Heat flow measurements

Boiling point elevation (ATb) Increase in the boiling point caused by addition of a nonvolatile solute, 269-271 Bomb calorimeter Device used to measure heat flow, in which a reaction is carried out within a sealed metal container, 202-203... 

A calorimeter Is a device used to measure heat flows that accompany chemical processes. The basic features of a calorimeter include an Insulated container and a thermometer that monitors the temperature of the calorimeter. A block diagram of a calorimeter appears in Figure 6-15. In a calorimetry experiment, a chemical reaction takes place within the calorimeter, resulting in a heat flow between the chemicals and the calorimeter. The temperature of the calorimeter rises or falls in response to this heat flow. 

Usually, isothermal calorimeters are used to measure heat flow in batch and semi-batch reactions. They can also measure the total heat generated by the reaction. With careful design, the calorimeter can simulate process variables such as addition rate, agitation, distillation and reflux. They are particularly useful for measuring the accumulation of unreacted materials in semi-batch reactions. Reaction conditions can be selected to minimize such accumulations. 

Getting a brief overview of thermodynamics Using heat capacity and calorimetry to measure heat flow Keeping track of the heats involved in chemical and physical changes Adding heats together with Hess s law... 

As pointed out in Section 8.2, most physical and chemical processes, not just the chemical transformation of reactants into products, are accompanied by heat effects. Thus, if calorimetry is used as an analytical tool and such additional processes take place before, during, or after a chemical reaction, it is necessary to separate their effects from that of the chemical reaction in the measured heat-flow signals. In the following, we illustrate the basic principles involved in applying calorimetry combined with IR-ATR spectroscopy to the determination of kinetic and thermodynamic parameters of chemical reactions. We shall show how the combination of the two techniques provides extra information that helps in identifying processes additional to the chemical reaction which is the primary focus of the investigation. The hydrolysis of acetic anhydride is shown in Scheme 8.1, and the postulated pseudo-first-order kinetic model for the reaction carried out in 0.1 M aqueous hydrochloric acid is shown in Equation 8.22 ... 

When the measured heat-flow rate (qtot) curve at 25°C is compared with the results obtained from the IR signal (see Fig. 8.5), it again becomes clear that the initial peak of the qtot curve, which is not visible in the IR signal, is not related to the chemical reaction but to the mixing. In this simple case, we have an excellent example of how the simultaneous... 

Of course, there are some reactions that occur at room temperature for which microcalorimeters are not able to detect any measurable heat flow. This may be because the reaction proceeds at too slow a rate, because the reaction enthalpy is very small or simply because there may not be a sufficient number of moles of material reacting. In some of these cases, it may be possible to accelerate any reactions occurring by increasing the temperature of the sample such that a measurable signal is obtained. The data are... 

Most calorimeters described above rely on a measurement of temperature (heat-Flow Calorimeters). The Tian182-Calvet183 calorimeters (some with a twin calorimeter design) use a thermopile (instead of a thermocouple) to measure heat flow directly (Fig. 11.78). 

Differential scanning calorimetry measures heat flow as a function of temperature. When starch is heated in the presence of excess water, a sharp peak (an endotherm) is obtained, which is caused by the disordering of... 

Calorimetry the science of measuring heat flow. (9.4) Capillary action the spontaneous rising of a liquid in a narrow tube. (16.2)... 

We measure heat flowing from a hot body to a cold one when they are brought into thermal contact, but we never detect heat flowing spontaneously in the opposite direction. 

From these equations it can be seen that key quantities that can affect the measured heat flow signal in a DSC are ... 

Having mentioned these broad limitations, it is also important to emphasize the remarkable versatility of the technique, again as a result of the fact that the method is essentially measuring heat flow processes that are applicable to virtually all systems. Indeed, it could well be argued that the method is underexploited within the pharmaceutical field, and it is hoped that this and the other texts mentioned... 

DSC instruments are sensitive pieces of modem equipment, having the capability to measure heat flows of the order of microwatts. This feature makes the applicability of the technique almost unlimited every physical change or chemical reaction takes place with a change of enthalpy and consequently absorption or release of heat. 

The value of AH can be determined experimentally by measuring the heat flow accompanying a reaction at constant pressure. Typically, we can determine the magnitude of the heat flow by measuring the magnitude of the temperature change the heat flow produces. The measurement of heat flow is calorimetry a device used to measure heat flow is a calorimeter. 

The isopeiibolic calorimeter has certain disadvantages-. The measured heat flow is non-linear at high power outputs because the temperature is allowed to rise, thus both reaction kinetics and heat capacity are distorted. It is difficult or impossible to ramp the reaction temperature and measure heat flow. It cannot be used easily to measure heat capacity. 

The three types of isothermal heat flow calorimeters described above can be used to measure heat flow in semi-batch reactions, where one or more reactants are charged to the reactor and the other reactants are added at controlled rates throughout the reaction. With careful design the heat flow calorimeters can simulate process variables such as feed rate, stirring, distillation and reflux . 

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