Basic concepts of calorimetry

Several subjects that might be considered under the title of thermochemistry are discussed in previous chapters. Such subjects are the heat capacities of a single-phase system, the dependence of the enthalpy of a single-phase system on temperature and pressure, and the dependence of the enthalpy of a one-component, multiphase system on the temperature, volume, and mole numbers. Here we are concerned with heat capacities of multiphase systems, with changes of enthalpy for the formation of a solution and for a change of concentration of the solution, and with changes of enthalpy of systems in which chemical reactions occur. First the basic concepts of calorimetry are reviewed. 

We may classify all calorimeters into two groups when we limit the processes to those that involve only the work of expansion or compression those that operate at constant volume and those that operate at constant pressure. The application of Equation (9.1) to constant-volume calorimeters shows that the heat absorbed by the system equals the change of energy of the system for the change of state that takes place in the system. Similarly, the heat absorbed by the system in constant-pressure calorimeters is equal to the change of enthalpy for the change of state that takes place in the system according to Equation (9.2). 

For the experimental determination, we use a bomb calorimeter and assume that the process takes place at constant volume, so that the volume of the chemical system remains constant. An excess of oxygen must be used to ensure complete combustion. The initial state of the chemical system is then a mixture of CH4 and 02 with known mole numbers of CH4 and 02 at a temperature equal or close to 298 K and at a measured total pressure. The individual gases as well as the gas mixture are not ideal gases. The final state is a two-phase system at a known temperature and measured (or calculated) 

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