Laplace's law (1780): The energy change accompanying any transformation is equal and opposite to energy change accompanying the reverse process.
#
Hess' law of constant heat summation (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
The first law of thermodynamics is a formulation of the law of conservation of energy in the context of thermodynamic processes. For a thermodynamic process affecting a thermodynamic system without transfer of matter, the law distinguishes two ...
(1845) and helped in its formulation.
Thermochemistry also involves the measurement of the
latent heat
Latent heat (also known as latent energy or heat of transformation) is energy released or absorbed, by a body or a thermodynamic system, during a constant-temperature process—usually a first-order phase transition, like melting or condensation. ...
of
phase transition
In physics, chemistry, and other related fields like biology, a phase transition (or phase change) is the physical process of transition between one state of a medium and another. Commonly the term is used to refer to changes among the basic Sta ...
s.
Joseph Black had already introduced the concept of latent heat in 1761, based on the observation that heating ice at its
melting point
The melting point (or, rarely, liquefaction point) of a substance is the temperature at which it changes state of matter, state from solid to liquid. At the melting point the solid and liquid phase (matter), phase exist in Thermodynamic equilib ...
did not raise the temperature but instead caused some ice to melt.
Gustav Kirchhoff
Gustav Robert Kirchhoff (; 12 March 1824 – 17 October 1887) was a German chemist, mathematician, physicist, and spectroscopist who contributed to the fundamental understanding of electrical circuits, spectroscopy and the emission of black-body ...
showed in 1858 that the variation of the heat of reaction is given by the difference in
heat capacity
Heat capacity or thermal capacity is a physical property of matter, defined as the amount of heat to be supplied to an object to produce a unit change in its temperature. The SI unit of heat capacity is joule per kelvin (J/K).
Heat capacity is a ...
between products and reactants: dΔH / dT = ΔC
p. Integration of this equation permits the evaluation of the heat of reaction at one temperature from measurements at another temperature.
[ Atkins P. and de Paula J., "Atkins' Physical Chemistry" (8th edn, W.H. Freeman 2006), p.56]
Calorimetry
The measurement of heat changes is performed using
calorimetry
In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in '' state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reac ...
, usually an enclosed chamber within which the change to be examined occurs. The temperature of the chamber is monitored either using a
thermometer
A thermometer is a device that measures temperature (the hotness or coldness of an object) or temperature gradient (the rates of change of temperature in space). A thermometer has two important elements: (1) a temperature sensor (e.g. the bulb ...
or
thermocouple
A thermocouple, also known as a "thermoelectrical thermometer", is an electrical device consisting of two dissimilar electrical conductors forming an electrical junction. A thermocouple produces a temperature-dependent voltage as a result of the ...
, 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
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 surroundings or environment. A system may be:
* a (completely)
isolated system
In physical science, an isolated system is either of the following:
# a physical system so far removed from other systems that it does not interact with them.
# a thermodynamic system enclosed by rigid immovable walls through which neither ...
which can exchange neither energy nor matter with the surroundings, such as an insulated
bomb calorimeter
A bomb is an explosive weapon that uses the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy. Detonations inflict damage principally through ground- and atmosphere-transmitted mechanic ...
* a
thermally isolated system In thermodynamics, a thermally isolated system can exchange no mass or heat energy with its environment. The internal energy of a thermally isolated system may therefore change due to the exchange of work energy. The entropy of a thermally isolated ...
which can exchange mechanical work but not heat or matter, such as an insulated closed
piston
A piston is a component of reciprocating engines, reciprocating pumps, gas compressors, hydraulic cylinders and pneumatic cylinders, among other similar mechanisms. It is the moving component that is contained by a cylinder (engine), cylinder a ...
or
balloon
A balloon is a flexible membrane bag that can be inflated with a gas, such as helium, hydrogen, nitrous oxide, oxygen, or air. For special purposes, balloons can be filled with smoke, liquid water, granular media (e.g. sand, flour or rice), ...
* a
mechanically isolated system
In thermodynamics, a mechanically isolated system is a system that is mechanically constrained to disallow deformations, so that it cannot perform any work on its environment. It may however, exchange heat across the system boundary.
For a simple ...
which can exchange heat but not mechanical work or matter, such as an uninsulated
bomb calorimeter
A bomb is an explosive weapon that uses the exothermic reaction of an explosive material to provide an extremely sudden and violent release of energy. Detonations inflict damage principally through ground- and atmosphere-transmitted mechanic ...
* a
closed system
A closed system is a natural physical system that does not allow transfer of matter in or out of the system, althoughin the contexts of physics, chemistry, engineering, etc.the transfer of energy (e.g. as work or heat) is allowed.
Physics
In cl ...
which can exchange energy but not matter, such as an uninsulated closed piston or balloon
* an
open system which it can exchange both matter and energy with the surroundings, such as a pot of boiling water
Processes
A system undergoes a process when one or more of its properties changes. A process relates to the change of state. An
isothermal
An isothermal process is a type of thermodynamic process in which the temperature ''T'' of a system remains constant: Δ''T'' = 0. This typically occurs when a system is in contact with an outside thermal reservoir, and a change in the sys ...
(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. A process is
adiabatic when no heat exchange occurs.
See also
*
Calorimetry
In chemistry and thermodynamics, calorimetry () is the science or act of measuring changes in '' state variables'' of a body for the purpose of deriving the heat transfer associated with changes of its state due, for example, to chemical reac ...
*
Chemical kinetics
Chemical kinetics, also known as reaction kinetics, is the branch of physical chemistry that is concerned with understanding the rates of chemical reactions. It is different from chemical thermodynamics, which deals with the direction in which a ...
*
Cryochemistry
*
Differential scanning calorimetry
Differential scanning calorimetry (DSC) is a thermoanalytical technique in which the difference in the amount of heat required to increase the temperature of a sample and reference is measured as a function of temperature. Both the sample and re ...
*
Isodesmic reaction
*
Important publications in thermochemistry
*
Photoelectron photoion coincidence spectroscopy
*
Principle of maximum work
*
Reaction Calorimeter
*
Thermodynamic databases for pure substances
*
Thermodynamics
Thermodynamics is a branch of physics that deals with heat, Work (thermodynamics), work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed b ...
*
Thomsen-Berthelot principle
*
Julius Thomsen
References
External links
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{{Authority control
Physical chemistry
Branches of thermodynamics