II . Read the text and find the answers to the questions above

TEXT A
PHYSICAL CHEMISTRY
Physical chemistry is the branch of chemistry concerned with the interpretation of the
phenomena of chemistry in terms of the underlying principles of physics. It lies at the
interface of chemistry and physics, in as much as it draws on the principles of physics
(especially quantum mechanics) to account for the phenomena of chemistry. It is also an
essential component of the interpretation of the techniques of investigation and their findings,
particularly because these techniques are becoming ever more sophisticated and because their
full potential can be realized only by strong theoretical backing. Physical chemistry also has
an essential role to play in the understanding of the complex processes and molecules
characteristic of biological systems and modern materials.
Physical chemistry is traditionally divided into a number of disciplines, but the
boundaries between them are imprecise. Thermodynamics is the study of transformations of
energy. Although this study might seem remote from chemistry, in fact it is vital to the study
of how chemical reactions yield work
and heat. Thermodynamic techniques
and analyses are also used to elucidate
the tendency of physical processes (such
as vaporization) and chemical reactions
to reach equilibrium — the condition
when there is no further net tendency to
change. Thermodynamics is used to
relate bulk properties of substances to each other, so that measurements of one may be used to
deduce the value of another. Spectroscopy is concerned with the experimental investigation of
the structures of atoms and molecules, and the identification of substances, by the observation
of properties of the electromagnetic radiation absorbed, emitted, or scattered by samples.
Microwave spectroscopy is used to monitor the rotations of molecules; infrared spectroscopy
is used to study their vibrations; and visible and ultraviolet spectroscopy is used to study
electronic transitions and to infer details of electronic structures. The enormously powerful
technique of nuclear magnetic resonance is now ubiquitous in chemistry. The detailed,
quantitative interpretation of molecular and solid-state structure is based in quantum theory
and its use in the interpretation of the nature of the chemical bond. Diffraction studies,
particularly x-ray diffraction and neutron diffraction studies provide detailed information
about the shapes of molecules, and x-ray diffraction studies are central to almost the whole of
molecular biology. The scattering of neutrons, in inelastic neutron scattering, gives detailed
information about the motion of molecules in liquids. The bridge between thermodynamics
and structural studies is called statistical thermodynamics, in which bulk properties of
substances are interpreted in terms of the properties of their constituent molecules. Another
major component is chemical kinetics, the study of the rates of chemical reactions; it
examines, for example, how rates of reactions respond to changes in conditions or the
presence of a catalyst. Chemical kinetics is also concerned with the detailed mechanisms by
which a reaction takes place, the sequences of elementary processes that convert reactants into
products, including chemical reactions at solid surfaces (such as electrodes).
There are further subdivisions of these major fields. Thermochemistry is a branch of
thermodynamics; its focus is the heat generated or required by chemical reactions.
Electrochemistry is the study of how chemical reactions can produce electricity and how
electricity can drive chemical reactions in "reverse" directions (electrolysis). Increasingly,
attention is shifting from equilibrium electrochemistry (which is of crucial importance in
interpreting the phenomena of inorganic chemistry) to dynamic electrochemistry, in which the
rates of electron-transfer processes are the focus. Chemical kinetics has divisions that are
52

based on the rates of reaction being studied. Special
techniques for studying atomic and molecular processes on
ever shorter time scales are being developed, and physical
chemists are now able to explore reactions on a femtosecond
(10
−15
second) timescale. Chemical kinetics studies are
theoretical as well as experimental. One goal is to understand
the course of reactions in step-by-step (and atomic) detail.
Techniques are available that allow investigators to study
collisions between individual molecules.
Physical chemistry is essential to understanding the
other branches of chemistry. It provides a basis for
understanding the thermodynamic
influences (principally, the entropy changes accompanying reactions)
that drive chemical reactions forward. It provides justifications for the
schemes proposed in organic chemistry to predict and account for the
reactions of organic compounds. It accounts for the structures and
properties of transition metal complexes, organometallic compounds, the
microporous materials known as zeolites that are so important for
catalysis, and biological macromolecules, such as proteins and nucleic
acids (including DNA). It is fair to say that there is no branch of
chemistry (including biochemistry) that can be fully understood without
interpretations provided by physical chemistry.
There is a distinction between physical chemistry and chemical physics, although the
distinction is hard to define and it is not always made. In physical chemistry, the target of
investigation is typically a bulk system (for example, chemical equilibrium, and colloids). In
chemical physics, the target is commonly an isolated, individual molecule.
Theoretical chemistry is a branch of physical chemistry in which quantum mechanics
and statistical mechanics are used to calculate properties of molecules and bulk systems. The
greater part of activity in quantum chemistry, as the former is commonly termed, is the
computation of the electronic structures of molecules and, often, their graphical
representation. This kind of study is particularly important to the screening of compounds for
potential pharmacological activity, and for establishing the mode of action of enzymes.

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