The constant-pressure reaction calorimeter

The contents of a constant-pressure calorimeter are usually open to the atmosphere, so this type of calorimeter is unsuitable for processes involving gases. It is, however, a convenient apparatus in which to study a liquid-phase chemical reaction, the dissolution of a solid or liquid solute in a liquid solvent, or the dilution of a solution with solvent. 

The process is initiated in the calorimeter by allowing the reactants to come into contact. The temperature in the reaction vessel is measured over a period of time starting before the process initiation and ending after the advancement has reached a final value with no further change. 

The value of A//(expt) at constant pressure would be zero if the process were perfectly adiabatic and the only work were expansion work, but this is rarely the case. There may be unavoidable work from stirring and from electrical temperature measurement. We can evaluate A//(expt) by one of the methods described in Sec. 7.3.2. For an adiabatic calorimeter, the appropriate expression is A//(expt) = er(t2 — h) (Eq. 7.3.19 on page 170 with Kiel set equal to zero), where e is the energy equivalent of the calorimeter, r is the slope of the heating curve when no reaction is occurring, and ti and t2 are the times at temperatures Ti 

Thermodynamics and Chemistry, second edition, version 3 2011 by Howard DeVoe. Latest version www.chem.vond.edu/theniiobook 

The enthalpy change we wish to find is the reaction enthalpy A77(rxn, T ), which is the change for the same advancement of the reaction at constant temperature T. The paths labeled A7/(expt) and A77(rxn, T ) in the figure have the same initial state and different final states. The path coimecting these two final states is for a change of the temperature from Ti to 72 with fixed at its final value the enthalpy change for this path is denoted A77(P). The value of A77(P) can be calculated from 

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Experimenters have used great ingenuity in designing calorimeters to measure reaction enthalpies and to improve their precision. In addition to the constant-pressure reaction calorimeter and bomb calorimeter described above, three additional types will be briefly mentioned. 

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