Thermodynamic Considerations

In a condensed system, if a phase change occurs in the heating mode where one phase is stable over a definite temperature range and the other phase is stable over, say, a higher temperature range, then the process of change at the transition temperature will be endothermic and reversible on the cooling mode with an exothermic character. This can be represented as

heating

> A(2) endothermic

This is true where both phase (1) and phase (2) are solids or phase (1) is solid and phase (2) is liquid. In the phase change from a liquid to a gas, the process is reversible, but because the gas is usually lost from the system, it is probably best to follow the process at different partial pressures of the vapor (p) when the relationship holds, where R is the gas constant, T is the temperature of the phase change (K), and AH is the enthalpy of fusion. The above phase transitions can all be investigated by DTA or DSC.

If an amorphous (energy-rich) solid phase undergoes transition, however, then a stable crystal may result. The amorphorus or glassy phase is metastable, and the process is exothermic and irreversible. This can be represented as

Metastable ^ Stable form (energy rich) (crystalline) Exothermic and irreversible

Such changes are shown in both inorganic and organic systems.

The above are all examples of transitions which are of first order. However, second-order transitions can also be followed and determine the position of the baseline in DSC. This leads to the calculation of heat capacity from DSC measurements and is the basis for the establishment of the glass transition temperature (Tg) in polymers. A further extension based on thermodynamic factors is purity determination based on the lowering of the freezing point caused by the presence of impurities.

In chemical reactions certain features may be noted. Thus, carbonate decompositions are endothermic, and loss of water is also endothermic. Obviously, such processes involving weight loss can be studied using both TG and DTA/DSC. It may also be advantageous to study such processes under partial pressure (P) of the gaseous products when one may expect the relationship

1 RT

similar to the liquid-gas phase change, but here A H is the enthalpy of the reaction.

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