thermochemistry

A branch of physical chemistry concerned with the absorption or evolution of heat that accompanies chemical reactions. Closely related topics are the latent heat associated with a change in phase (crystal, liquid, gas), the chemical composition of reacting systems at equilibrium, and the electrical potentials of galvanic cells. Thermodynamics provides the link among these phenomena.

A knowledge of such heat effects is important to the chemical engineer for the design and operation of chemical reactors, the determination of the heating values of fuels, the design and operation of refrigerators, the selection of heat storage systems, and the assessment of chemical hazards. Thermochemical information is used by the physiologist and biochemist to study the energetics of living organisms and to determine the calorific values of foods. Thermochemical data give the chemist an insight to the energies of, and interactions among, molecules. See also Chemical thermodynamics.

A calorimeter is an instrument for measuring the heat added to or removed from a process. There are many designs, but the following parts can generally be identified: the vessel in which the process is confined, the thermometer which measures its temperature, and the surrounding environment called the jacket. The heat associated with the process is calculated by the equation below, where T is the temperature. The quantity C, the energy equivalent of the calorimeter, is obtained from a separate calibration experiment. The work transferred to the process, w, is generally in the form of an electric current (as supplied to a heater, for example) or as mechanical work (as supplied to a stirrer, for example) and can be calculated from appropriate auxiliary measurements. The quantity q_ex is the heat exchanged between the container and its jacket during the experiment. It is calculated from the temperature gradients in the system and the measured thermal conductivities of its parts.

Two principal types of calorimeters are used to measure heats of chemical reactions. In a batch calorimeter, known quantities of reactants are placed in the vessel and the initial temperature is measured. The reaction is allowed to occur and then the final equilibrium temperature is measured. If necessary, the final contents are analyzed to determine the amount of reaction which occurred.

In a flow calorimeter, the reactants are directed to the reaction vessel in two or more steady streams. The reaction takes place quickly and the products emerge in a steady stream. The rate of heat production is calculated from the temperatures, flow velocities, and heat capacities of the incoming and outgoing streams, and the rates of work production and heat transfer to the jacket. Dividing this result by the rate of reaction gives the heat of reaction. See also Calorimetry.

In the past, thermochemical quantities usually have been given in units of calories. A calorie is defined as the amount of heat needed to raise the temperature of 1 gram of water 1°C. However, since this depends on the initial temperature of the water, various calories have been defined, for example, the 15° calorie, the 20° calorie, and the mean calorie (average from 0 to 100°C). In addition, a number of dry calories have been defined. Those still used are the thermochemical calorie (exactly 4.184 joules) and the International Steam Table calorie (exactly 4.1868 J).

The aspect of physical chemistry dealing with temperature changes that accompany chemical reactions.

For other uses, see Chemical thermodynamics.

The world’s first ice-calorimeter, used in the winter of 1782-83, by Antoine Lavoisier and Pierre-Simon Laplace, to determine the heat evolved in various chemical changes; calculations which were based on Joseph Black’s prior discovery of latent heat. These experiments mark the foundation of thermochemistry.

In thermodynamics and physical chemistry, thermochemistry is the study of the energy evolved or absorbed in chemical reactions and any physical transformations, such as melting and boiling. Thermochemistry, generally, is concerned with the energy exchange accompanying transformations, such as mixing, phase transitions, chemical reactions, and including calculations of such quantities as the heat capacity, heat of combustion, heat of formation, enthalpy, and free energy.

Contents 1 History 2 Calorimetry 3 Systems 4 Processes 5 See also 6 References 7 External links

History

Thermochemistry rests on two generalizations. Stated in modern terms, they are as follows:^[1]

1. Lavoisier and Laplace’s law (1780): The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process.^[2]
2. Hess's law (1840): The energy change accompanying any transformation is the same whether the process occurs in one step or many.

These statements preceded the first law of thermodynamics (1845) and helped in its formulation.

Edward Diaz and Hess also investigated specific heat and latent heat, although it was Joseph Black who made the most important contributions to the development of latent energy changes.

Calorimetry

The measurement of heat changes is performed using calorimetry, usually an enclosed chamber within which the change to be examined occurs. The temperature of the chamber is monitored either using a thermometer or thermocouple, and the temperature plotted against time to give a graph from which fundamental quantities can be calculated. Modern calorimeters are frequently supplied with automatic devices to provide a quick read-out of information, one example being the DSC or differential scanning calorimeter.

Systems

Several thermodynamic definitions are very useful in thermochemistry. A system is the specific portion of the universe that is being studied. Everything outside the system is considered the surrounding or environment. A system may be: Isolated system - when it cannot exchange energy or matter with the surroundings, as with an insulated bomb reactor; Closed system - when it can exchange energy but not matter with the surroundings, as with a steam radiator; Open system - when it can exchange both matter and energy with the surroundings, as with a pot of boiling water.

Processes

A system undergoes a process when one of more of its properties changes. A process relates to the change of state. An isothermal (same temperature) process occurs when temperature of the system remains constant. An isobaric (same pressure) process occurs when the pressure of the system remains constant. An adiabatic (no heat exchange)process occurs when no heat exchange occurs.

See also

Science portal

• Differential scanning calorimetry
• Important publications in thermochemistry
• Isodesmic reaction
• Principle of maximum work
• Reaction Calorimeter
• Thomsen-Berthelot principle
• Julius Thomsen
• Thermodynamic databases for pure substances
• Calorimetry

References

1. ^ Perrot, Pierre (1998). A to Z of Thermodynamics. Oxford University Press. ISBN 0-19-856552-6. 
2. ^ See page 290 of Outlines of Theoretical Chemistry] by Frederick Hutton Getman (1918)

External links

• Thermochemistry - Britannica (1911)
• Thermochemistry - an overview

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