Work and heat are the transformations of energy. Applied thermodynamics is the science of the relationship
between heat, work, and systems that analyze energy processes. The energy process that converts heat energy from available
sources such as chemical fuels into mechanical work is the major concern of this science. Thermodynamics consists of a number
of analytical and theoretical methods which may be applied to machines for energy conversion
The first law states that the amount of energy added to a system is equal to the sum of its increase in heat energy and the work done on the system. Energy can be changed from one form to another, but it cannot be created or destroyed. The change in energy of a system is due to transfer of heat energy (q), plus the work energy (w) done by the system on its surroundings, which is given by the following equation:
ΔE = q + W
The value of (heat energy) q has a positive sign (+ve), if heat is added to the system. Similarly, the value of (work energy) W has a positive sign, if the work is done by the surroundings.
The value of (heat energy) q has a negative sign (−ve), if heat is released by the system into the surroundings. Similarly, the value of (work energy) W has a negative sign, if the work is done by the system on the surroundings.
The sign conventions for q, W, ΔE
For q: +ve means system gains heat; −ve means system loses heat.
For W: +ve means work done on system; −ve means work done by system.
According to first law of thermodynamics, energy can change from one form to other with variations in state functions. Pressure, volume and temperature are the state functions. Therefore, the change in energy of a system, with variations in state functions can be studied as follows:
The change in energy of a system, ΔE at constant temperature, in terms of potential energy is the difference between the final and initial states of potential energies of a system. Since, at constant temperature the kinetic energy of the system is negligible, as the movement of molecules is less at constant temperatures.
ΔE = PE final − PE initial
| q | W | ΔE |
|---|---|---|
| + | + | + |
| + | − | Depends on size of q and W |
| − | + | Depends on size of q and W |
| − | − | − |
If potential energy of the final state (PE final) of a system is greater than the potential energy of the initial state (PE initial ) of a system than the change in energy of a system, ΔE value is positive, indicating that the process is an endothermic process. It means that that the system gains energy from the surroundings.
If potential energy of the final state (PE final) of a system is less than the potential energy of the initial state (PE initial) of a system than the change in energy of a system, ΔE value is negative, indicating that the process is an exothermic process. It means that that the system loses its energy to the surroundings.
The change in energy of a system, ΔE at constant pressure, in terms of work is given by,
ΔE = qp − W
The (−) minus sign enters the equation because at constant pressure, the system does work on the surroundings, and such work has been defined as a negative quantity.
Work done by the system
A sample of hot water is a system, the beaker containing it and the rest of the lab are surroundings.
The water transfers energy as heat to the surroundings until the temperature of the water equals that of surroundings. Here,
the heat (q) is released by the system into the surroundings, and the work (w) is done by the system on the surroundings.
Work done by a system is defined as the pressure exerted over a given area, therefore
work = Pressure × Area × Distance moved
Multiplying the area by the distance results in volume units or an overall volume change in a system given by:
work = Pressure × Volume change
W = − PΔV
Therefore, the change in energy of a system, ΔE at constant pressure is given by,
ΔE = q p − PΔV
Where qp→ is the heat at constant pressure
PΔV → is the work done by the system on the surroundings.
The change in energy of a system, ΔE at constant volume, in terms of heat is given by the following equation:
ΔE = qv
At constant volume, the work is done by the surroundings, it means that transfer of energy is from the surroundings into the system. Therefore, work done by the system becomes zero, i.e., PΔV is zero and the change in energy, ΔE is written as–
ΔE = qv (∵ ΔE = qv − PΔv).
Where q v → is the heat at constant volume.
The change in energy of a system, ΔE at constant pressure is given by,
ΔE = qp − PΔV
Here qpis replaced by ΔH, change in enthalpy, as the change in energy of a system is due to the change in heat content of the substance in a system. Therefore, the equation can be written as:
ΔE = ΔH − PΔV
Change in Enthalpy:
The thermodynamic variable ΔH, which is called the change in enthalpy of a system is said to be an extensive property,
as it varies with the quantity of the substance and state functions of the substance, i.e., pressure and temperature.