Hess's law

Germain Henri Hess reported calorimetric analyses of a number of reactions of sulfuric acid and several of its hydrates. Among them were

(9)     H2SO4 + 2 H2O --- > H2SO4.2H2O
(10)     H2SO4.H2O + H2O --- > H2SO4.2H2O

(The numbering is from Hess's paper.) He reported that the heat evolved in reaction (9) was 116.7 and that of reaction (10) 38.56. (The units are not important, but they refer to a fixed quantity of H2SO4.) Use Hess's law to predict the heat evolved in the following reaction:

(7)     H2SO4 + H2O --- > H2SO4.H2O

Copyright 2003 by Carmen Giunta. Permission is granted to reproduce for non-commercial educational purposes.

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