Class 11 Chapter 6 – Thermodynamics

Class 11 Chapter 6: Thermodynamics

System:

A system is that part of universe in which we made observations is called system.

Surroundings:

All remaining part of the universe, other than system is called surroundings.
Universe = System + Surroundings

Types of System:

The system is classified on the basis of movement of energy and matter inside or outside the system. I.e. open system, closed system and isolated system.

1.     Open System:

A system in which exchange of matter and energy between the system and surroundings take place, is called open system.
Example:- Reaction in open beaker.

2.     Closed System:

A system in which exchange of energy between the system and surroundings take place but there is no exchange of matter between the system and surroundings takes place, is called closed system.
Example:- Reaction in closed conducting vessel of steel.

3.     Isolated System:

A system, in which no exchange of matter and energy between the system and surroundings take place, is called isolated system.
Example:- Reaction in isolated vessel like thermos flask.

The state of the system

In chemistry, the state of thermodynamic system is described by the measurable or macroscopic i.e. bulk properties of the thermodynamic system.
To learn better, if someone asks, how we can describe the state of a gas?
So to answer this question, we describe the state of a gas by getting information about pressure (p), volume (V), temperature (T) and amount (n) etc. of gas.
So, these variables i.e. p, V, T are known as state variables or state functions.
Why these variables (p, V, T) are called state variables or state functions?
These variables are called state variables or state functions because their values depends only on the state of the system and do not depend on how they are reached on that point.

Internal Energy

Sum of all energies (like chemical, electrical, mechanical etc.) of the system is called internal energy (U) of the system.
Now question is when internal energy of the system changes?
Internal energy of the system changes when,
·        Heat passes into or heat passes out of the system.
·        Work is done on the system or work is done by the system.
·        Matter enters the system or matter leaves the system.

i.       Work

Before learning about effect of work on internal energy we first understand what is Adiabatic system.

Adiabatic system is a system which does not allow transfer of heat through its boundary; means heat can not enters or leave the system.

If 1kJ mechanical work done (case 1) on the system and 1kJ electrical work done (case 2) on the system than change in temperature is same in bath cases.
So, amount of work done on the system produces the same change of state no matter how this work was done.
So,
U = U2 – U1 = Wad
Where,
U = internal energy
U2= internal energy at state 2 (final state)
U1 = internal energy at state 1 (initial state)
a.     If Wad is positive, then work is done on the system.
b.     If Wad is negative, then work is done by the system.

ii.     Heat

What is heat?

The exchange of energy due to difference in temperature is called heat.
If system allows exchange of heat (conducting walls of the system) then change in internal energy depends on amount of heat transfer between system and surroundings at constant volume when no work is done.
So,
U = q
Where,
U = internal energy
q = heat
Sign of q
a.     q is positive, if heat is transferred from the surroundings to the system.
b.     q is negative, if heat is transferred from the system to surroundings.

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