Chemistry ppt

In thermodynamics, the internal energy is one of the two cardinal state functions of the state variables of a thermodynamic system. It refers to energy contained within the system, while excluding the kinetic energy of motion of the system as a whole and the potential energy of the system as a whole due to external force fields. It keeps account of the gains and losses of energy of the system.

Internal energy

• In thermodynamics, the internal energy is one of the two cardinal state functions of the state variables of a thermodynamic system. It refers to energy contained within the system, while excluding the kinetic energy of motion of the system as a whole and the potential energy of the system as a whole due to external force fields. It keeps account of the gains and losses of energy of the system.

The internal energy of a system can be changed by heating the system, or by doing work on it, or by adding or taking away matter. When matter transfer is prevented by impermeable walls containing the system, it is said to be closed. Then the first law of thermodynamics states that the increase in internal energy is equal to the total heat added and work done on the system by the surroundings. If the containing walls pass neither matter nor energy, the system is said to be isolated. Then its internal energy cannot change.

• The internal energy of a system can be changed by heating the system, or by doing work on it, or by adding or taking away matter. When matter transfer is prevented by impermeable walls containing the system, it is said to be closed. Then the first law of thermodynamics states that the increase in internal energy is equal to the total heat added and work done on the system by the surroundings. If the containing walls pass neither matter nor energy, the system is said to be isolated. Then its internal energy cannot change.

• The internal energy of a given state of a system cannot be directly measured. It is determined through some convenient chain of thermodynamic operations and thermodynamic processes by which the given state can be prepared, starting with a reference state which is customarily assigned a reference value for its internal energy. Such a chain, or path, can be theoretically described by certain extensive state variables of the system, namely, its entropy, S, its volume, V, and its mole numbers, {Nj}. The internal energy, U(S,V,{Nj}), is a function of those. Sometimes, to that list are appended other extensive state variables, for example electric dipole moment.

Work

• In thermodynamics, work performed by a system is the energy transferred by the system to another that is accounted for by changes in the external generalized mechanical constraints on the system. As such, thermodynamic work is a generalization of the concept of mechanical work in physics.

• The external generalized mechanical constraints may be chemical, electromagnetic, (including radiative), gravitational or pressure/volume or other simply mechanical constraints, including momental, as in radiative transfer. Thermodynamic work is defined to be measurable solely from knowledge of such external macroscopic constraint variables. These macroscopic variables always occur in conjugate pairs, for example pressure and volume, magnetic flux density and magnetization, mole fraction and chemical potential. In the SI system of measurement, work is measured in joules (symbol: J). The rate at which work is performed is power.

• It is customary to calculate amount of energy transferred as work through quantities external to the system of interest, and thus belonging to its surroundings. Nevertheless, for historical reasons, the customary sign convention is to consider work done by the system on its surroundings as positive. Although all real physical processes entail some dissipation of kinetic energy, it is matter of principle that the dissipation that results from transfer of energy as work occurs only inside the system; energy dissipated outside the system, in the process of transfer of energy, is not counted as thermodynamic work. Thermodynamic work does not account for any energy transferred between systems as heat.

• Mechanical thermodynamic work is performed by actions such as compression, and including shaft work, stirring, and rubbing. In the simplest case, for example, there are work of change of volume against a resisting pressure, and work without change of volume, known as isochoric work. An example of isochoric work is when an outside agency, in the surrounds of the system, drives a frictional action on the surface of the system. In this case the dissipation is not necessarily actually confined to the system, and the quantity of energy so transferred as work must be estimated through the overall change of state of the system as measured by both its mechanically and externally measurable deformation variables (such as its volume), and its non-deformation variable (usually internal to the system, for example its empirical temperature, regarded not as a temperature but simply as a mechanically measurable variable).

• In a process of transfer of energy by work, the internal energy of the final state of the system is then measured by the amount of adiabatic work of change of volume that would have been necessary to reach it from the initial state, such adiabatic work being measurable only through the externally measurable mechanical or deformation variables of the system, but including also full information about the forces exerted by the surroundings on the system during the process. In the case of some of Joule's measurements, the process was so arranged that heat produced outside the system by the frictional process was practically entirely transferred into the system during the process, so that the quantity of work done by the surrounds on the system could be calculated as shaft work, an external mechanical variable. For closed systems, internal energy changes in a system other than as work transfer are as heat.

• In physics, heat is the transfer of energy other than by work or transfer of matter. Heat flows spontaneously from a hotter body to a colder one whenever a suitable physical pathway exists between the bodies, and always results in a net increase in entropy. The pathway can be direct, as in conduction and radiation, or indirect, as in convective circulation. Because it refers to a process, heat is not a property of a system.

Heat

• Kinetic theory explains heat as a macroscopic manifestation of the motions and interactions of microscopic constituents such as molecules and photons.The quantity of energy transferred as heat is a scalar expressed in an energy unit such as the joule (J) (SI), with a sign that is customarily positive when a transfer adds to the energy of a system. It can be measured by calorimetry,[10] or determined by calculations based on other quantities, relying on the first law of thermodynamics. In calorimetry, latent heat changes a system's state without temperature change, while sensible heat changes its temperature.

• If latent heat is defined with respect to a change of a particular state variable of the system, then a specifically corresponding variety of constrained sensible heat can be defined for change of temperature, leaving that particular state variable unchanged. For infinitesimal changes, the total incremental heat transfer is then the sum of the latent and sensible heat increments. This is a basic paradigm for thermodynamics, and was important in the historical development of the subject.

• Referring to conduction, Partington writes: "If a hot body is brought in conducting contact with a cold body, the temperature of the hot body falls and that of the cold body rises, and it is said that a quantity of heat has passed from the hot body to the cold body.“ Referring to radiation, Maxwell writes: "In Radiation, the hotter body loses heat, and the colder body receives heat by means of a process occurring in some intervening medium which does not itself thereby become hot."

• Maxwell writes that convection as such "is not a purely thermal phenomenon". In thermodynamics, convection in general is regarded as transport of internal energy. If, however, the convection is enclosed and circulatory, then it may be regarded as an intermediary that transfers energy as heat between source and destination bodies, because it transfers only energy and not matter from the source to the destination body.