crystallization, decomposition, oxidation, alterations of crystal structure, and 

 development of new compounds. Each reaction which involves a definite amount 

 of heat energy, will take place at a definite temperature that depends upon the 

 type of compounds involved and the pressure of the particular gaseous environ- 

 ment, thus providing the means of identification. 



THERMOCHEMISTRY We shall consider a small insulated container 



full of ice in which a sensitive thermometer is 

 embedded. Initially the temperature will be somewhat below OC. If heat is 

 applied at a uniform rate, the temperature will rise to OC, at which time melting 

 will commence. As heat is applied continuously, the temperature will remain at 

 OC until melting is completed. Only then will the temperature of the resulting 

 water start to rise. A reaction has taken place due to the application of heat — a 

 physical change of state involving the absorption of energy. 



According to the first law of thermodynamics (conservation of energy), 

 energy can be neither created nor destroyed, but only changed from one form to 

 another. Thus, the liquid water in the preceding experiment absorbed heat en- 

 ergy by melting. During absorption there was no rise in temperature. Until 

 the water cooled to the freezing point, it retained this energy. However, if heat 

 is removed at OC, the water would again freeze, thus returning the energy ab- 

 sorbed during melting. This, then, is an example of a reversible reaction induced 

 by temperature change. The temperature of the reaction and the amount of en- 

 ergy involved is characteristic of the compound H 2 and can serve to identify it. 



We shall consider another slightly more complicated example. Sulfur is 

 placed in a closed insulated container with an inert gas such as nitrogen. A 

 thermocouple is embedded in the sulfur, and leads are brought out to a py- 

 rometer (millivoltmeter calibrated to read directly in temperature). If heat is 

 applied to the container at a uniform rate, temperature as indicated by the py- 

 rometer will rise steadily. This fact is represented in the plot of temperature vs. 

 time, Figure 7-1A, as the line AB. When the melting point is reached, tempera- 

 ture will remain constant, line BC, until melting is complete. The temperature, 

 CD, of molten sulfur then will rise to the boiling point. Evaporation takes place 

 between D and E, again appearing as a horizontal line. The temperature, EF, of 

 sulfur vapor then will rise. If at point F, air or oxygen is added to the container, 

 the sulfur will oxidize immediately; heat from the reaction will cause a steeper 

 temperature rise, FG. The reactions indicated by BC and DE are endothermic, 

 meaning that they represent absorption of heat. The reaction FG is exothermic, 

 meaning that it gives off heat. 



121 



