mass
amount
molar mass
concentration
solution volume
gas volume
molar gas volume
Avogadro
constant, L
number of
entities, N
CALCULATING ON THE SIDE OF SUCCESS – Module H(2024): MANIPULATING STOICHIOMETRIC RATIOS
H. CHEMICAL REACTION STOICHIOMETRY
H0. BACKGROUND TO THE QUANTITATIVE NATURE OF CHEMISTRY
By the late c.18th, the early chemists needed to account for the relationships between the masses of
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Although it was clear to chemists that atoms of different elements possess differing masses, they were not able fully to understand the reasons why, for example, in compound formation, one atom of sodium combines with one atom of chlorine, whereas one atom of carbon combines with four atoms of chlorine.
In chemistry, we know now that interactions
are based on atom-atom, molecule-molecule, ion-ion, for instance, rather than g-g, mg-mg,
kg-kg, etc. In other words, nanoscopic particles react on an amount of substance basis, NOT
on a mass basis, this being referred to as stoichiometry, and recognized by the importance of the mole as the SI unit for the base physical quantity amount of substance.
reacting substances that they observed in their experiments, e.g., whenever two elements combined in a chemical reaction, they did so in a definite, fixed proportion by mass:
Fe:S ratio 7 g:4 g and Mg:O ratio 3 g:2 g, for instance.
With the advent of reliable balances, accurate and reproducible measurements of mass had only recently become available - up until then the Science of Chemistry arguably did not exist.
By the middle of the c.19th, confusion still reigned supreme regarding the chemical formulae of many substances that we now regard as relatively simple. Kekulé's Lehrbuch der Organischen Chemie (1861) gave 19 different formulae in use by chemists for acetic acid, as below.
Not only do atoms of different elements possess different masses, but also, crucially, their bonding capacity or valence depends upon their electronic structures, governing the formulae exhibited by their compounds.
A wide body of complementary experimental work, carried out between 1860 & 1920, provided
our modern understanding of the periodic nature of many relatively simple chemical formulae. Amongst the most prominent were
1. the development and construction of glassware that could operate at very low pressures and create high vacuum - the Geissler & Sprengel pumps (1860s).
2. Mendeléev's development of the Periodic Table of (known) chemical elements (1869-71) following Döbreiner's Triads (1829), Newlands' Octaves (1864) and concurrent with Lothar Meyer's Curves (1868-9).
3. elucidating the electronic structures of atoms (from the 1880s) from electronic line spectra.
4. discovering the existence in air of noble gases like argon (1890s) by William Ramsay & Lord Rayleigh
(John William Strutt). Helium had been discovered independently (1868) by astrophysicist Jules Janssen & astronomer Norman Lockyer (working with Edward Frankland) when, looking down their spectroscopes, they observed - within 63 days of each other - an unexpected yellow spectral line emanating from the Sun's chromosphere. While Jansen mistook this for sodium, Frankland & Lockyer posited that it was due to an unknown element which they christened Helios (Greek 'Sun'). Electromagnetic spectroscopy as a technique was pioneered by Gustav Kirchhoff and Robert Bunsen (1860), arguably the dawn of the most powerful scientific methods in history.
5. the accidental discovery of radioactivity (1896) by Henri Becquerel.
Deutsche Bundespost commemoration stamp of Fraunhofer's discovery of dark spectral lines from the Sun
6. the famous Geiger & Marsden experiment (1909-10) - firing a-particles at thin gold foils - that led to the Rutherford nuclear model of the atom (1911) proposing a phenomenally dense, tiny, positively charged nucleus orbited by extra-nuclear electrons.
7. the discovery of isotopes for many of the naturally occurring elements (from 1912 for neon) by F.W. Aston, J.J. Thomson and Frederick Soddy.
8. Moseley's X-ray spectra (1913) which demonstrated that the fundamental properties of an atom are determined by its serial number - aka atomic number - rather than its atomic mass.