HEAT OF FORMATION 99 



atomic linkages. There are some points to be borne in 

 mind in applying thermal data for this purpose. 



The first is that if the reaction studied in the calorimeter 

 is accompanied by a change of volume, and is not per- 

 formed in a bomb, a part of the thermal effect is due to 

 external work, so that the value obtained will be too high 

 if the reaction takes place with contraction. The correc- 

 tion to be made on this account is very simple, since only 

 the work done in changing the volume of a gas has to be 

 considered, the changes in the liquid and solid phases being 

 negligible in amount. Take for example the combustion 

 of carbon monoxide 



2 CO-f0 2 =2C0 2 . 



Under constant pressure this is accompanied by a contrac- 

 tion, since three molecules are converted into two. The 

 work thus converted into heat is per kilogram-molecule 

 APV= 2 T calories; 



so that in the foregoing case in which 2(12 + 1 6) = 56 

 kilograms of carbon monoxide are burnt at ordinary 

 temperature (17) 



2 (273 4- 17) = 580 calories 



of heat are gained by the contraction, and are consequently 

 to be subtracted from the heat of combustion to obtain the 

 number relating to the actual chemical process. If as the 

 data here used and to be used, we employ the gram-molecule, 

 then for each molecule of gas that disappears 058 calories 

 must be subtracted, for each molecule formed, that amount 

 is to be added. 



A second point of much greater magnitude in effect is 

 the state of aggregation at the moment, since alteration of 

 it is accompanied by a very considerable thermal effect, 

 e.g. the solidification of water evolves 008 x 18 = 1-44 

 calories, and its evaporation absorbs 06 x 1 8 = 10-8. In 

 the numerical comparison Thomsen dealt with the gaseous 

 state. The direct method of judging the additivity, by 

 starting from the heat of formation of simple binary com- 



G 2 



