Calorimetry

In essence, a calorimeter is any device in which the temperature before and after some kind of change can be accurately measured. Probably the simplest of such devices is the coffee cup calorimeter so-called because it is made of a styrofoam cup such as the ones in which coffee is commonly served. A styrofoam cup is used because styrofoam is a relatively good insulating material. Heat given off within it as a result of some physical or chemical change will not be lost to the surrounding environment. To use the coffee cup calorimeter, one simply carries out the reaction to be studied inside the coffee cup, measures the temperature changes that take place, and then calculates the amount of heat lost or gained during the change.

The type of calorimeter more commonly used for precise work is called the bomb calorimeter. A bomb calorimeter designed to measure heat of combustion, as an example, consists of a strong-walled metal container set inside another container filled withwater. The inner container is fitted with an opening through which oxygen can be introduced and with electrical leads to which a source of electricity can be connected.

The object to be studied is then placed in a combustion crucible within the bomb and ignited. The reaction occurs so quickly within the reaction chamber that it is similar to the explosion of a bomb. Hence theinstrument's name. Surrounding the bomb in this arrangement is a jacket filled with (usually)water. Heat given off or absorbed within the bomb heats up the water in the jacket, a change that can readily be measured with a thermometer inserted into the water.

Many variations in the basic design described here are possible. For example, the use of liquids other than water in the insulating jacket can permit the study of heat changes at higher temperatures than the boiling point of water 212°F (100°C). Aneroid (without liquid) calorimeters are also used for special purposes, such as the measurement of heat changes over very large temperature ranges. Such calorimeters use metals with a high coefficient of thermal conductivity, like copper, to measure the gain or loss of heat in some type of change.