Calorimeter surroundings

Measuring the gross heating value (mass) is done in the laboratory using the ASTM D 240 procedure by combustion of the fuel sample under an oxygen atmosphere, in a bomb calorimeter surrounded by water. The thermal effects are calculated from the rise in temperature of the surrounding medium and the thermal characteristics of the apparatus. 

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). 

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. 

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 ... 

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,... 

In the combustion reaction as carried out in the calorimeter of Figure 7-2, the volume of the system is kept constant and pressure may change because the reaction chamber is sealed. In the laboratory experiments you have conducted, you kept the pressure constant by leaving the system open to the surroundings. In such an experiment, the volume may change. There is a small difference between these two types of measurements. The difference arises from the energy used when a system expands against the pressure of the atmosphere. In a constant volume calorimeter, there is no such expansion hence, this contribution to the reaction heat is not present. Experiments show that this difference is usually small. However, the symbol AH represents the heat effect that accompanies a chemical reaction carried out at constant pressure—the condition we usually have when the reaction occurs in an open beaker. 

The pattern we have identified is one version of the second law of thermodynamics. The natural progression of a system and its surroundings (which together make up the universe") is from order to disorder, from lower to higher entropy. For practical measurements, a small isolated region, such as a thermally insulated, sealed flask or a calorimeter, is considered to represent the universe. 

Calorimeters are insulated to ensure that negligible heat is transferred between the calorimeter and its surroundings during the experiment. As a result, the only heat exchange is between the chemicals and the calorimeter ... 

Figure 6-17 illustrates a constant-volume calorimeter of a type that is often used to measure q for combustion reactions. A sample of the substance to be burned is placed inside the sealed calorimeter in the presence of excess oxygen gas. When the sample bums, energy flows from the chemicals to the calorimeter. As in a constant-pressure calorimeter, the calorimeter is well insulated from its surroundings, so all the heat released by the chemicals is absorbed by the calorimeter. The temperature change of the calorimeter, with the calorimeter s heat capacity, gives the amount of heat released in the reaction. 

All calorimeters are composed of an inner vessel (the calorimeter vessel, A in Fig. 1), in which the thermal phenomenon under study is produced, and of a surrounding medium (shields, thermostat, etc., B in Fig. 1). Depending upon the intensity of the heat exchange between the inner vessel and its surroundings, three main types of calorimeters may be distinguished theoretically as indicated in Fig. 1. 

When there is no heat exchange between the inner vessel and its surroundings (adiabatic calorimeter, 1 in Fig. 1), the temperature of the calorimeter vessel varies when heat is liberated or absorbed. The quantity of heat produced or absorbed may be calculated from this temperature change, if the heat capacity of the inner vessel and of its contents is known. 

When heat is liberated or absorbed in the calorimeter vessel, a thermal flux is established in the heat conductor and heat flows until the thermal equilibrium of the calorimetric system is restored. The heat capacity of the surrounding medium (heat sink) is supposed to be infinitely large and its temperature is not modified by the amount of heat flowing in or out. The quantity of heat flowing along the heat conductor is evaluated, as a function of time, from the intensity of a physical modification produced in the conductor by the heat flux. Usually, the temperature difference 0 between the ends of the conductor is measured. Since heat is transferred by conduction along the heat conductor, calorimeters of this type are often also named conduction calorimeters (20a). 

It appears therefore that during the operation of all usual calorimeters, temperature gradients are developed between the inner vessel and its surroundings. The resulting thermal head must be associated, in all cases, to heat flows. In isoperibol calorimeters, heat flows (called thermal leaks in this case) are minimized. Conversely, they must be facilitated in isothermal calorimeters. All heat-measuring devices could therefore be named heat-flow calorimeters. However, it must be noted that in isoperibol or isothermal calorimeters, the consequences of the heat flow are more easily determined than the heat flow itself. The temperature decrease... 

In this microcalorimeter, the heat sink is not a massive metal block but is divided into several parts which are mobile with respect to each other. Each thermoelectric element (E) and a cell guide (D) are affixed to a fluxmeter holder (C). The holder (C) is mobile with respect to a massive arm (B) which, in turn, rotates around a vertical axle (A). All parts of the heat sink are made of brass. Surfaces in contact are lubricated by silicone grease. Four thermoelectric elements (E) are mounted in this fashion. They enclose two parallelepipedic calorimetric cells, which can be made of glass (cells for the spectrography of liquids are particularly convenient) or of metal (in this case, the electrical insulation is provided by a very thin sheet of mica). The thermoelectric elements surrounding both cells are connected differentially, the Petit microcalorimeter being thus a twin differential calorimeter. 

The relation between the emf of the thermoelectric pile and the heat flux from the calorimeter cell will be first established. Let us suppose (Fig. 8) that the process under investigation takes place in a calorimeter vessel (A), which is completely surrounded by n identical thermoelectric junctions, each separated from one another by equal intervals. The thermocouples are attached to the external surface of the calorimeter cell (A), which constitutes the internal boundary (Eint) of the pile and to the inside wall of the heat sink (B), constituting the external boundary (Eext) of the thermoelectric pile. The heat sink (B) is maintained at a constant temperature (6e). 

B The assumptions include no heat loss to the surroundings or to the calorimeter, a solution density of 1.00 g/mL, a specific heat of 4.18 J g 1 °C 1, and that the initial and final solution volumes are the same. The equation for the reaction that occurs is NaOH(aq) + HCl(aq) - NaCl(aq) + H20(l). Since the two reactants combine in a one to one mole ratio, the limiting reactant is the one present in smaller amount. 

The heat of combustion of solids or liquids is usually measured in a device known as an oxygen bomb calorimeter. Such a device operates at a constant volume between states 1 and 2, and its heat loss is measured by means of the temperature rise to a surrounding water-bath. This is schematically shown in Figure 2.2. The combustion volume is charged with oxygen and a special fuel is added to ensure complete combustion of the fuel to be measured. Since the process is at constant volume (V), we have... 

The measurement of an enthalpy change is based either on the law of conservation of energy or on the Newton and Stefan-Boltzmann laws for the rate of heat transfer. In the latter case, the heat flow between a sample and a heat sink maintained at isothermal conditions is measured. Most of these isoperibol heat flux calorimeters are of the twin type with two sample chambers, each surrounded by a thermopile linking it to a constant temperature metal block or another type of heat reservoir. A reaction is initiated in one sample chamber after obtaining a stable stationary state defining the baseline from the thermopiles. The other sample chamber acts as a reference. As the reaction proceeds, the thermopile measures the temperature difference between the sample chamber and the reference cell. The rate of heat flow between the calorimeter and its surroundings is proportional to the temperature difference between the sample and the heat sink and the total heat effect is proportional to the integrated area under the calorimetric peak. A calibration is thus... 

Measurements based on the law of conservation of energy are of two main types. In phase change calorimetry the enthalpy of the reaction is exactly balanced by the enthalpy of a phase change of a contained compound surrounded by a larger reservoir of the same compound used to maintain isothermal conditions in the calorimeter. The latter enthalpy, the measurand, is often displayed indirectly through the change in the volumetric properties of the heat reservoir compound, e.g. ice/water. 

Adiabatic calorimetry uses the temperature change as the measurand at nearly adiabatic conditions. When a reaction occurs in the sample chamber, or energy is supplied electrically to the sample (i.e. in heat capacity calorimetry), the temperature rise of the sample chamber is balanced by an identical temperature rise of the adiabatic shield. The heat capacity or enthalpy of a reaction can be determined directly without calibration, but corrections for heat exchange between the calorimeter and the surroundings must be applied. For a large number of isoperibol... 

The overall heat capacity of the calorimeter is a simple function of the amount of steel the bomb comprises and the amount of water surrounding it. If the mass is m and the heat capacity is C, then the overall heat capacity is expressed by... 

It is quite difficult to measure an accurate enthalpy of solution A//( olutioni with a calorimeter, but we can measure it indirectly. Consider the example of sodium chloride, NaCl. The ions in solid NaCl are held together in a tight array by strong ionic bonds. While dissolving in water, the ionic bonds holding the constituent ions of Na+ and Cl- in place break, and new bonds form between the ions and molecules of water to yield hydrated species. Most simple ions are surrounded with six water molecules, like the [Na(H20)6]+ ion (VI). Exceptions include the proton with four water molecules (see p. 235) and lanthanide ions with eight. 

So that there is no gain nor loss of heat, thermochemical reactions that are called adiabatic are carried out in well-insulated containers called calorimeters. The reactants and products in the calorimeter are called the chemical system. Everything else is called the surroundings, and includes the air above the calorimeter, the water in the calorimeter, the thermometer and stirrer, etc. 

Some of the heat transferred to the surroundings during an exothermic reaction are absorbed by the calorimeter and its parts. In order to account for this heat, a calorimeter constant or heat capacity of the calorimeter is required and usually expressed in J °C 1. 

In this context, the term adiabatic refers to calorimetry conducted under conditions that minimize heat losses to the surrounding environment to better simulate conditions in the plant, where bulk quantities of stored or processed material tend to minimize cooling effects. This class of calorimetry includes the accelerating rate calorimeter (ARC), from Arthur D. Little, Inc., and PHI-TEC from Hazard Evaluation Laboratory Ltd. 

The coffee-cup calorimeter can be used to measure the heat changes in reactions that are open to the atmosphere, qp, constant pressure reactions. We use this type of calorimeter to measure the specific heats of solids. We heat a known mass of a substance to a certain temperature and then add it to the calorimeter containing a known mass of water at a known temperature. The final temperature is then measured. We know that the heat lost by the added substance (the system) is equal to the heat gained by the surroundings (the water and calorimeter, although for simple coffee-cup calorimetry the heat gained by the calorimeter is small and often ignored) ... 

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