CHAPTER 11 REACTIONS AND OTHER CHEMICAL PROCESSES
11.3 MOLARREACTIONENTHALPY 321
BIOGRAPHICAL SKETCH
Germain Henri Hess (1802–1850)Hess was a Russian chemist and physician
whose calorimetric measurements led him to
formulate the law of constant heat summation,
now known as Hess’s law. His given name had
several versions: “Germain Henri” in French,
as shown above; “Hermann Heinrich” in Ger-
man; and “German Iwanowitsch” in Russian.
He was born in Geneva, Switzerland, the
son of an artist. The family moved to Rus-
sia when he was three years old; his father
had found work there as a tutor on an es-
tate. Hess studied medicine at the University
of Tartu in Estonia (then part of the Russian
empire) and received his doctor of medicine
degree in 1825. In addition to his medical stud-
ies, Hess took courses in chemistry and geol-
ogy and wrote a doctoral dissertation on the
composition of mineral waters in Russia.a
Hess was more interested in chemistry than
medicine. He briefly studied with the fa-
mous Swedish chemist Jons Jakob Berzelius ̈
in Stockholm, and they became life-long
friends. Hess then practiced medicine in
Irkutsk, Siberia, while continuing his interests
in mineral chemistry and chemical analysis.
In 1829, after being being elected an adjunct
member of the St. Petersburg Academy of Sci-
ences, he gave up his medical practice and be-
gan teaching chemistry at various institutions
of higher learning in St. Petersburg. He wrote
a two-volume textbook,Fundamentals of Pure
Chemistry, in 1831, followed by a one-volume
abridgment in 1834 that became the standard
Russian chemistry textbook and went throughseven editions. He became a full member of
the St. Petersburg Academy Academy in 1834.
Hess published the results of his thermo-
chemical research between 1839 and 1842.
His 1840 paperbdescribes his measurements
of the heat evolved when pure sulfuric acid,
H 2 SO 4 , is mixed with various amounts of wa-
ter, and another series of measurements of the
heat evolved when the acid in H 2 SO 4 -water
mixtures is neutralized with aqueous ammo-
nia. The following table from this paper is of
historical interest, although it is a bit difficult
to decipher:
Acid Heat evolved by Sum
ammonia water
HP ́S 595:8 595:8
HP^2 ́S 518:9 77:8 596:7
HP^3 ́S 480:5 116:7 597:2
HP^6 ́S 446:2 155:6 601:8
Average597:9
The first column gives the relative amounts of
acid and water in the notation of Berzelius:HP
is H 2 O, ́S is SO 3 , andHP ́S is H 2 SO 4. Thus
HP^3 ́S, for example, is H 2 SO 4 C2H 2 O. The sec-
ond and third columns show the heat evolved
( q) in units of calories per gram of the SO 3
moiety of the H 2 SO 4. The near-equality of the
sums in the last column for the overall reaction
H 2 SO 4 .l/C2 NH 3 .aq/! .NH 4 / 2 SO 4 .aq/
demonstrates Hess’s law of constant heat sum-
mation, which he stated in the words:c
The amount of heat evolved during the formation
of a given compound is constant, independent of
whether the compound is formed directly or in-
directly in a series of steps.
Hess confirmed this law with similar exper-
iments using other acids such as HCl(aq) and
other bases such as NaOH(aq) and CaO(s).
After 1848 Hess’s health began to deterio-
rate. He was only 48 when he died. His role
as the real founder of thermochemistry was
largely forgotten until the influential German
physical chemist Wilhelm Ostwald drew atten-
tion to it about forty years after Hess’s death.
aRefs. [ 94 ] and [ 101 ]. bRef. [ 77 ]. cRef. [ 77 ]; translation in Ref. [ 41 ].