Table 303 

 THERMOCHEMISTRY. CHEMICAL ENERGY DATA 



311 



The total heat generated in a chemical reaction is independent of the steps from initial to final 

 state. Heats of formation may therefore be calculated from steps chemically impracticable. 

 Chemical symbols now represent the chemical energy in a gram-molecule or mol(^) ; treat re- 

 action equations like algebraic equations : CO 4- O = C0 2 -)- 68 Kg-cal ; subtract C + 2 O = C0 2 

 — |— 97 Kg-cal, then C4-0 = C04-29 Kg-cal. We may substitute the negative values of the 

 formation heats in an energy equation and solve MgCl 2 4- 2 Na= 2 NaCl -|- Mg4- x Kg-cal; 

 — 151 =— 196 4- x ; x = 45 Kg-cal. Heats of formation of organic compounds can be found 

 from the heats of combustion since burned to H 2 and C0 2 . When changes are at constant 

 volume, energy of external work is negligible; also generally for solid or liquid changes in vol- 

 ume. When a gas forms a solid or liquid at constant pressure, or vice versa, it must be allowed 

 for. For N mols of gas formed (disappearing) at T|^ & the energy of the substance is decreased (in- 

 creased) by 0.002 • N .• T K Kg-cal. H 2 4- = H 2 4- 67.5 Kg-cal. at i8°C at constant volume ; 

 1(2 H 2 -f 2 — 2 H,0= 1 35.0 +0.002X3X291 = 136.7) =68.4 Kg-cal. 



The heat of solution is the heat, 4- or — , liberated by the solution of 1 mol of substance in so 

 much water that the addition of more water will produce no additional heat effects. Aq. signifies 

 this amount of water; H 2 0, one mol. ; NH 3 4- Aq = NH,OH • Aq. 4- 8 Kg-cal. 



Heats of Formation from Elements in Kilogram-Calories 

 At ordinary temperatures. 



am =r amorphous: di — diamond; gr = graphite ; cr =: crystal ; g = gas; 1 = liquid ; s =: solid; y = yellow (gold); 

 rh = rhombic (sulphur). * Heats of formation not from elements but as indicated. 



Smithsonian Tables. 



