• A state function describes the equilibrium state of a system and thus also describes the types of system.
• The cyclic integral involving a state function is always zero.
• All the thermodynamics property satisfy the requirements of state function.
• U = q + w         change in thermodynamic energy
S = qrev/T            entropy
H = U = PV         enthalpy
G = H – TS          Gibb’s free energy
A = U – TS          Helmholtz free energy
• Internal energy, enthalpy, entropy are the example of state functions.

## Path Function:

• Path function depends on the path taken to reach that specific value, not on the initial and final state of that value.
• Path function needs multiple integral and limits of the integration in order to integrate.
• It is based on how the state of a system was established.
• Work, heat, arc length are the example of path function.

## Thermodynamic process:

It is the path or operation by which a system changes from one state to another state.

### 1. Isothermal process:

The temperature of the system remains constant during each step.
T = constant => dT = 0

#### Details:

• For such change system should be contained in a perfectly conducting container.
• Perfect isothermal change is impossible but when a change is carried out very slowly approximate isothermal change occurs.
• It follows Boyle’s law.
• Work done in the isothermal process is graphically given by the area under the P-V curve.
• ∆H = nCp∆T  and  ∆H = nCv∆T
In isothermal process ∆T = 0
=> ∆H = 0 and ∆E = 0
• Specific change at constant T is infinitely great in the isothermal process.

There is no heat exchange between the system and surroundings.
q = constant
=> dq = 0
Perfectly adiabatic change is impossible but when a process is carried out very rapidly fairly

### 3. Isobaric process:

It is the system in which pressure of the system remains constant during each step.
P = constant , dp = 0

### 4. Polytropic process:

In this process heat capacity of the body remains constant.
Cp = constant  and  Cv = constant

=> dCp = 0  and  dC = 0

### 5. Quasistatic process:

The process in which the deviation from thermodynamic equilibrium is infinitesimal and all the states through which the system passes can be considered as equilibrium states.

### 6. Isochoric process :

When there is no change in the volume of the system during various operations the change is said to be isochoric.
V = constant  => dv = 0
For example, the combustion of a substance in a bomb calorimeter is an isochoric process.

### 7. Cyclic process:

The process which brings back a system to its original state after a series of changes is called acyclic process.
As the E and H depend only on their states, E and H are constant.
So, dE = 0 and  dH = 0

## Reversiable and Irreversiable process

### Reversible Process:

A thermodynamically reversible process is one in which all the changes occurring in any part of the process are exactly reversed when it is carried out in opposite direction.

### Characteristic of the reversible process:

• It has to be carried out in infinitesimal amount and hence require infinite time. Therefore changes in this process are very slow.
• At all time during the process, the driving force for the change is opposed by a restraining force infinitesimally smaller than the driving force.
• It can be reversed by an infinitesimal increase in opposing force.
• The work produced in the reversible process is maximum.

## Irreversible process:

The process which occurs suddenly or spontaneously without the restriction of occurring in the successive stage of infinitesimal quantities.

### Characteristic of the irreversible process:

• In an irreversible process work done in the forward direction and in the backward direction are not equal.
• If the initial and final stages are specified, the internal energy change would always be the same, whether the process has been affected by reversibly or irreversibly.
• In an irreversible process, since the work done(dw) in two opposite direction are unequal, the heat transfer(dq)would also be unequal.
• All the natural processes are irreversible.
e.g.=>  (1) flow of heat from high temperature to low temperature
(2) expansion of gas from high pressure to low pressure