PHYSICAL CHEMISTRY
UNIVERSITY OF NAIROBI
1ST LAW OF THERMODYNAMICS
The first law of thermodynamics expresses the conservation of energy principles where one
form of energy can change to another form but the total amount of energy remains the
same.
This law helps to balance the internal energy that releases and the work done in the process
Limitations
It says nothing about whether the process of change specifying can in fact occur and if so in
which direction. This quiz can only be answered during the second law.
When a system is changed from state A to state B it undergoes a change in internal energy
from A to B therefore AU=UB-UA
This energy change is brought by the evolution or absorption of heat or by work being done
by the system because the total energy of the system must remain constant the
mathematical statement of the 1st law is given by:
ΔU=Q+W e.g. 1
Where p is the amount of that supplied in the system and w is work done by the system.
The gas expands against an applied constant pressure by change in volume and therefore
total mechanical work done is
W=-P ΔV equation 2
Combining equations 1 and 2
ΔU=Q-P ΔV
Infinitesimal change in energy can be expressed by
Du=dq-pdv
And this is referred to as differential quality
Case 1
for cyclic process involving isothermal expansion of ideal gas, ΔT=0
And the 1st law reduces to q=-w
UNIVERSITY OF NAIROBI
1ST LAW OF THERMODYNAMICS
The first law of thermodynamics expresses the conservation of energy principles where one
form of energy can change to another form but the total amount of energy remains the
same.
This law helps to balance the internal energy that releases and the work done in the process
Limitations
It says nothing about whether the process of change specifying can in fact occur and if so in
which direction. This quiz can only be answered during the second law.
When a system is changed from state A to state B it undergoes a change in internal energy
from A to B therefore AU=UB-UA
This energy change is brought by the evolution or absorption of heat or by work being done
by the system because the total energy of the system must remain constant the
mathematical statement of the 1st law is given by:
ΔU=Q+W e.g. 1
Where p is the amount of that supplied in the system and w is work done by the system.
The gas expands against an applied constant pressure by change in volume and therefore
total mechanical work done is
W=-P ΔV equation 2
Combining equations 1 and 2
ΔU=Q-P ΔV
Infinitesimal change in energy can be expressed by
Du=dq-pdv
And this is referred to as differential quality
Case 1
for cyclic process involving isothermal expansion of ideal gas, ΔT=0
And the 1st law reduces to q=-w
