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Calorimetry ­ Accessscience from Mcgraw­Hill Education 3/20/2017 Calorimetry ­ AccessScience from McGraw­Hill Education (http://www.accessscience.com/) Calorimetry Article by: Marsh, Kenneth M. Thermodynamic Research Center, Texas A & M University, College Station, Texas. Last updated: 2014 DOI: https://doi.org/10.1036/1097­8542.104500 (https://doi.org/10.1036/1097­8542.104500) Content Hide Types of calorimeters Calorimeter components Thermometry Calorimetric measurements Heat­capacity calorimeter Enthalpy­of­vaporization calorimeter Links to Primary Literature Additional Readings The measurement of the quantity of heat energy involved in processes such as chemical reactions, changes of state, and mixing of substances, or in the determination of heat capacities of substances. The unit of energy in the International System of Units is the joule, symbolized J. Another unit still being used, but is strongly discouraged, is the calorie, symbolized cal and defined as 4.184 J. Most calorimetric measurements are made at constant pressure, and the measured change is called the enthalpy change. See also: Enthalpy (/content/enthalpy/234900); Heat capacity (/content/heat­capacity/310600) Types of calorimeters A calorimeter is an apparatus for measuring the quantity of heat energy released or absorbed during a process. Since there are many processes that can be studied over a wide range of temperature and pressure, a large variety of calorimeters have been developed. Nonisothermal calorimeters These instruments measure the temperature change that occurs during the process. An aneroid (containing no liquid) nonisothermal (temperature varying during the experiment) calorimeter is normally constructed of a material having a high thermal conductivity, such as copper, so that there is rapid temperature equilibration. It is isolated from its surroundings by a high vacuum to reduce heat leaks. This type of calorimeter can be used for determining the heat capacity of materials at low temperatures. Aneroid nonisothermal calorimeters have also been developed for measuring the energy of combustion for small samples of rare materials. Another type of nonisothermal calorimeter uses a large quantity of liquid as a heat sink, so that even if there is a large amount of energy released in the process the temperature rise is not excessive. The liquid is kept at a uniform temperature by stirring, and the change in temperature can be measured accurately. In general, the size of the liquid heat sink is designed so that the temperature change is of the order of 5 K (9°F). For measurements near room temperature, water is commonly used as the working fluid; measurements at higher temperatures use less volatile organic liquids. http://www.accessscience.com/content/calorimetry/104500 1/5 3/20/2017 Calorimetry ­ AccessScience from McGraw­Hill Education With most nonisothermal calorimeters, it is necessary to relate the temperature rise to the quantity of energy released in the process. This is done by determining the calorimeter constant, which is the amount of energy required to increase the temperature of the calorimeter itself by 1 K. This value can be determined by measurement on a well­defined test system or by electrical calibration. In bomb calorimetry, for example, the calorimeter constant is usually determined from the temperature rise that occurs when a known mass of a very pure standard sample of benzoic acid is burned. Electrical calibration, however, is complex and often does not mimic the way that energy is released during the reaction. Isothermal calorimeters These instruments make measurements at constant temperature. The simplest example is a calorimeter containing an outer annular space filled with a liquid in equilibrium with a crystalline solid at its melting point, arranged so that any volume change will displace mercury along a capillary tube. The Bunsen ice calorimeter operates at 0 °C (32 °F) with a mixture of ice and water. Changes resulting from the process being studied cause the ice to melt or the water to freeze, and the consequent volume change is determined by measurement of the movement of the mercury meniscus in the capillary tube. While these calorimeters can yield accurate results, their operation is limited to the equilibrium temperature of the two­ phase system. Other types of isothermal calorimeters use electrical energy to achieve exact balance of the heat absorption that occurs during an endothermic process. Generally, calorimeters based on this principle are more accurate than similar nonisothermal calorimeters. Many mixing and flow calorimeters operate in this mode. Calorimeter components All calorimeters consist of the calorimeter proper and a jacket or a bath, which is used to control the temperature of the calorimeter and the rate of heat leak to the environment. For temperatures not too far removed from room temperature, the jacket or bath contains liquid at a controlled temperature. For measurements at extreme temperatures, the jacket usually consists of a metal block containing a heater to control the temperature. With nonisothermal calorimeters, where the jacket is kept at a constant temperature, there will be some heat leak to the jacket when the temperature of the calorimeter changes. It is necessary to correct the temperature change observed to the value it would have been if there were no leak. This is achieved by measuring the temperature of the calorimeter before and after the process and applying Newton's law of cooling. This correction can be avoided by using the technique of adiabatic (no energy loss or gain with the surroundings) calorimetry, where the temperature of the jacket is kept equal to the temperature of the calorimeter as a change occurs. This technique requires more elaborate temperature control, and its primary use is for accurate heat capacity measurements at low temperatures. Thermometry In calorimetric experiments, it is necessary to measure temperature differences accurately; in some cases, the temperature itself must be accurately known. Modern calorimeters use resistance thermometers to measure both temperatures and temperature differences, while thermocouples or thermistors are used to measure smaller temperature differences. A calibrated standard platinum resistance thermometer can measure temperature differences to 0.00001°C and temperatures ranging from −260°C to 630°C (−436°F to 1166°F) with an uncertainty of ±0.001°C. Multiple­junction thermocouples and thermistors can measure smaller temperature differences, allowing precise measurements of very small heat effects. See also: Temperature measurement (/content/temperature­measurement/683700); Thermistor (/content/thermistor/690100); Thermocouple (/content/thermocouple/690500); Thermometer (/content/thermometer/691400) Calorimetric measurements http://www.accessscience.com/content/calorimetry/104500 2/5 3/20/2017 Calorimetry ­ AccessScience from McGraw­Hill Education Heat capacities of materials and energies of combustion are processes that are routinely measured with calorimeters. Calorimeters are also used to measure the heat involved in phase changes, for example, the change from a liquid to a solid (fusion) or from a liquid to a gas (vaporization). Calorimetry has also been applied to the measurement of the enthalpy of hydrogenation of unsaturated organic compounds, the enthalpy of dissolution of a solid in a liquid, or the enthalpy change on mixing two liquids. Heat­capacity calorimeter Heat capacities of gases or liquid can be determined in a flow calorimeter. Some essential features are shown in Fig. 1. A known mass of fluid flowing at a constant rate is pumped through the calorimeter. The inlet temperature is measured with thermometer 1. The fluid is heated with a known amount of electrical energy, and the change in temperature is observed at thermometer 2. The radiation shields reduce heat losses to the surroundings. Fig. 1 High­pressure flow calorimeter for measurement of heat capacity. (After G. Ernst, G. Maurer, and E. Wiederoh, Flow calorimeter for the accurate determination of the isobaric heat capacity at high pressures: Results for carbon dioxide, J. Chem. Thermodyn., 21:53–65, 1989) Enthalpy­of­vaporization calorimeter A modern enthalpy­of­vaporization calorimeter is shown in Fig. 2. The material in the vaporization vessel is heated with a known amount of electrical energy, and the mass of material vaporized is determined from the change in mass of the glass collection vessel. The thermostated shields, which are maintained at the same temperature as the vaporization vessel, ensure that all the electrical energy goes to vaporize the sample and none is lost to the surroundings. See also: Chemical thermodynamics (/content/chemical­thermodynamics/128200); Heat transfer (/content/heat­transfer/311100); Thermochemistry (/content/thermochemistry/690300); Thermodynamic principles (/content/thermodynamic­ principles/690700) http://www.accessscience.com/content/calorimetry/104500 3/5 3/20/2017 Calorimetry ­ AccessScience from McGraw­Hill Education Fig. 2 Heat­of­vaporization calorimeter. (After L. Sváb et al., A calorimeter for the determination of enthalpies of vaporization at high temperatures and pressures, J. Chem. Thermodyn., 20:545–550, 1988) Kenneth N. Marsh Links to Primary Literature S. Duane et al., An absorbed dose calorimeter for IMRT dosimetry, Metrologia, 49(5):S168, 2012 DOI: https://doi.org/10.1088/0026­1394/49/5/S168 (https://doi.org/10.1088/0026­1394/49/5/S168) C. Pascot et al., Calorimetric measurement of the heat generated by a Double­Layer Capacitor cell
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