22 CALORIMETRY 



does not hinder the banker from entering sterling in his books. 

 So calories may be the units employed, although heat may not 

 necessarily enter the reaction. 



A. Measurement of energy-value (E.V.) of foods by ultimate analysis. 



The energy of a pure chemical compound may be calculated 

 from its chemical formula. The amount of heat evolved when 

 C is oxidised to CO 2 and when 2H is combined with O to form 

 water has been determined. The equations of these two reactions 

 could therefore be written : 



C +O 2 = CO 2 + 94-31 cals. 

 3 +117-4 cals. 



(A horizontal line above the formula of a substance in a thermo-chemical 

 equation indicates that the substance is in the gaseous state, the absence 

 of any line indicates the liquid state, while a line below the formula indi- 

 cates the solid state. The suffix aq is intended to convey the idea that 

 the substance is in solution in such a large volume of water that the addition 

 of more water would not produce any appreciable effect that is, the 

 substance is so dilute that its heat of dilution on the further addition of 

 water would be negligible.) 



One must note that any alteration of gaseous volume or of 

 any other physical characteristic of any of the reacting units 

 would, by utilising some energy as positive or negative work, 

 produce an alteration in the amount of heat evolved. Welter 

 enunciated a rule whereby one might arrive at an approximate 

 value of the heat of oxidation of a compound containing oxygen 

 as well as carbon and hydrogen. According to this rule the 

 oxygen is subtracted from the molecular formula with as much 

 hydrogen as would serve to convert it completely into water, 

 the heat of oxidation of the carbon and hydrogen in the residue 

 then gives a rough value of the heat of oxidation in the whole 

 compound. For example : 



A fat, tripalmitin, has the following formula C 51 H gs O G 

 Deduct intramolecular water HO. 



Leaving for oxidation - - C 51 H SO 



QiH 86 +145O., = 51 CO 2 +43 H 2 O 



= 51 X94-31 +43x58-7 

 = 7333-9 cals. 



That is, a gram molecule (806 grams) of tripalmitin in being 

 completely oxidised to CO 2 +H 2 O would liberate 7333-9 cals. of 

 heat. Similarly, the energy stored in the form of carbohydrate 

 may be calculated. A difficulty, however, occurs with proteins. 



