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Introductory University Chemistry I

University Chemistry: An Introduction

Two Centuries of Transition: 1600-1800

Between 1500 and 1600, we enter perhaps the most interesting phase of the
development of chemical concepts, a period of transition which represents
the transition from earlier ideas, and from alchemy, to a chemistry whose
ideas are essentially those of modern chemistry. This occurred by about
1800, so that this period begins with Johann Van Helmont and ends with
Antoine L. Lavoisier (AD 1743 - 1794). Much of this study involved gases and
the nature of combustion. The erroneous concept of phlogiston is perhaps the
best-known concept of the period.

Johann Baptista Van Helmont (1579-1644) of Brussels studied several subjects
before finally choosing to follow a career in medicine. He became a medical
chemist as well as an ardent follower of Paracelsus. The chemical side of
medicine finally took over from the medical side and he devoted much of his
life to chemical experiments. He has been described as the last alchemist
and the first chemist, because although he believed in alchemy, including
the production of gold from lead which he claimed to have performed, his
emphasis upon experiment rather than argument is a great advance.

Van Helmont is an important figure in the development of chemical concepts
because it is impossible to separate an understanding of the nature of air
from an understanding of the nature of combustion, or burning in air. Air
had been considered by Aristotle and the Greek philosophers as one of the
four elements, with real "airs" or what we call gases all being more or less
contaminated ideal air. The concept of different gases was not clearly
understood; all known gases or vapors were considered as different mixtures
of air and earth or air and fire. This Greek understanding of the nature of
air persisted through the Middle Ages and through the period of alchemy.

Van Helmont was probably the first to recognize, and was the first to state
in print, that there existed or could be created several specific different
kinds of gas each with different properties. Indeed, the word gas was first
used by Van Helmont. Many of the 15 or so "different" gases mentioned by Van
Helmont we now know to be mixtures of gases, or were carbon dioxide obtained
in different ways, but the major advance of recognition of the existence,
and some of the properties, of different gases we owe to him. We owe also
other advances in quantitative measurements, including the use of the
balance for precise weighings, to him, as well as some advances in medicine.
(Van Helmont is also known for an experiment in which he planted a tree in a
pot of earth and weighed both the earth and the tree after five years. Since
the weight of the earth had decreased by at most a few ounces while the
weight of the tree was about 170 pounds, Van Helmont concluded that the tree
arose from water only, since he had added nothing else to the pot!) The
period of Van Helmont's chemical contributions began in the seventeenth
century, probably around 1609, but his work was not generally published
until 1648.

Robert Boyle (1627 - 1691) is best known as the discoverer of Boyle's Law
which relates the pressure upon a sample of gas to its volume, as we shall
see in Chapter 6. Boyle was also responsible for the first clear statement
of the modern definition of a chemical element: a chemical element is a pure
substance which cannot be broken down into any simpler substance by chemical
means.

Combustion, Air, and Phlogiston

We now know that the air we breathe is a mixture of gases, primarily oxygen
and nitrogen. In the eighteenth century, however, the discovery that air was
such a mixture, and the characterization of its components, was modern
chemical research. Van Helmont's understanding that there were different
gases with different chemical properties led to attempts to separate gases
from air and react gases with air. These studies are the basis of our modern
understanding of the nature of the atmosphere.

The erroneous doctrine of phlogiston, introduced by Georg Ernest Stahl,
enabled chemists to explain metal reduction and oxidation by the same
mechanism. It was held by most eighteenth-century chemists including Joseph
Priestley (1733 - 1804) and the Swedish apothecary Carl Wilhelm Scheele
(1742 - 1786). The doctrine of phlogiston did not fade until its replacement
by the oxygen chemistry approach of Lavoisier and his colleagues around
1800.

Although Scheele and Priestley did not use a symbol for phlogiston, it is
convenient for us to write chemical reactions using symbols. If phlogiston
is symbolized by X, the oxidation or rusting of iron as understood by
Priestley would be written Iron --> Calx of iron + X. The reaction of carbon
is similar, Carbon --> Calx of carbon + X. The reduction of the calx of tin
can be, then, Calx of tin + X --> Tin, and the description of the actual
smelting process of tin or iron is:

Calx of tin + Carbon --> Tin + Calx of carbon

Calx of iron + Carbon --> Iron + Calx of carbon

Scheele interpreted his experiments in the light of this phlogiston theory.
He thought that hydrogen might be phlogiston, or perhaps a compound of
phlogiston with some unknown substance, for it was clear that many metals
could be used in the reaction Calx of metal + Hydrogen --> Metal + Heat. For
example, when Scheele burned a hydrogen flame in a glass globe of air
standing over water, he observed that the water rises. He reasoned,
correctly, that this meant that combustion must use up the fire air
(oxygen). His further interpretation of this experiment is an interesting
piece of erroneous reasoning: since no product of this reaction was observed
other than heat (water did not condense since hot water was used in his
water bath), the reaction taking place would have to be fire air + X -->
heat. It is therefore reasonable to attempt to reverse the reaction, to
decompose heat and produce fire air. To do this, one must present to heat a
substance having a greater attraction for phlogiston than does fire air.
Such a substance is nitric acid, because it readily attacks metals taking
out their phlogiston:

(Calx of M + X) + nitric acid --> Calx of M + (nitric acid + X)

where (Calx of M + X) is equivalent to the metal M itself and (nitric acid +
X) is taken as the red fumes of NO2. Then, since nitric acid boils away on
heating, one must fix it by combination with potash:

H+NO3- + KOH --> KNO3 + H2O

and then distill it with oil of vitriol (H2SO4) in a retort:

4KNO3 + H2SO4 --> 4NO2 + O2 + 2K2SO4 + H2O

If one then absorbs the NO2 gas in limewater, while the K2SO4 remains behind
in the flask, an empty bladder can be filled with the fire air gas. This
erroneous interpretation of a correctly planned experiment is worthy of some
thought on the part of the reader.

Other methods of preparing oxygen were also used by Scheele, who interpreted
the results of heating the red oxide of mercury according to the phlogiston
theory using (fire air + X) as equivalent to heat and (calx of mercury + X)
as equivalent to mercury metal:

Calx of mercury + (X + fire air) <--> fire air + (calx of mercury + X)

Common Gases and the Atmosphere

The atmosphere or air which surrounds us is an almost uniform gas mixture of
roughly 80% nitrogen and 20% oxygen, with traces of other gases. Once Van
Helmont had shown that different gases existed, the chemists ofthe
eighteenth century quickly characterized four: carbon dioxide, nitrogen,
hydrogen, and oxygen.

Carbon Dioxide

The first gas prepared and truly characterized as a pure substance was
carbon dioxide, CO2(g). Carbon dioxide was carefully studied around 1750 by
Joseph Black (1728 - 1799, Edinburgh), who named it fixed air. Black, a
medical man, presented his studies in 1754 as a doctoral dissertation
entitled "On the Acid Humour Arising from Food, and Magnesia Alba". Magnesia
alba, or white magnesia, is a complex compound whose structure is
xMgCO3.yMg(OH)2.zH2O;the values of x, y and z can vary. Suspensions of this
material are still sold, under the name of milk of magnesia, as a remedy for
acid stomach. Black realized that the reaction which took place on heating
magnesia alba were analogous to those which took place on heating limestone.
That is, the reactions:

magnesia alba --> quicklime + fixed air (on heating)

limestone + acid --> a salt + fixed air

quicklime + acid --> a salt

We now know that the reactions which take place are actually:

xMgCO3.yMg(OH)2.zH2O --> (x+y)MgO +(y+z)H2O + yCO2

xMgCO3.yMg(OH)2.zH2O + (2x+2y)HCl -->(2x+2y+z)H2O + xCO2 + (x+y)MgCl2

MgO + 2HCl --> MgCl2 + 2H2O and CaCO3 --> CaO + CO2

CaCO3 + 2HCl --> CaCl2 + 2H2O + CO2

CaO + 2HCl --> CaCl2 + H2O

The alkalies then known included these materials and comparatively few
others; all existed both as a "mild form" and a "caustic form". The older
names and modern formulas for these are given in the table below.
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Table: Alkalies Known At The Time of Joseph Black

Alkali                                        Modern Formula

Vegetable alkali (potash), mild form          K2CO3
Vegetable alkali (potash), caustic form       KOH

Marine alkali (soda), mild form               Na2CO3
Marine alkali (soda), caustic form            NaOH

Volatile alkali (ammonia), mild form          (NH4)2CO3
Volatile alkali (ammonia), caustic form       "NH4OH", actually NH3 + H2O

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Only recently has it been realized that NH4OH actually exists as hydrated
ammonia, NH3.H2O, rather than as a hydroxide species. Black could use some
of them to obtain information about fixed air. Fixed air is, or behaves as,
an acid. Since:

limestone --> quicklime + fixed air, and

quicklime + mild alkali --> limestone + caustic alkali, then

mild alkali = caustic alkali + fixed air.

Although fixed air had no commercial use when it was discovered, carbonated
beverages or "mephitic juleps" were first prepared by Joseph Priestley, the
discoverer of oxygen, in London in 1772. The taste of carbonated water is
due partly to the evolution of carbon dioxide from its solution but mostly
to the formation of carbonates and hydrogen carbonates in solution.

Nitrogen

Most of our atmosphere is made up of nitrogen, N2. The credit for the
discovery of nitrogen is unknown, although it was clearly distinguished
around 1772. In addition to studies by Cavendish and by Priestley at this
time, work was done in 1770 - 1773 by Scheele and by one of Black's
students, Daniel Rutherford, in his thesis of 1772. Scheele noted that air
could be trapped and a part of the air removed by a chemical reaction, such
as absorption by linseed oil or reaction with moist iron filings; the
remaining air will not support combustion. Daniel Rutherford, who later
became a professor of botany, permitted mice to breathe air until
suffocation; the fixed air produced by them was removed by caustic potash.
The remaining air, which does not support life or combustion, is not fixed
air because it does not give a precipitate with limewater. However, like
fixed air, it does not support life or combustion and so it was called dead
air.

Hydrogen

The gas we know as hydrogen, H2(g), was once called inflammable air. Credit
for the discovery of hydrogen goes to Henry Cavendish (London, 1731 - 1810)
who described it in his work "On Factitious Airs" published in 1766. By
factitious airs Cavendish meant airs, or gases, produced by chemical art
rather than found in nature. Hydrogen gas is not a normal constituent of the
atmosphere of the earth. The gas was produced by reacting an acid with any
of several metals:

H2SO4 or HCl + Zn or Fe or Sn --> inflammable air + a salt

This gas had a very low density, which Cavendish measured. Cavendish was
also responsible for development of a method for measuring the weight of
fixed air present by absorbing it on pearl-ash (KOH) which was weighed
before and after. This method is still used in the analysis of organic
compounds, as we shall see in the following sections.

Oxygen

From many points of view the most important gas we know is oxygen, O2(g).
Oxygen, which makes up about one-fifth of our atmosphere, was given the
early names of fire air and dephlogisticated air because it has such an
obvious and central role in combustion. Credit for the discovery and
preparation of oxygen is generally given to Joseph Priestley (England, 1733
- 1804), who prepared it by heating the red oxide of mercury, HgO, in 1774.
In modern terms the reaction is HgO + heat --> Hg + 1/2 O2(g), but this was
not Priestley's interpretation. He considered air to acquire or or lose
phlogiston; thus:

air + X = phlogisticated air (nitrogen, N2)

air - X = dephlogisticated air (oxygen, O2)

The measure of phlogiston gained or lost required a method for measurement
of the degree of phlogistication of air, or a measure of the goodness of
air. The method used by Priestley to measure this was its slow reaction with
NO2:

4NO2(g) + O2(g) --> 2N2O5(g)

N2O5(g) + H2O --> 2HNO3

The N2O5 formed immediately dissolves in the water above which the gas is
confined. Thus, the relative decrease in volume on reaction with nitrogen
dioxide gas is a measure of the goodness, or the degree of phlogistication,
of a sample of air.

Compounds Are Made Up of Elements

The work of chemists from Boyle onward has been based on the idea that a
chemical element is a pure substance which can not be broken down into any
simpler substances by chemical means. Although we now know that nuclear
reactions can actually convert one element into another, these lie outside
the normal province of chemistry. Most of the substances found on earth are
mixtures containing several different substances. Mixtures can be separated
by physical means into the different pure substances which are their
components. The trinity of principles of the alchemists, mercury, sulfur,
and salt, could all be prepared as pure substances, as could water and
metals. Reactions of pure metals with air produced pure metal oxides, and
reactions of these oxides with water produced other pure compounds. Through
the seventeenth century, it became clear that most pure substances are not
elements but compounds. Compounds could be made or synthesized from their
pure component elements, and in many cases broken back down to elements
again or analyzed. The number of pure compounds known grew quickly, and the
number of elements known grew more slowly, through both the seventeenth and
eighteenth centuries.

Pure substances, both elements and compounds, engage in well-defined
chemical reactions with each other to produce new pure substances or, in
many cases, mixtures of substances. Pure substances which engage in chemical
reactions came to be known as reagents. Substances of the highest available
purity, which are the most desirable for the quantitative study of chemical
reactions, became known as reagent-grade materials. The specifications for
modern reagent-grade chemicals are published in most countries by their
national governmental standards office or by their national society of
chemists because they are so important.
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