Thermodynamics

Some Physical Insight to Internal EnergyInternal energy is defined earlier as the sum of all the microscopic forms ofenergy of a system. It is related to the molecular structure and the degree ofmolecular activity and can be viewed as the sum of the kinetic and potentialenergies of the molecules.To have a better understanding of internal energy, let us examine a systemat the molecular level. The molecules of a gas move through space withsome velocity, and thus possess some kinetic energy. This is known as thetranslational energy. The atoms of polyatomic molecules rotate about anaxis, and the energy associated with this rotation is the rotational kineticenergy. The atoms of a polyatomic molecule may also vibrate about theircommon center of mass, and the energy associated with this back-and-forthmotion is the vibrational kinetic energy. For gases, the kinetic energy ismostly due to translational and rotational motions, with vibrational motionbecoming significant at higher temperatures. The electrons in an atom rotateabout the nucleus, and thus possess rotational kinetic energy. Electrons atouter orbits have larger kinetic energies. Electrons also spin about theiraxes, and the energy associated with this motion is the spin energy. Otherparticles in the nucleus of an atom also possess spin energy. The portion ofthe internal energy of a system associated with the kinetic energies of themolecules is called the sensible energy (Fig. 2–5). The average velocity andthe degree of activity of the molecules are proportional to the temperature ofthe gas. Therefore, at higher temperatures, the molecules possess higherkinetic energies, and as a result the system has a higher internal energy.The internal energy is also associated with various binding forces betweenthe molecules of a substance, between the atoms within a molecule, andbetween the particles within an atom and its nucleus. The forces that bind themolecules to each other are, as one would expect, strongest in solids andweakest in gases. If sufficient energy is added to the molecules of a solid orliquid, the molecules overcome these molecular forces and break away, turningthe substance into a gas. This is a phase-change process. Because of thisadded energy, a system in the gas phase is at a higher internal energy levelthan it is in the solid or the liquid phase. The internal energy associated withthe phase of a system is called the latent energy. The phase-change processcan occur without a change in the chemical composition of a system. Mostpractical problems fall into this category, and one does not need to pay anyattention to the forces binding the atoms in a molecule to each other.An atom consists of neutrons and positively charged protons boundtogether by very strong nuclear forces in the nucleus, and negativelycharged electrons orbiting around it. The internal energy associated with theatomic bonds in a molecule is called chemical energy. During a chemicalreaction, such as a combustion process, some chemical bonds are destroyedwhile others are formed. As a result, the internal energy changes. Thenuclear forces are much larger than the forces that bind the electrons to thenucleus. The tremendous amount of energy associated with the strong bondswithin the nucleus of the atom itself is called nuclear energy (Fig. 2–6).Obviously, we need not be concerned with nuclear energy in thermodynamicsunless, of course, we deal with fusion or fission reactions. A chemicalreaction involves changes in the structure of the electrons of the atoms, buta nuclear reaction involves changes in the core or nucleus. Therefore, anMoleculartranslation+–ElectrontranslationElectronspin–Chapter 2 | 55MolecularrotationMolecularvibration+NuclearspinFIGURE 2–5The various forms of microscopicenergies that make up sensible energy.Sensibleand latentenergyChemicalenergyNuclearenergyFIGURE 2–6The internal energy of a system is thesum of all forms of the microscopicenergies.