162 SMITH'S INTERMEDIATE CHEMISTRY 



The chemical interactions to be studied thermally are arranged 

 so that they may be carried out in a small vessel which can be 

 placed inside another containing water. The whole apparatus 

 is called a calorimeter (Greek, heat-measurer). The heat de- 

 veloped raises the temperature of this water. Where gases like 

 oxygen are concerned, a closed bulb of platinum forms the inner 

 vessel. The quantity of heat capable of raising one gram of water 

 one degree in temperature at 15 Centigrade is called a calorie. 



Thermochemical Equations. While in physics the unit of 

 quantity is the gram, in chemistry the unit which we select is 

 naturally a gram-atomic weight or a gram-molecular weight of the 

 substance. Thus, the heat of combustion of carbon means the 

 heat produced by combining twelve grams of carbon with thirty- 

 two grams of oxygen, and is sufficient to raise nearly 100,000 

 grams of water one degree. This is expressed as follows: 



C + 2 -> C0 2 + 96,900 cal. 



In other words, the combustion of less than half an ounce of car- 

 bon will raise over two pounds of water from to the boiling- 

 point. 



When the action is one which absorbs heat, this fact is indicated 

 by the negative sign preceding the number of calories. Thus for 

 the dissociation of water vapor into hydrogen and oxygen we 

 have the thermochemical equation: 



2H 2 O -r* 2H 2 + 2 - 116,200 cal. 



If the action is reversible, as this one is, the heat absorbed when 

 it proceeds in one direction is equal to that liberated when it goes 

 in the other direction: 



2H 2 + 2 -> 2H 2 O + 116,200 cal. 





 Answers to Possible Questions. It is always found that 



the same quantities of any given chemical substances, undergoing 

 the same chemical change under the same conditions, produce 



