Engineering Chemistry S Datta

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Thermodynamics
Thermodynamics is concerned with the flow of heat and therefore deals with the quantitative
relationship between heat and work. The science of thermodynamics actually covers a much
wider field covering energy-changes in all physical and chemical processes as well as mutual
transformations of all the different forms of energy. Thermodynamic deductions are not made
on the basis of any hypothesis about the molecular structure of matter but the three simple
laws of thermodynamics are based on human experience and these relations were tested
experimentally for centuries starting from heat engines to cellular enzymatic processes and
are considered inviolable. Thus, Einstein comments: “It (thermodynamics) is the only physical
theory of universal content concerning which I am convinced that, within the framework of
applicability of its basic concepts, it will never be overthrown”.
• Before beginning with the laws of thermodynamics following terms are needed to be
defined.
1. Thermodynamic systems. A system is defined as any matter or its part, chosen for
study and separated from the rest of the universe by a real or imaginary boundary. The rest of
the universe is called the surroundings.
A system is said to be an isolated system when it is capable of exchanging either energy
or matter with the rest of the universe. Hence, the energy and matter remain constant for the
system. There is no such perfectly isolated system other than our universe itself.
A closed system is one, which is capable of exchanging only energy with the surroundings
but no mass transfer across the boundary is permitted; example is a sealed flask containing
matter in a closed system.
An open system is a system, which can exchange both mass and energy across the
boundary; example is a plant or any living being.
2. Properties of a system. The physical characteristics which are experimentally
measurable and which enable us to define a system are called thermodynamic parameters or
properties of the system. Composition, volume, pressure and temperature are the fundamentally
important properties. Refractive index, viscosity, dielectric constant are also other properties.
Extensive property. Properties like volume, surface area, energy, etc., are dependent
upon the mass of the system and are called extensive property.
Intensive property. Properties like temperature, pressure, surface tension, viscosity,
specific heat, density, refractive index, etc., are independent of the mass of the system and are
called intensive properties.
3. State of a system. The set of thermodynamic variables such as pressure, volume,
temperature, composition, etc., which describe a system is called the “state of the system”.
When one or more of these variables undergo change, the system is said to have undergone
a change of state.
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