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
Adiabatic
system is a system which does not allow transfer of heat through its boundary;
means heat can not enters or leave the system.
Work on adiabatic 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)
Wad = adiabatic work
Sign of Wad
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.
