Calorie, defined standard

The calorie defined by the U.S. National Bureau of Standards as 4.18400 J and used in thermochemical data tables. 

Some Sl-derived units with special names are included in Table 2.2. The standard atmosphere may be used temporarily with SI units it is dehned to be equal to 1.01325 X 10 Pa. The thermochemical calorie is no longer recommended as a unit of energy, but it is defined in terms of an SI unit, joules, symbol J, as 4.184 J [4]. The unit of volume, liter, symbol L, is now defined as Idm. ... 

Useful quantitation of heat q as a quantity of energy can be traced to the studies of Joseph Black around 1803. Black recognized that different substances vary in their capacity to absorb heat, and he undertook systematic measurements of the heat capacity C (the ratio of heat absorbed to temperature increase) for many substances. He recognized that a fixed quantity of any pure substance (e.g., 1 g of water) has a unique value of C, which can be chosen as a calorimetric standard for defining quantity of heat in a convenient way. In this manner, he introduced the calorie as a unit of heat ... 

The standard or 15" calorie is nnw defined as the rate of absorption of heal per gram per degree at 15°C and in practice is essentially the same as the average caloric over the 1° interval front 14.5 lo I5.5°C. 

The data are referred to a standard temperature of 25° C. The energy unit used is the calorie which is defined as equal to 4.1840 absolute joules. 

These results are strictly true only if the ampere, volt and ohm are in absolute units there is a slight difference if international units are employed, the absolute volt-coulomb or joule being different from the international value. The United States Bureau of Standards has recommended that the unit of heat, the calorie, should be defined as the equivalent of 4.1833 international joules, and hence... 

While thermal energy units were defined for many years as indicated above, it is now customary to define them arbitrarily in terms of mechanical units, with no reference to the heating of water. At the present time, there are two defined gram-calories in wide use, the United States National Bureau of Standards thermochemical... 

For the values of the principal physical constants see R. T. Birge, Phys. Soc. Rep. Prog. Phys., 8, 90 (1941). Note also that in conformity with the recommendation of the International Union of Physics (S. G. 48-6, 1948) the calorie is defined as 4 1840 abs. joules. The above table has been revised in accordance with the latest recommendations of the Bureau of Standards, Tables of Selected Values of Chemical Thermodynamic Properties (Washington, 1947). 

In order to convert the results obtained by the electrical heating method into calories, it is necessary to know the relationship between joules and calories. That there is such an exact connection is really an aspect of the first law of thermodynamics ( 6a), which will be tacitly assumed for the present. Because of a slight uncertainty, of about two parts in 10,000, concerning the relationship between the standard (K5 ) calorie, as defined in 3d, and the int. joule ( 3b), a defined calorie,... 

The U.S. National Institute of Standards and Technology defines the calorie as joules. 

The calorie is so defined that the specific heat of water is approximately 1 cal/g-°C. The standard acceleration of free fall in cgs units is... 

Heat is quantitatively defined by measurement with a calorimeter, that is, by the temperature increment produced in its transfer to a standard body in the course of a change of state of the system. The unit of heat is the calorie the mean gram-calorie is one one-hundredth of the heat absorbed by one gram of liquid water under a pressure of one atmosphere on raising its temperature from 0 to 100°C. The 15° gram-calorie is defined as the heat required to raise the temperature of one gram of water from 14.5 to 15.5°C. The ratio of the latter to the former is 1.00024. 

Investigators must take care to read the foreword of the particular table they use so that they know which standard state has been employed, because most thermodynamic properties are calculated with respect to convenient scales. For example, the standard enthalpy of formation of a compound, AHf, is almost always quoted for a temperature of 298.15 K, and the enthalpy of formation of an element in its standard state must by definition be zero. It is therefore practically useful to look at a table and find, for an element, where a zero entry occurs. For example, the following values might appear C(graphite), A/ff = 0.000 kcalthmol" C(diamond), AHf = 0.4532 kcalthmol". It is clear that C(graphite) is the standard state adopted for carbon in the table under consideration. Entropy, on the other hand, is usually defined by taking as zero the entropy, at T = 0, of the crystalline form in which all the molecules are orientated regularly. Because many of the extant tables have used thermochemical calories, care will also have to be taken in the future to see that values taken from different tables are corrected to the same units. 

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