According to first law of thermodynamics, it is possible to create or destroy the energy but energy changes form one form to another form, and the total quantity of energy in the universe remains constant. It is based on conservation of energy.
The internal energy of a system can be changed by adding heat to a system or by removing heat from a system and by doing work on a system or work done by the system.
It deals with energy exchanges between a system and its surroundings. The total internal energy in a system is the sum of all the kinetic energies and potential energies of its particles.
Suppose,
Internal energy of a system = E1
Internal energy of a system after chemical reaction = E2
Heat obtained or lost by the system = q
Work done on the system or by the system = w
So:
E2 = E1 + q + w
E2 - E1 = q + w
ΔE = q + w (1)
This equation is the symbolic form of the first law of thermodynamics.
When heat is added to the system = value of q is positive (endothermic reaction q is positive)
Heat is lost by the system = value of q is negative (exothermic reaction q is negative)
In the same way:
Work done on the system = w is positive
Work done by the system = w is negative
However, work done on the system and work done by the system is depending on the volume of a system, specifically for gaseous compound.
The volume of a system decreases = work done on the system
The volume of a system increases = work done by the system
Suppose that.
Initial volume of a system = V1
Final volume after reaction = V2
External pressure = P (atmospheric pressure – constant)
Then, work done by the system,
-w = P (V2-V1)
-w = P ΔV
w = - P ΔV (2)
According to first law of thermodynamics, the value of w is replace in equation (1) then,
ΔE = q – P ΔV
Now, there is no change in volume during chemical reaction then, ΔV = 0, so that: ΔE = q
Thus, the change in energy of a system is equal to the heat obtained or lost by a system at constant volume.