Calorimetry

Calorimetry is the science of measuring the heat of chemical reactions or physical changes. Calorimetry is performed with a calorimeter. The word calorimetry is derived from the Latin word calor, meaning heat. Scottish physician and scientist Joseph Black, who was the first to recognize the distinction between heat and temperature, is said to be the founder of calorimetry.

Calorimetry is the quantitative measurement of the heat required or evolved during a chemical process.

Indirect calorimetry calculates heat that living organisms produce from their production of carbon dioxide and nitrogen waste (frequently ammonia in aquatic organisms, or urea in terrestrial ones), OR from their consumption of oxygen. Lavoisier noted in 1780 that heat production can be predicted from oxygen consumption this way, using multiple regression. The Dynamic Energy Budget theory explains why this procedure is correct. Of course, heat generated by living organisms may also be measured by direct calorimetry, in which the entire organism is placed inside the calorimeter for the measurement.


A calorimeter is a device used for calorimetry, the science of measuring the heat of chemical reactions or physical changes as well as heat capacity. The word calorimeter is derived from the Latin word calor, meaning heat. Differential scanning calorimeters, isothermal microcalorimeters, titration calorimeters and accelerated rate calorimeters are among the most common types. A simple calorimeter just consists of a thermometer attached to a metal container full of water suspended above a combustion chamber.

Different Calorimeters

Adiabatic calorimeters
An Adiabatic calorimeter is a calorimeter used to examine a runaway reaction. Since the calorimeter runs in an adiabatic environment, any heat generated by the material sample under test causes the sample to increase in temperature, thus fuelling the reaction.
No adiabatic calorimeter is truly adiabatic - some heat will be lost by the sample to the sample holder. Examples of adiabatic calorimeters are:-
• THT EV-Accelerating Rate Calorimeter
• HEL Phi-Tec
• A simple Dewar flask
• Systag FlexyTSC a successor of their SIKAREX unit - the electronics of which could be used to apply a feedback system to heat the sample holder to give a result closer to true adiabaticy, however as the sample holder is an open ended glass tube, one soon loses the sample as a great deal of smoke.

Reaction calorimeters
A reaction calorimeter is a calorimeter in which a chemical reaction is initiated within a closed insulated container. Reaction heats are measured and the total heat is obtained by integrating heatflow versus time. This is the standard used in industry to measure heats since industrial processes are engineered to run at constant temperatures. Reaction calorimetry can also be used to determine maximum heat release rate for chemical process engineering and for tracking the global kinetics of reactions. There are four main methods for measuring the heat in reaction calorimeter:

Heat flow calorimetry
The cooling/heating jacket controls either the temperature of the process or the temperature of the jacket. Heat is measured by monitoring the temperature difference between heat transfer fluid and the process fluid. In addition fill volumes (i.e. wetted area), specific heat, heat transfer coefficient have to be determined to arrive at a correct value. It is possible with this type of calorimeter to do reactions at reflux, although the accuracy is not as good.

Heat balance calorimetry
The cooling/heating jacket controls the temperature of the process. Heat is measured by monitoring the heat gained or lost by the heat transfer fluid.

Power compensation
Power compensation uses a heater placed within the vessel to maintain a constant temperature. The energy supplied to this heater can be varied as reactions require and the calorimetry signal is purely derived from this electrical power.

Constant flux
Constant flux calorimetry (or COFLUX as it is often termed) is derived from heat balance calorimetry and uses specialized control mechanisms to maintain a constant heat flow (or flux) across the vessel wall.

A calorimeter may be operated under constant (atmosphere) pressure, or constant volume. Whichever kind to use, we first need to know its heat capacity. The heat capacity is the amount of heat required to raise the temperature of the entire calorimeter by 1 K, and it is usually determined experimentally before or after the actual measurements of heat of reaction.

The heat capacity of the calorimeter is determined by transferring a known amount of heat into it and measuring its temperature increase. Because the temperature differences are very small, extreme sensitive thermometers are required for these measurements. Example 1 shows how it is done.

[Example]

The temperature of a calorimeter increases 0.10 K when 7.52 J of electric energy is used to heat it. What is the heat capacity of the calorimeter?

Solution:
Dividing the amount of energy by the temperature increase yields the heat capacity, C,

C = 7.52 / 0.10 = 75.2 J/K.

Discussion:
We often compare the heat capacity of a calorimeter to that of a definite amount of water. The heat capacity of 75.2 J/K for the calorimeter is equivalent to the heat capacity of 1 mole (18 g) of water (18 g mol^-1*1 cal (g K)^-1*4.184 J cal^-1 = 75.3 J (K mol)^-1.

Do you know that the electric energy = q * dV = i * dt * dV, where q is charge; dV, voltage; i, current; and t, time.