The calorimeter

The Calorimeter.—The heat of combustion is measured by the elevation of temperature produced in a given weight of water in a calorimeter. The calorimeter used for this purpose is essentially the same as that previously described (p. 276). It consists (Fig. 95) of a large nickel-plated metal vessel, G, containing the water in which the bomb is immersed, and of a water-mantle, W. The water in the jacket can be stirred by means of the stirrers R R, and the water of the calorimeter by the stirrer r, which can be worked by hand, or driven by a motor or hot-air engine. The stroke of the stirrer should be so arranged that at its lowest point it almost touches the bottom of the calorimeter, and at its highest point rises almost to, but not above, the surface of the water. 

Substance. Approxiiiiate amount to betake. Heat of combustion per X gm. 

Making a Determination.— The substance to be burned is first compressed into a tabloid by means of a press (p. i3 )  

The calorimeter may now be got ready. The water-mantle having been filled with water (preferably some hours previously), a thermometer is hung in the air-space inside. After it has taken the temperature of the enclosure, the temperature is read. A Beckmann thermometer is now set (p. 130) so that the lower end of the scale represents a temperature of about i 5-2 degrees below that found in the enclosure of the water-mantle. 

The calorimeter vessel is then tared, a quantity of water placed in it, and the weight of the water determined on a balance accurate to about i gm. The amount of water used should be such that when the bomb is immersed, the water 

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A calorimeter is a device used to measure the work that would have to be done under adiabatic conditions to bring about a change from state 1 to state 2 for which we wish to measure AU= U -U This work is generally done by passing a known constant electric current 3 for a known time t through a known resistance R embedded in the calorimeter, and is denoted by where... 

In general it is difficult to construct a calorimeter that is truly adiabatic so there will be unavoidable heat leaks q. It is also possible that non-deliberate work is done on the calorimeter such as that resulting from a change in volume against a non-zero external pressure / Pk i dk>, often called /iFwork. Additional work w ... 

The p V work temi is not nomially measured. It can be eliminated by suspending the calorimeter in an evacuated space (p = 0) or by holding the volume of tire calorimeter constant (dF= 0) to give... 

All calorimeters consist of the calorimeter proper and its surround. This surround, which may be a jacket or a batii, is used to control tlie 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 usually contains a stirred 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 non-isothemial calorimeters (calorimeters where the temperature either increases or decreases as the reaction proceeds), if the jacket is kept at a constant temperature there will be some heat leak to the jacket when the temperature of the calorimeter changes. 

Hence, it is necessary to correct the temperature change observed to the value it would have been if there was no leak. This is achieved by measuring the temperature of the calorimeter for a time period both before and after the process and applying Newton s law of cooling. This correction can be reduced by using the teclmique of adiabatic calorimetry, where the temperature of the jacket is kept at the same temperature as the calorimeter as a temperature change occurs. This teclmique requires more elaborate temperature control and it is prunarily used in accurate heat capacity measurements at low temperatures. 

With most non-isothemial calorimeters, it is necessary to relate the temperature rise to the quantity of energy released in the process by determining the calorimeter constant, which is the amount of energy required to increase the temperature of the calorimeter by one degree. This value can be detemiined by electrical calibration using a resistance heater or by measurements on well-defined reference materials [1], For example, in bomb calorimetry, the calorimeter constant is often detemiined from the temperature rise that occurs when a known mass of a highly pure standard sample of, for example, benzoic acid is burnt in oxygen. 

The energy released when the process under study takes place makes the calorimeter temperature T(c) change. In an adiabatically jacketed calorimeter, T(s) is also changed so that the difference between T(c) and T(s) remains minimal during the course of the experiment that is, in the best case, no energy exchange occurs between the calorimeter (unit) and the jacket. The themial conductivity of the space between the calorimeter and jacket must be as small as possible, which can be achieved by evacuation or by the addition of a gas of low themial conductivity, such as argon. 

This type of calorimeter is nomrally enclosed in a themiostatted-jacket having a constant temperature T(s). and the calorimeter (vessel) temperature T(c) changes tln-ough the energy released as the process under study proceeds. The themial conductivity of the intemiediate space must be as small as possible. Most combustion calorimeters fall into this group. 

A liquid serves as the calorimetric medium in which the reaction vessel is placed and facilitates the transfer of energy from the reaction. The liquid is part of the calorimeter (vessel) proper. The vessel may be isolated from the jacket (isoperibole or adiabatic), or may be in good themial contact (lieat-flow type) depending upon the principle of operation used in the calorimeter design. 

The selection of the operating principle and the design of the calorimeter depends upon the nature of the process to be studied and on the experimental procedures required. Flowever, the type of calorimeter necessary to study a particular process is not unique and can depend upon subjective factors such as teclmical restrictions, resources, traditions of the laboratory and the inclinations of the researcher. 

Batch calorimeters are instmments where there is no flow of matter in or out of the calorimeter during the time the energy change is being measured. Batch calorimeters differ in the way the reactants are mixed and in the method used to detennine the enthalpy change. Enthalpy changes can be measured by the various methods... 

Calorific Value. To determine calorific value, a sample is placed in a bomb, pressurized with oxygen, and ignited. The temperature rise in the water bath of the calorimeter surrounding the bomb is used to determine the calorific value (D2015, D3286, or D1989) (18). 

Heat-flux data obtained from calorimeters present in the fire-affected area revealed maximum heat fluxes of 160-300 kW/m. Figure 5.1 shows the calorimeter positions, the final contours of the flash fire, and heat-flux data from calorimeters positioned near or in the flames. No data are available on flame propagation during the vapor-bum tests. 

Calit. a ceramic for insulators. T. N. Calorienwert, m. caloric value, calorimetrieren, v.t. measure with the calorimeter. 

To measure the heat flow in a reaction, a device known as a calorimeter is used. The apparatus contains water and/or other materials of known heat capacity. The walls of the calorimeter are insulated so that there is no exchange of heat with the surrounding air. It follows that the only heat flow is between the reaction system and the calorimeter. The heat flow for the reaction system is equal in magnitude but opposite in sign to that of the calorimeter ... 

Notice that if the reaction is exothermic ( reaction < 0), calorimeter must be positive that is, heat flows from the reaction mixture into the calorimeter. Conversely, if the reaction is endothermic, the calorimeter gives up heat to the reaction mixture. 

The equation just written is basic to calorimetric measurements. It allows you to calculate the amount of heat absorbed or evolved in a reaction if you know the heat capacity, Ccd, and the temperature change, At, of the calorimeter. 

A calorimeter contains 50.0 g of water at 25.00°C. When 1.00 g of calcium chloride is added to the calorimeter, the temperature rises to 28.51°C. Assume that all the heat given off by the reaction is transferred to the water. 

Bomb calorimeter. The heat flow, q, for the reaction is calculated from the temperature change multiplied by the heat capacity of the calorimeter, which is determined in a preliminary experiment... 

All of the heat given off by the reaction is absorbed by the calorimeter, which consists of the metal bomb and the water that surrounds it. In other words,... 

The heat absorbed by the calorimeter is equal to the product of its heat capacity, Ccai, and the temperature change, At. Hence... 

Knowing the heat capacity of the calorimeter, the heat flow for any reaction taking place within the calorimeter can be calculated (Example 8.3). 

A simple experiment with a coffee-cup calorimeter shows that when one gram of NH4N03 dissolves, fraction = 351 J. The calorimeter is open to the atmosphere, the pressure is constant, and... 

In earlier times, ethyl ether was commonly used as an anesthetic. It is, however, highly flammable. When five milliliters of ethyl ether, C HuQC/), (d - 0.714 g/mL) is burned in a bomb calorimeter, die temperature rises from 23-5°C to 39.7°Q The calorimeter heat capacity is 10.34 k)/°C. 

WEB When one mole of caffeine (CgHlaN402) is burned in air, 4.96 X 103 kj ofheat is evolved. Five grams of caffeine is burned in a bomb calorimeter. The temperature is observed to increase by 11.37°Q What is die heat capacity of the calorimeter in J/°C ... 

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