Temperature-time curve combustion calorimeter

The basic output from a combustion experiment made with an isoperibol calorimeter is a temperature-time curve, such as the one represented in figure 7.2. In the initial or fore period (between ta and tf and in the final or after period (between tf and tf), the observed temperature change is governed by the heat of stirring, the heat dissipated by the temperature sensor, and the heat transfer between the calorimeter proper and the jacket. The reaction or main period begins at tu when, on ignition, a rapidtemperature rise results from the exothermic... 

Figure 7.2 Atypical temperature-time curve from a combustion reaction studied with an isoperibol combustion calorimeter.

The experiments are usually carried out at atmospheric pressure and the initial goal is the determination of the enthalpy change associated with the calorimetric process under isothermal conditions, AT/icp, usually at the reference temperature of 298.15 K. This involves (1) the determination of the corresponding adiabatic temperature change, ATad, from the temperature-time curve just mentioned, by using one of the methods discussed in section 7.1 (2) the determination of the energy equivalent of the calorimeter in a separate experiment. The obtained AT/icp value in conjunction with tabulated data or auxiliary calorimetric results is then used to calculate the enthalpy of an hypothetical reaction with all reactants and products in their standard states, Ar77°, at the chosen reference temperature. This is the equivalent of the Washburn corrections in combustion calorimetry... 

In an isoperibol calorimeter the heat, q, released in the combustion reaction produces an increase of the temperature of the water inside it, which is measured by means of a suitable thermometer (usually a platinum resistance thermometer, a quartz thermometer or a thermistor). The stirrer ensures that the energy released in the combustion reaction is transferred to the calorimeter so that the combustion bomb, the water and the can are quickly brought to the same temperature. The heater is used to take the temperature of the calorimeter to the starting point of the experiment. It is desirable to have an experimental temperature increment of at least 1 K. Figure 2 shows a typical temperature-time curve obtained in an isoperibol combustion calorimeter. 

Figure 2. Temperature-time curve obtained in an isoperibol combustion calorimeter.

Enthalpies of reaction in solution are generally measured in an isothermal jacketed calorimeter. This consists of a calorimetric vessel that contains a certmn amount of one of the reactants that is either a liquid or, if a solid is involved, it has been dissolved in a suitable solvent. The other reactant is sealed in a glass ampoule that is placed in a holder. The vessel is enclosed in a container, which is placed in a thermostatted bath with the temperature controlled to 0.001 °C. When the system has reached thermal equilibrium, the ampoule is broken and the reaction is initiated. Throughout the experiments the temperature is measured as a function of the time and a temperature-time curve with approximately the same shape as the ones obtmned in combustion calorimetry, vdth fore-period, reaction-period and after-period is obtained. The observed temperature rise is due to several sources die heat transferred from the thermostatted bath, the energy of the reaction and the stirring energy. To correct... 

Adiabatic and Isoperibol Calorimeters.—Most calorimeters used in combustion and reaction calorimetry undergo a change of temperature when reaction takes place. If the calorimeter is surrounded by a jacket, the temperature of which is controlled to be the same as that of the calorimeter, no heat-exchange occurs between the siuroundings and the calorimeter, which is then described as adiabatic. However, if the temperature of the environment is maintained constant (in a type of calorimeter conveniently described as isoperibol and sometimes, incorrectly, as isothermal) some heat-exchange occurs between the calorimeter and its surroundings, but may be accurately determined by analysis of the temperature-time curves before and after reaction takes place, provided the reaction is of short duration (say not exceeding 15 min). With slower processes, isoperibol calorimeters are less useful, and the adiabatic principle is easier to effect and yields more accurate results. 

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