Calorimetry
In chemistry and thermodynamics, calorimetry (from Latin calor 'heat' and Greek μέτρον (metron) 'measure') 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 reactions, physical changes, or phase transitions under specified constraints.
Scottish physician and scientist Joseph Black, who was the first to recognize the distinction between heat and temperature, is said to be the founder of the science of calorimetry.
[2] Indirect calorimetry calculates heat that living organisms produce by measuring either their production of carbon dioxide and nitrogen waste (frequently ammonia in aquatic organisms, or urea in terrestrial ones), or from their consumption of oxygen.
Lavoisier noted in 1780 that heat production can be predicted from oxygen consumption this way, using multiple regression.
The dynamic energy budget theory explains why this procedure is correct.
A widely used modern instrument is the differential scanning calorimeter, a device which allows thermal data to be obtained on small amounts of material.
Calorimetry requires that a reference material that changes temperature have known definite thermal constitutive properties.
The classical rule, recognized by Clausius and Kelvin, is that the pressure exerted by the calorimetric material is fully and rapidly determined solely by its temperature and volume; this rule is for changes that do not involve phase change, such as melting of ice.
There are many materials that do not comply with this rule, and for them, the present formula of classical calorimetry does not provide an adequate account.
It can be said to be 'measured along an isotherm', and the pressure the material exerts is allowed to vary freely, according to its constitutive law
For a given material, it can have a positive or negative sign or exceptionally it can be zero, and this can depend on the temperature, as it does for water about 4 C.[10][11][12][13] The concept of latent heat with respect to volume was perhaps first recognized by Joseph Black in 1762.
This means that for an increase in the temperature of a body without change of its volume, heat must be supplied to it.
, gained by the body of calorimetric material, is given by where The new quantities here are related to the previous ones:[3][7][17][18] where and The latent heats
[19] It is common to refer to the ratio of specific heats as An early calorimeter was that used by Laplace and Lavoisier, as shown in the figure above.
The latent heat involved in this calorimeter was with respect to phase change, naturally occurring at constant temperature.
This kind of calorimeter worked by measurement of mass of water produced by the melting of ice, which is a phase change.
For a time-dependent process of heating of the calorimetric material, defined by a continuous joint progression
The idea that heat was a conservative quantity was invented by Lavoisier, and is called the 'caloric theory'; by the middle of the nineteenth century it was recognized as mistaken.
The terms 'rapidly' and 'very small' call for empirical physical checking of the domain of validity of the above rules.
Empirically, it is convenient to measure properties of calorimetric materials under experimentally controlled conditions.
For measurement at constant experimentally controlled volume, the isochoric coefficient of pressure rise with temperature, is defined by [30] For measurements at experimentally controlled pressure, it is assumed that the volume
The quantity that is conveniently measured at constant experimentally controlled pressure, the isobar volume expansion coefficient, is defined by [30][31][32][33][34][35][36] For measurements at experimentally controlled temperature, it is again assumed that the volume
The quantity that is conveniently measured at constant experimentally controlled temperature, the isothermal compressibility, is defined by [31][32][33][34][35][36] Assuming that the rule
through the mathematically deducible relation Thermodynamics developed gradually over the first half of the nineteenth century, building on the above theory of calorimetry which had been worked out before it, and on other discoveries.
It tells about the heat absorbed or emitted in the isothermal segment of a Carnot cycle.
A Carnot cycle is a special kind of cyclic process affecting a body composed of material suitable for use in a heat engine.
Such a material is of the kind considered in calorimetry, as noted above, that exerts a pressure that is very rapidly determined just by temperature and volume.
, is equal to zero, because the material of the working body has the special properties noted above.
No work is performed in constant-volume calorimetry, so the heat measured equals the change in internal energy of the system.
If there is a pressure difference between initial and final states, the heat measured needs adjustment to provide the enthalpy change.
