Classical Thermodynamics

# Introduction

Thermodynamics is the science of energy[1]. It deals with energy transfer, conversion, and quantification. Energy is an agent of change in the universe. Thermodynamics is an essential science because our daily processes require the conversion of energy from one form to another. For example, to move yourself out of bed you require energy. To open the door to your car you require energy. Literally, everything in the universe is governed by an exchange of energy, hence, the science of thermodynamics is at the heart of studying all these processes. Of course, thermodynamics is not exclusive in this study. Nonetheless, it addresses an essential component and dictates the laws that govern energy exchange in nature.

# Pure Substances

A pure substance is one that has a uniform chemical composition throughout.

## Compresssed Liquid

A compressed liquid is one that is NOT about to evaporate. An increase in temperature will not bring about evaporation.

## Saturated Liquid

A Saturated liquid is one that is about to evaporate. Any slight increase in temperature (or decrease in pressure) will cause evaporation.

## Saturated Vapor

A Saturated vapor is one that is about to condense. Any slight decrease in temperature (or increase in pressure) will cause condensation.

## Superheated Vapor

A Superheated vapor is one that is not about to condense. Any decrease in temperature will not bring about condensation.

# Energy

According to Cengel and Boles [1], The term energy was coined in 1807 by Thomas Youg, and English polymath. use in thermodynamics was promoted by Lord Kelvin circa 1852. The term internal energy and its symbol $U$ first appeared in the works of Rudolph Clausius and William Rankine in the second half of the nineteenth century, and it eventually replaced the alternative terms inner work, internal work, and intrinsic energy commonly used at the time.

## Forms of Energy

Energy can exist in several forms such as electrical, thermal, chemical, mechanical, and nuclear. Their sum represents the total energy $E$ of a system. It is also convenient to define the specific total energy as

(1)
\begin{align} e = \frac{E}{m};\quad \left(\mathrm{kJ/kg\right}) \end{align}

or, on a molar basis

(2)
\begin{align} \bar{e} = \frac{E}{n}; \quad \left(\mathrm{kJ/kmol}\right) \end{align}

The science of thermodynamics does NOT provide any information about the absolute value of the total energy of a system. Instead, it grants us the ability to calculate the change in total energy given an initial state of a system. For all practical purposes, the total energy associated with the initial state of a system can be set to zero.

The total energy of a system can be decomposed into two forms, macroscopic and microscopic or internal energies. Each corresponds to different agents causing a change in the energy of the system.

### Macroscopic Energy

Macroscopic contributions to the energy of a system include all those external agents that influence the system as a whole. These are usually defined using an external frame of reference. Macroscopic energy includes the work done by forces at a distance such as gravity and magnetism. The kinetic energy associated with motion is also a macroscopic form of energy. In most situation, only the kinetic and potential energies are the most dominant.
The kinetic and potential energies can be defined on a mass and molar basis.

### Microscopic Energy

Microscopic energy has to do with the energies associated with the constituent molecules or atoms of the system being analyzed. For exampled, one may take into account the kinetic and potential energies of individual molecules. Also, vibrational, rotational, and electron spin are other forms of microscopic energies. For convenience, the sum of all microscopic energies is referred to as the internal energy $U$ of the system

(3)
\begin{align} U = \sum \text{Microscopic energy} \end{align}

Likewise, the internal energy can be defined on a mass or molar basis,

(4)
\begin{align} u = \frac{U}{m}; \quad \bar{u} = \frac{U}{n} \end{align}

## Total Energy

The total energy of a system is then defined as

(5)
\begin{align} E = \text{Microscopic energy} + \text{Macroscopic energy} = U + \mathrm{K.E.} + \mathrm{P.E.} \end{align}

# Pure Substances

## Definitions

• Pure Substance: A pure substance is one that has a uniform and fixed chemical composition throughout. CO2, Water, Nitrogen, Helium are examples of pure substances. A pure substance may be composed of different chemical compounds. For example, Air is usually considered as a pure substance although it is made up of a variety of chemical compounds. A mixture of two or more phases of a pure substance is also a pure substance. For instance, water and ice is a pure substance. However, a mixture of liquid air and air is not a pure substance since the composition of liquid air is different from that of gaseous air.
• Heat of Evaporation: The heat of evaporation or $h_{\text{fg}}$ is the amount of heat required to evaporate one unit of mass of a liquid. It is also the same amount of heat required to be extracted to condense one unit of mass of the vapor.

## Solutions - Pure Substances - Cengel Thermodynamics Book

2.1 Is Iced water a pure substance? Why?
Yes, because it has a uniform chemical composition, H2O.

2.2 What is the difference between sautrated liquid and compressed liquid?
A saturated liquid is one that is about to evaporate. Any increase in temperature (@ fixed P) or decrease in pressure (@ fixed T) will bring about evaporation. A compressed liquid or subcooled liquid on the other hand is not about to evaporate.

2.3 What is the difference between saturated vapor and superheated vapor?
Saturated vapor is one that is about to condense, any decrease is temperature or pressure will cause condensation to occur. Superheated vapor on the other hand is not about to condense. Any decrease in temperature (above the saturation T at that pressure) will not bring about condensation.

2.4 Is there a difference between saturated vapor at a given T and the vapor in a saturated mixture at the same T?
No. The properties are the same.

2.5 Same as previous for liquid.
No. The properties are the same.

2.6 Is it true that water boils at higher temperatures at higher pressures? Explain.
Yes. At higher pressure, the saturation temperature of water is higher. This is due to the fact that at higher pressures, the molecules of water are closely packed together requiring higher energy to separate them from each other to cause a phase change.

2.7 If the pressure of a substance is increased during a boiling process, will the temperature also increase or will it remain constant? Why?
Boiling occurs at a solid liquid interface where the solid surface is at a higher temperature than the saturation temperature of the liquid at that pressure. If the pressure is increased, so long as the temperature of the solid surface is higher than the saturation temperature of the liquid at that pressure, then boiling will occur. Therefore, as the pressure is increased, and as boiling is maintained, the temperature of the liquid will also increase because the saturation temperature is also increasing.

2.8 Why are the pressure and temperature dependent in a saturated mixture region?
The pressure and temperature are dependent in this region because the latent heat of vaporization (or condensation) depends on both pressure and temperature. This latent heat is require to evaporate or condense saturated vapors and liquids.

2.9 What is the difference between the critical point and the triple point?
The critical point is the point at which the phase change process is indistiguishable. The specific volume increases continuously.

# First Law of Thermodynamics

The first law of thermodynamics is a statement of conservation of energy. It describes how the total energy of a system changes in conjunction with boundary work and heat transfer. The formal statement is given as: the net change in total energy of a system during a process is equal to the difference between the total energy entering and the total energy leaving the system during that process.

# Second Law of Thermodynamics

Bibliography
1. Cengel, Boles, Thermodynamics, an Engineering Approach.
page revision: 21, last edited: 03 Jan 2011 18:52