1.3.2: Calorimetry- Isobaric- General Operation

Calorimetry, particularly isobaric calorimetry, is a key technique in chemical thermodynamics, for studying the properties of liquid mixtures and solutions. Numerous designs for calorimeters have been published. [1,2] The operation of a classic calorimeter involves two key steps.

Step 1. Known amounts of two liquids (e.g. solvent and solution) are mixed in a thermally insulated reaction vessel at constant pressure. The rise in temperature is recorded.

Step 2. A known electric current is passed for a recorded length of time through an electric resistance in the reaction vessel to produce a comparable rise in temperature.

By proportion the required amount of energy to produce the measured rise in temperature in step 1 is obtained. [Complications emerge by the need to take account of spontaneous cooling in both steps when the temperature of the calorimeter exceeds ambient temperature; cf. Newton’s Law of Cooling.]

Another type of isobaric calorimeter involves injecting aliquots of one liquid (solution or solvent) into sample cell containing another liquid, recording the rise in temperature accompanying injection of each aliquot. The calorimeter is again calibrated electrically.

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