Page 21 - Physical Chemistry

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Chapter 1 Organic chemists use kinetics studies to figure out the mechanisms of reactions,
Thermodynamics use quantum-chemistry calculations to study the structures and stabilities of reaction
intermediates, use symmetry rules deduced from quantum chemistry to predict the
course of many reactions, and use nuclear-magnetic-resonance (NMR) and infrared
spectroscopy to help determine the structure of compounds. Inorganic chemists use
quantum chemistry and spectroscopy to study bonding. Analytical chemists use spec-
troscopy to analyze samples. Biochemists use kinetics to study rates of enzyme-
catalyzed reactions; use thermodynamics to study biological energy transformations,
osmosis, and membrane equilibrium, and to determine molecular weights of biological
molecules; use spectroscopy to study processes at the molecular level (for example, in-
tramolecular motions in proteins are studied using NMR); and use x-ray diffraction to
determine the structures of proteins and nucleic acids.
Environmental chemists use thermodynamics to find the equilibrium composition
of lakes and streams, use chemical kinetics to study the reactions of pollutants in the
atmosphere, and use physical kinetics to study the rate of dispersion of pollutants in
the environment.
Chemical engineers use thermodynamics to predict the equilibrium composition
of reaction mixtures, use kinetics to calculate how fast products will be formed, and
use principles of thermodynamic phase equilibria to design separation procedures
such as fractional distillation. Geochemists use thermodynamic phase diagrams to un-
derstand processes in the earth. Polymer chemists use thermodynamics, kinetics, and
statistical mechanics to investigate the kinetics of polymerization, the molecular
weights of polymers, the flow of polymer solutions, and the distribution of conforma-
tions of a polymer molecule.
Widespread recognition of physical chemistry as a discipline began in 1887 with
the founding of the journal Zeitschrift für Physikalische Chemie by Wilhelm Ostwald
with J. H. van’t Hoff as coeditor. Ostwald investigated chemical equilibrium, chemi-
cal kinetics, and solutions and wrote the first textbook of physical chemistry. He was
instrumental in drawing attention to Gibbs’ pioneering work in chemical thermody-
namics and was the first to nominate Einstein for a Nobel Prize. Surprisingly, Ostwald
argued against the atomic theory of matter and did not accept the reality of atoms
and molecules until 1908. Ostwald, van’t Hoff, Gibbs, and Arrhenius are generally
regarded as the founders of physical chemistry. (In Sinclair Lewis’s 1925 novel
Arrowsmith, the character Max Gottlieb, a medical school professor, proclaims that
“Physical chemistry is power, it is exactness, it is life.”)
In its early years, physical chemistry research was done mainly at the macroscopic
level. With the discovery of the laws of quantum mechanics in 1925–1926, emphasis
began to shift to the molecular level. (The Journal of Chemical Physics was founded
in 1933 in reaction to the refusal of the editors of the Journal of Physical Chemistry
to publish theoretical papers.) Nowadays, the power of physical chemistry has been
greatly increased by experimental techniques that study properties and processes at the
molecular level and by fast computers that (a) process and analyze data of spec-
troscopy and x-ray crystallography experiments, (b) accurately calculate properties of
molecules that are not too large, and (c) perform simulations of collections of hun-
dreds of molecules.
Nowadays, the prefix nano is widely used in such terms as nanoscience, nano-
technology, nanomaterials, nanoscale, etc. A nanoscale (or nanoscopic) system is one
with at least one dimension in the range 1 to 100 nm, where 1 nm 10 9 m. (Atomic
diameters are typically 0.1 to 0.3 nm.) A nanoscale system typically contains thou-
sands of atoms. The intensive properties of a nanoscale system commonly depend
on its size and differ substantially from those of a macroscopic system of the same
composition. For example, macroscopic solid gold is yellow, is a good electrical con-
ductor, melts at 1336 K, and is chemically unreactive; however, gold nanoparticles of