State functions
State of a system
A system is said to be in a particular physical
state when specific values of the macroscopic properties of the system are
known. For eg. The gaseous state of matter can be described by parameters like
Pressure (P), Volume (V), Temperature (T) etc. The values of these parameters
change when the matter is in liquid state. Thus, the state of a system is defined by specific measurable macroscopic
properties of the system.
The initial
state of system refers to the starting state of the system before any kind
of interaction with its surroundings.
The final
state of system refers to the state after the interaction of system with
its surroundings. A system can interact with its surroundings by means of
exchange of matter or heat or energy or all.
The variables like P,V,T, composition (no. of moles) `n' that are used
to describe the state of a system are called as state variables or state functions.
When the state of the system changes, the values of the state variables of the system also change.
Thus, state functions depend only on the initial and final states of system and
not on how the changes occur. Also, if the values of state functions of a
system are known, all other properties like mass, viscosity, density etc. of
the system become specified.
For specifying a state of the system, it is not
necessary to know all the state variables, since they are interdependent and
only a few of them (state variables) are sufficient. A system which satisfies
the conditions of thermal, mechanical and chemical equilibria and contains the
macroscopic properties which are independent of time is said to be in thermodynamic equilibrium.
Thermodynamic
equilibrium sets the condition that there should be no flow of heat from one portion or
part of the system to another portion or part of the same system. ie.
temperature of the system remaining constant at every point of the system.
Mechanical equilibrium implies that there is no work done by one portion or part of the system over another portion or part of the
same system. ie. Pressure of the system being constant at all its points.
Chemical
equilibrium demands that the composition of one or more phases of chemicals present in the
system should remain constant.
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