Heat measures, conversion factors

For chemical reactions and phase transformations, the energy absorbed or liberated is measured as heat. The principal unit for reporting heat is the calorie, which is defined as the energy needed to raise the temperature of 1 gram of water at l4.5° C by a single degree. The term kilocalorie refers to 1,000 calories. Another unit of energy is the joule (rhymes with school), which is equal to 0.239 calories. Conversely, a calorie is 4.184 joules. The translation of calories to joules, or kilocalories to kilojoules, is so common in chemical calculations that you should memorize the conversion factors. 

In order to assign and compare catalyst reactivity rates, measured conversions were "normalized" to 3000 GHSV by multiplying observed conversions by the factor actual GHSV/3000. The normalized conversions were used to specify rates to individual products and rates for overall CO conversion. The reaction has b n shown not to be mass or heat transfer limited (12). CO and irreversible H2 chemisorption were measured at room temperature, the former using a pulse injection system and a thermal conductivity detector, and the latter using a static system. Prior to measurements, catalysts were reduced under the same schedule as for reactor runs. 

The heat transfer was originally measured in units of calories, where one calorie was defined as the quantity of energy required to raise one gram of pure water from 14.5 to 15.5 °C at one atmosphere. This definition has been supplanted by the introduction of the joule, which represents the energy specified by the conversion factor 1 cal = 4.184 joules. One joule is also equivalent to the energy developed in a circuit by an electric current of one ampere flowing through a resistance of one ohm (driven by a potential difference of one volt) in one second. 

Data for such a direct comparison have been pubhshed elsewhere (13), along with a complete description of the tests. A summary of the results is shown in Figure 8, where the conversion efficiency for each of the square channel substrates is plotted against the conversion efficiency for each of the sinusoidal channel substrates, for the peirticular FTP test portion and for each of the three measured gases (HC, CO, and NOx). Here, only the latter part of the first test cycle (bag IB), all of the second cycle (bag 2), and only the latter part of the third test cycle (bag 3B) are included in the comparison. The slope of the best-fit fine to these data is 1.004 0.003, demonstrating that the test results are similar for the two structures, as the Heat Mass Transfer Factor predicts. 

Finally, the world literature on energy production and consumption is plagued by a proliferation of measurement units. Variously, data are presented in terms of the International System of Units (SI, e.g., metres, pascals, joules), traditional industry-based units e.g., barrels of oil, kilowatt hours of electricity, million tonnes of oil equivalent) and, especially in the USA, Imperial units e.g., miles, British thermal units of heat, quads of energy, cubic feet of natural gas, bars of pressure). For the expression of time, however, units of days and years are generally more appropriate than the SI unit (seconds) in this field. In order to assist readers in translating units into those with which they are familiar, a set of conversion factors has been included. 

Measurement of energy is the most important issue from the viewpoints of efficiency and cost. To measure the energy, heating value, of a fuel, a certain amount of consumed fuel and certain amount of converted heat data are utilized. Following are units and conversion factors that are commonly used (Tables 2.1 and 2.2). 

Mechanical equivalent of heat n. A conversion factor that transforms work or kinetic energy into heat. Probably the best known one is 788ft-lb per British thermal unit others are 2545 Btu per horsepower-hour, 4.186 X lO ergs/cal, and3413Btu/kWh. In SI there is no need for such factors because work, heat, and electrical energy are all measured in joules (IJ = ImN = IWs). 

A good laboratory balance can measure mass routinely to within 10 kg, and use of a microbalance with considerable precautions can lead to mass measurements to within 10 kg or so (Section 2.3). This is stiU three orders of magnitude larger than the mass changes equivalent to heats of reaction, so the physicists argument is valid from this point of view. (Note that the above calculation of Am exemplifies an important property of the SI, its coherence, by which we mean that if all quantities in a formula are expressed in SI units without prefixes the result of the calculation is also expressed in the appropriate SI unit, with no need for conversion factors). 

Strictly speaking, DTA measures the difference in temperature (AT) between sample and reference, but it is possible to convert AT into absorbed or evolved heat via a mathematical procedure. The conversion factor is temperature-dependent. However, a DTA which accurately measures calorimetric properties is referred to as a differential scanning calorimeter. A DSC is thus a DTA that provides calorimetric... 

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