Контрольная Термодинамика, 2 вопроса, 2 задачи. Учебная работа № 187849

Учебная работа № 187849. Контрольная Термодинамика, 2 вопроса, 2 задачи

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Термодинамика и термохимия

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to position, heat energy as measured by the temperature, electrical energy,
chemical energy, etc. Chemical and physical processes are almost invariably
accompanied by energy changes, and results of considerable importance have been
obtained studying the laws underlying these changes. It is this study of energy
transformation which constitutes the subject matter of thermodynamics. Although
thermodynamics may appear to be somewhat theoretical in nature, the two laws
have led to results of fundamental importance to chemistry, as well as to
physics.
Conservation
of Energy: The First Law of Thermodynamics.
Many attempts
have been made from time to time to realize «perpetual motion», that
is, the continuous production of mechanical work without supplying an
equivalent amount of energy from another source. The failure of all such
efforts has led to the universal acceptance of the principle of conservation of
energy. This principle has been stated in many forms, but essentially they
amount to the fact that although energy can be converted from one form to
another, it cannot be created or destroyed or, alternatively, whenever a
quantity of one kind of energy is produced, an exactly equivalent amount of
other kinds must disappear. It is evident that perpetual motion, in the
generally accepted sense of the term, would be contrary to this principle, for
it would involve the creation of energy. Further, the exact equivalence of
mechanical or electrical work and heat, as found by Joule and others, is a
necessary consequence of the same principle.
The law of
conservation of energy is purely the result of experience, no exception to it
having as yet been found. The assumption that it is of universal applicability
is the basis of the first law of thermodynamics. This law can be stated in any
of the ways given above for the principle of the conservation of energy, or
else it may be put in the following form. The total energy of a system and its
surroundings must remain constant, although it may be changed from one form, to
another.
Heat
Changes in Chemical Reactions.
The subject of
thermochemistry deals with the heat changes accompanying chemical reactions. As
will be seen shortly the laws of thermochemistry are based-largely on the
principle of the conservation of energy or the first law of thermodynamics.
Different substances have different amounts of internal (chemical) energy, and
so the total energy of the products of a reaction is generally different from
that of the reactants; hence, the chemical change will be accompanied by the
liberation or absorption of energy, which may appear in the form of heat. If
heat is liberated in the reaction the process is said to be exothermic, but if
heat is absorbed it is described as endothermic. The majority of, although not
all, chemical reactions which go to virtual completion at ordinary temperatures
are exothermic in character, since they are accompanied by an evolution of
heat. If a chemical reaction is associated with a volume change, as is
particularly the case for many processes involving the combination of gases,
the magnitude of the heat change will depend on whether the reaction is carried
out at constant pressure or at constant volume. Since many reactions normally
occur at constant (atmospheric) pressure it is the usual practice to record
heat changes by quoting the value of qp, the heat absorbed at
constant pressure; this may, of course, be identified with ΔH , the
increase of heat content under the same conditions. This quantity is often
referred to as the heat of reaction; it represents the difference in the heat
contents of the reaction products and of the reactants, at constant pressure
and at definite temperature, with every substance in a definite physical state.
From the value of qp (or ΔH) the value of gv
(or ΔE) can be readily determined if the volume change ΔV
at the constant pressure P is known as will be seen below.
The heat change
accompanying a reaction, for example, that between solid carbon (graphite) and
gaseous oxygen to yield carbon …