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Wednesday, 2 December 2015

FSc Notes Chemistry Part 1 Chapter 7 Thermochemistry Lecture 2

FSc Notes Chemistry Part 1 Chapter 7 Thermochemistry Lecture 2

State & State Function:

The physical appearance of a system at particular conditions of pressure, volume and temperature, is known as state of the system.
Or            
The state of a system is the condition of the system which is specified by mentioning pressure, volume etc.
Other properties specifying the state of a system are, enthalpy, internal energy, force etc.

State Function:

Any property which specifies the state of a system is known as state property or state variable.
For example:
  • temperature
  •  pressure
  •  volume
  •  internal energy 
  •  enthalpy etc.

Characteristics of State Function:

State functions have two characteristics.

A state function depends only upon the initial and final states of the system and is independent of the path followed.
For example if water is heated from 10 to 150c then the change in temp. ( ) a state function depends only on T2 (final state ) and T1 ( initial state ) of the system but is independent of the path followed, ie Here it is not needed that how the temperature is changed.

If we know the values of other state functions of a system, them the value of any particular state function (which is not know ) can easily determined.
For example, if we have one mole of a gas whose values of “V” and “Temp” are known : then the value of pressure (P) can be calculated as
PV = RT or P = RT/V

It is important to note that there are some properties which depend upon the path through which the state change is brought. Such properties which are path dependent are not termed as state function.
Examples of such proportions are heat (q) work (w) etc.


Internal Energy:
The total amount of energy possessed by a system, is known as internal energy of that system or sum of all kinds of energies , possessed by the particles ( atoms, ions or molecules ) of a system. The particles ( atoms, ions or molecules ) possess, K.E due to their motion ( which may be vibration, rotational or translation ) an dP.T which is due to the binding forces ( intramolecular or inter-molecular attractive forces). Internal energy of a system is represented by “E”.
Internal energy depends upon the state ( like pressure, temp, volume etc ) of the system, therefore it is a state function.
When we increase the internal energy of a system, following facts occur: K.E of the particles of the system increases and hence temperature of the system increases.
If change in internal energy is enough , then the chemical bonds of reactants may bear and thus a chemical reaction may occur.
It is important to note that decrease in Internal Energy of the system cannot cause a chemical reaction.

Written by: Asad Hussain

 

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