Download 서울대학교 일반화학실험

EXPT 1. Atomic Weight of Barium
[Key Contents]
- atomic theory, atomic weight, valency, equivalent weight
- precipitation equilibrium, gravimetric analysis
- precipitate chloride ion in barium chloride as silver chloride,
determine atomic weight of barium
- discovery of radium by Marie Curie, Nobel Prize-winning work of
Richards
[References]
Principles of Modern Chemistry, 6th Ed. (Oxtoby et al.)
Ch 2. Chemical Formulas, Chemical Equations,
and Reaction Yields
Ch 16. Solubility and Precipitation Equilibria
Chemistry for Life, Chemistry for Better Life (Kim et al.)
Ch 3. Atoms and Molecules
Ch 9. Equilibrium Reactions
[Goal]
- to determine atomic weight of barium following the procedure
used by Marie Curie in her discovery of radium and, thereby, learn
principles and techniques involved in the historic experiment
- to observe images of atoms using scanning tunneling microscope
(STM)
[Background]
The world around us is the world of atoms. In the world of atoms,
atomic weight is a fundamental quantity that distinguishes the atom of
one element from the atom of another. In other words, different
elements differ in atomic weight. And the atomic weight of an
element is a constant over time and space. In a sense, the number of
protons in an atom (atomic number) is more fundamental than the
atomic weight.
Creation of particles with different number of protons either in the
big bang universe or in stars implies creation of nuclei of different
chemical
elements.
Neutrons
make
heavy
elements
possible
by
binding repelling protons together through strong nuclear force. Thus
the number of neutrons could differ in a given element leading to
isotopes of the same element. Changes in the number of electrons
lead to ions of different charge; however, identity of the element
remains unchanged. Changes in the number of protons deep in the
atomic nucleus results in the creation of entirely different elements
with new chemical and physical properties.
Hydrogen made in the big bang universe and heavy elements made
later in stars and spewed out into the interstellar space by the
supernova explosion encounter each other and are used collectively
to make the solar system including the earth inhabited by living
creatures. Consider the hydrogen bond in DNA. There is no that life
on earth is a result of 13.7 billion years evolution of the universe. In
the
evolution of the universe, isotopes of an element are mixed
uniformly giving rise to the observed atomic weight, which is the
weighted average of isotopic masses.
Atomic weights of many elements were known with a fairly good
accuracy even before the discovery of the subatomic particles and
Mosely's
determination
of
the
atomic
number,
which
is
why
Mendeleev was able to come up with the periodic table arranging the
elements in the order of increasing atomic weight.
Two
different
methods
were
available
to
the
19th
century
scientists. In the first method, Avogadro's principle proposed in 1811
played
a
key
role.
Based
on
Avogadro's
principle,
one
could
determine molecular weight from density measurements. Knowing how
atoms combine, than one could determine atomic weight of the
combining atoms. Thus, Cannizzaro determined atomic weight of
several elements based on the molecular weight of hydrogen as 2.
Avogadro's principle could not be used for metallic elements that
do not readily form gaseous compounds. An empirical law discovered
by Dulong and Petit turned out to be quite useful in this case. Dulong
and Petit showed in 1819 that the constant volume molar heat
capacity, Cv, of solid elements is about the same.
dE/dT ≒ 25 J․mol-1․K-1
The observed value of 25 J․mol-1․K-1 turned out to be 3R, which
comes from the fact that the sum of the kinetic energy and the
potential energy in a 3-dimensional network of atoms connect by
springs is 3RT. There would have been no Mendeleev without Dulong
and Petit.
Majority
Richards
of
of
the
current
Harvard
and
atomic
his
weights
associates
were
determined
primarily
by
the
by
AgCl
precipitation method. Marie Curie and her husband, Pierre Curie,
determined the atomic weight of radium before they announced the
new element.
Marie Curie (1867-1934, 1903 Nobel Physics Prize,
1911년 Nobel Chemistry Prize) and Pierre Curie
(1859-1906, 1903 Nobel Physics Prize)
You will be repeating what Curies did in their atomic weight
determination of radium. In April, 1902, they isolated about 120 mg of
radium from about a ton of pitchblend and announced its atomic
weight in July. In the purification, barium (which is right above
radium in the periodic table and thus have almost identical chemical
properties)
went
with
radium.
Barium
was
finally
removed
by
fractional crystallization. Then the atomic weight of radium was
determined by measuring the weight of chlorine as AgCl in a known
amount of RaCl2.
Curie's lab notebook shows several key figures such as 0.10925 g
RaCl2, 0.10647 g AgCl, Ra/Cl = 3.154. The atomic weight of radium
was reported as 223.3.
Let's consider the significance of Ra/Cl = 3.154. The weight of Cl
in 0.10925 g RaCl2 must have been derived from the weight of Cl in
AgCl. The difference from 0.10925 g should be the weight of Ra.
weight of Ra in 0.10925 g RaCl2 : 0.10925 x (3.154)/(4.154) =
0.08295
weight of Cl in 0.10925 g RaCl2 : 0.10925 x (1)/(4.154) = 0.02630
What is the atomic weight of Cl Curies used in their determination
of Ra atomic weight as 223.3?
0.08295/(223.3/2) = 0.02630/X
X = 35.40
The weight of Cl in 0.10925 g RaCl2 should be the same as the
weight of Cl in 0.10647 g AgCl. What is the atomic weight of Ag
Curies used?
0.02630/35.40 = (0.10647 - 0.02630)/Y
Y = 107.91
The same principles apply in the atomic weight determination of Ba.
The key reaction is the following.
Ag+ + Cl- → AgCl(s)
You will be determining the atomic weight of Ba from known
atomic weights of Ag and Cl. Thereby, you will learn principles and
techniques involved in the historic experiment by Curie.
[Apparatus and Chemicals]
oven, two 100 mL beakers, 10 mL pipette, pipette filler, glass rod,
filter paper, Buchner funnel, Buchner flask, vacuum pump, forceps,
spatula
BaCl2·2H2O solution (to be provided by TA, make 1 L solution
containing about 12 g
BaCl2·2H2O weighed to 0.01 g), AgNO3 (169.87
g/mol)
[Procedure]
Expt 1. Atomic Weight of Barium
1) Using a pencil, mark a filter paper with your group number and
dry in a 120oC oven for more than 10 minutes. Then, weigh it to 0.001 g
accuracy.
2) Transfer 10.0 mL of the barium chloride solution to a 100 mL
beaker.
3) Weigh about 0.5 g AgNO3 and dissolve with distilled water in a
100 mL beaker.
4) Without delay, slowly add all of the AgNO3 solution into the
barium solution constantly stirring with a glass rod.
5) Stir for about 10 min. In the mean time, set up the evacuation
system with the Buchner funnel, Buchner flask, and the vacuum pump.
6) Place the oven-dried filter paper on the Buchner funnel, turn on
the vacuum pump, and use distilled water to fix the paper on the
funnel.
7) Pour the solution containing AgCl precipitate to the filter paper
on the funnel. Let not the precipitate collect on the wall of the
funnel. Use spatula and wash bottle to transfer all of the precipitate.
Incomplete transfer is the major source of error.
8) If the filtrate is cloudy or any precipitate is visible in the
filtrate, repeat filtration.
9) Wash the precipitate several times with distilled water. Make
sure that all precipitate remain on the filter paper.
10) Dry the precipitate together with the filter paper in a 120oC
oven and weigh to 0.001 g accuracy after brief cooling.
[Caution]
- Avoid skin contact with the AgNO3 solution.
- Do not hold the filter paper with your finger. Use forcept.
[Data Analysis]
1) Calculate the weight of Ba and Cl in the BaCl2 used.
2) Calculate the atomic weight of Ba and compare againt the book
value.
The moisture content of the filter paper is about 4%. Take this into
account when calculating the weight of AgCl from the total weight of
the precipitate and the dried filter paper if the filter paper is not
dried initially.
[Additional Material]
Assignment for Internet Search
1. Nobel Prize for Theodore Richards
Theodore Richards (1868-1928)
Professor, Harvard University
1914 Nobel Prize in Chemistry
2. Methods for Atomic Weight Determination
♦ gas density
Cannizzaro (1826-1910)
relative weight
oxygen
-------------------------------------------------------H2O
18
16
NO
30
16
N2O
44
16
NO2
46
32
SO2
64
32
CO2
44
32
O2
32
32
O3
48
48
♦ Dulong-Petit's law
♦ gravimetric analysis
3. Scanning Tunneling Microscopy