Thermodynamics Properties of pure fluids

Classification of TD properties:-
TD properties of fluids can be classified into 3 broad groups:
• The reference props
• Energy props and
• The derived props

Reference Properties:- Also known as primary props, that are used to define the state of the system. They have absolute values, which are measured relative to some arbitray reference state.

Ex:- Temperature, Pressure, Volume and Entropy.
Where Tempr and Pressur are intensive and Volume and Entropy are Extensive props.
In solution the composition is also treated as a reference property.

Energy Properties:- The four energy props are internal energy (u), Enthalpy (H), the Helmholtz free energy (A), and Gibbs free energy (G) and all are extensive TD props and known as relative to some ref. State.

These are referred as energy props, because the changes in the these TD function indicate useful work under certain condition of restraint.

Derived properties:- These are partial derivatives of energy props or the reference props.
EX:- Specific height (C) , co-efficient of expansion ( β) , Joule –Thomson coeff(μ) & Co-efficient of compressibility ( K).


The Helmholtz free energy (A) of a system is defined as A=U-TS where U, T & S are the I.E , Temperature and Entropy of the system resp.
A is an extensive property , Since U & S are both extensive & it is also a state function.

Thermodynamics properties or Fundamentals Property rules:-
WKT First Law for Closed System is ΔU=Q-W => du =dq-dw .
By Defination dq = ds / T => dq = TdS and also dw=pdv

There fore du=TdS – pdv ------------------------- (1)
First law for open System :- ΔH-q- ws , dh= dq-dws
By defn. dq = TdS , dws = -vdp

There fore dH=TdS+ vdp----------------------------(2)

Fluid Properties are

* Density

The density of a material is defined as its mass per unit volume. The symbol of density is ρ (the Greek letter rho).

* Newtonian fluid

A Newtonian fluid (named for Isaac Newton) is a fluid whose stress versus rate of strain curve is linear and passes through the origin. The constant of proportionality is known as the viscosity.

* Non-Newtonian fluid

A non-newtonian fluid is a fluid whose flow properties are not described by a single constant value of viscosity. Many polymer solutions and molten polymers are non-newtonian fluids, as are many commonly found substances such as ketchup, starch suspensions, paint, blood and shampoo. In a newtonian fluid, the relation between the shear stress and the strain rate is linear (and if one were to plot this relationship, it would pass through the origin), the constant of proportionality being the coefficient of viscosity. In a non-newtonian fluid, the relation between the shear stress and the strain rate is nonlinear, and can even be time-dependent. Therefore a constant coefficient of viscosity can not be defined. A ratio between shear stress and rate of strain (or shear-dependent viscosity) can be defined, this concept being more useful for fluids without time-dependent behavior.

Although the concept of viscosity is commonly used to characterize a material, it can be inadequate to describe the mechanical behavior of a substance, particularly non-newtonian fluids. They are best studied through several other rheological properties which relate the relations between the stress and strain rate tensors under many different flow conditions, such as oscillatory shear, or extensional flow which are measured using different devices or rheometers. The properties are better studied using tensor-valued constitutive equations, which are common in the field of continuum mechanics.

* Surface tension

Surface tension is an attractive property of the surface of a liquid. It is what causes the surface portion of liquid to be attracted to another surface, such as that of another portion of liquid (as in connecting bits of water or as in a drop of mercury that forms a cohesive ball).

Applying Newtonian physics to the forces that arise due to surface tension accurately predicts many liquid behaviors that are so commonplace that most people take them for granted. Applying thermodynamics to those same forces further predicts other more subtle liquid behaviors.

Surface tension has the dimension of force per unit length, or of energy per unit area. The two are equivalent — but when referring to energy per unit of area, people use the term surface energy — which is a more general term in the sense that it applies also to solids and not just liquids.

In materials science, surface tension is used for either surface stress or surface free energy.

* Viscosity

Viscosity is a measure of the resistance of a fluid which is being deformed by either shear stress or extensional stress. In everyday terms (and for fluids only), viscosity is "thickness". Thus, water is "thin", having a lower viscosity, while honey is "thick" having a higher viscosity. Viscosity describes a fluid's internal resistance to flow and may be thought of as a measure of fluid friction. For example, high-viscosity magma will create a tall, steep stratovolcano, because it cannot flow far before it cools, while low-viscosity lava will create a wide, shallow-sloped shield volcano. All real fluids (except superfluids) have some resistance to stress, but a fluid which has no resistance to shear stress is known as an ideal fluid or inviscid fluid. The study of viscosity is known as rheology.

When looking at a value for viscosity, the number that one most often sees is the coefficient of viscosity. There are several different viscosity coefficients depending on the nature of applied stress and nature of the fluid. They are introduced in the main books on hydrodynamics and rheology.

* Dynamic viscosity (or absolute viscosity) determines the dynamics of an incompressible Newtonian fluid;
* Kinematic viscosity is the dynamic viscosity divided by the density for a Newtonian fluid;
* Volume viscosity (or bulk viscosity) determines the dynamics of a compressible Newtonian fluid;
* Shear viscosity is the viscosity coefficient when the applied stress is a shear stress (valid for non-Newtonian fluids);
* Extensional viscosity is the viscosity coefficient when the applied stress is an extensional stress (valid for non-Newtonian fluids).

* Vapor pressure

Vapor pressure (also known as equilibrium vapor pressure), is the pressure of a vapor in equilibrium with its non-vapor phases. All liquids and solids have a tendency to evaporate to a gaseous form, and all gases have a tendency to condense back into their original form (either liquid or solid). At any given temperature, for a particular substance, there is a pressure at which the gas of that substance is in dynamic equilibrium with its liquid or solid forms. This is the vapor pressure of that substance at that temperature. The equilibrium vapor pressure is an indication of a liquid's evaporation rate. It relates to the tendency of molecules and atoms to escape from a liquid or a solid. A substance with a high vapor pressure at normal temperatures is often referred to as volatile.

The vapor pressure of any substance increases non-linearly with temperature according to the Clausius-Clapeyron relation. The atmospheric pressure boiling point of a liquid (also known as the normal boiling point) is the temperature where the vapor pressure equals the ambient atmospheric pressure. With any incremental increase in that temperature, the vapor pressure becomes sufficient to overcome atmospheric pressure and lift the liquid to form bubbles inside the bulk of the substance. Bubble formation deeper in the liquid requires a higher pressure, and therefore higher temperature, because the fluid pressure increases above the atmospheric pressure as the depth increases.

* Compressibility

In thermodynamics and fluid mechanics, compressibility is a measure of the relative volume change of a fluid or solid as a response to a pressure (or mean stress) change.