Calorimetry

This isn’t seen in singular molecules and is only seen in two degrees in linear molecules because they rotate only in two degrees. For this reason, a diatomic molecule has only two degrees of freedom in the rotational sense. This gives the diatomic molecule like N2 or diatomic nitrogen a total of five degrees of freedom.

The degrees of freedom corresponding to translations and rotations are called the rigid degrees of freedom, since they do not involve any deformation of the molecule. Vibrational degrees of freedom are not rigid with these degrees of freedom depending on the shape and characteristics of the molecule. Vibrational degrees involve deformations and changes between atoms of a molecule. An example is nitrous oxide or N2O, which is linear so it has two rotational degrees of freedom but 8 vibrational degrees of freedom. This leads to a total of 13 degrees of freedom (2 rotational, 3 translational, and 8 vibrational) and a greater heat capacity.

CALORIMETRY

Calorimetry means “heat-measurement” in Latin and Greek, which basically means that this is the science of measuring the amount of heat in a substance. It can measure the amount of heat consumed (in an endothermic process), the amount of heat produced (in an exothermic process), or the amount of heat dissipated by a sample. Calorimetry can be used in a vast number of chemical and biological systems as it can be used whenever heat is exchanged, generated, or lost.

A calorimeter is a device that measures the heat of a reaction. Reactants can be mixed in a calorimeter, allowed to react, and then there will be a measurement of the temperature difference before and after the reaction. A calorimeter does not have to be fancy or complex. It simply has to contain a reaction and measure temperatures.

A calorimeter can be operated under constant pressure (such as atmospheric pressure) or constant volume. It can measure the heat capacity of the calorimeter itself by adding heat to it and measuring the amount of heat that is required to raise the temperature of the entire calorimeter by 1-degree Kelvin. This will be an extremely small number, requiring very sensitive thermometers.

A bomb calorimeter is an explosive-proof steel container called the bomb that does not change in volume during the reaction. It is submerged in liquid (usually water) that absorbs the heat of the reaction taking place. The volume does not change so that the heat evolved under constant volume is the change in temperature multiplied by a constant. When using water in a calorimeter, the change in temperature of the water after a chemical reaction will be detected and will be used to calculate the energy added to or given off by a system.

In bomb calorimetry, an object is placed in a chamber along with air or oxygen. An electrical signal is used to ignite the object in order to burn it completely. The energy given off when the item is completely combusted is transferred to the surrounding water, changing its temperature and becoming a measurable amount. Figure 19 shows what a calorimeter looks like:

Figure 19.