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Atoms, Dalton, conservation of mass
1. You have a chemical in a sealed glass container filled with air. The system has a mass of
250.0g. The chemical is ignited by means of a magnifying glass focusing sunlight on the
reactant. After the chemical is completely burned, what is the mass of the setup? Explain
your answer.
Basically, this is Lavoisier experiment. He studied the true nature of combustion;
His careful weightings showed that mass was conserved in chemical reactions and
that combustion involves reaction with oxygen.
2. Heat is applied to an ice cube in a closed container until only steam is present. Draw a
representation of this process, assuming you can see it at an extremely high level of
magnification. What happens to the size of the molecule? What happens to the total mass
of the sample?
If we were to look at this system under a microscope, we would first see the water
molecules in the ice cube in a well-ordered system. There are two types of forces
holding the system together. The first is intramoleuclar bonds. These are the
bonds that hold the atoms together to make molecules. The second type of bond is
called an intermolecular bond. These bonds are weak attractive forces between
molecules. The water molecules in this solid state are restricted to vibration only.
The intermolecular forces are very strong because the molecules are moving so
slowly. The water molecules cannot move around at all. As heat is added to the
system, the water molecules gain more energy and start to vibrate faster until the
vibrations separate the individual water molecules. Now they have enough energy to
move around (rotate and vibrate), but they are still in really close proximity. The
intermolecular forces are strong but not as strong as in a solid. You can pour a liquid
because the molecules are able to move more freely. Finally, enough energy is so
that the water molecules are able to move with even more energy. They are moving
so fast that they cannot exert any type of intermolecular forces on each other.
They are separated from each other. Through this entire process the molecules do
not change size or mass. They merely move more.
3. Dalton assumed that all atoms of the same element were identical in all their properties.
Explain why this assumption is not valid.
Dalton was unable to “see” into the atom. Individual atoms cannot vary by the
number of protons, but they can vary by the number of neutrons. This changes the
atomic mass for atoms in an element. These atoms are isotopes of each other.
4. What evidence led to the conclusion that cathode rays had a negative charge?
The ray was repelled by the negative pole of an applied electric field, attracted to a
positive pole of an applied field. The rays traveled in straight lines from the
cathode when the current flowed. Some other characteristic properties of the rays
are: the ray was independent of current sources, tube material, cathode material
and the gas that partially filled the tube. The rays were invisible, but gave off light
when they strike glass coated with zinc sulfide and the particles had mass.
5. What discoveries were made by J.J. Thomson, R.A. Millikan, and Lord Rutherford? How did
Dalton’s model of the atom have to be modified to account for these discoveries?
In Thomson’s experiment, a beam of electrons of known energy is deflected
by a magnetic field—the beam curves. The radius of the curve depends on the
energy of the electrons, the mass of the electrons and the strength of the magnetic
field. The energy of the electrons in turn depends on their charge. Thomson knew
the magnetic field strength but neither the mass nor the charge of the electron.
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To make the experiment clearer, consider an experiment very similar in principle,
Attach a weight to a spring and hold the other end of the spring, whirl the weigh
about you over your head. The distance between your head and the mass (that is
the radius of the curve depends on three things, the mass of the spring , the speed
of weight, the stiffness of the spring. To make the radius of the circle bigger, you
can increase the weight, increase the speed, or use a slacker spring. In Thomson
experiment, the mass is the mass of the electron, the speed is analogous to the
energy of the electron, and the spring is the magnetic field strength.
Millikan’s experiment is designed to measure the fundamental unit of charge.
Oil drops are charged with X-rays and allowed to fall between two charged palates.
the charge on each drop can be found by varying the voltage between the plates
until electrical force felt by the charged drop just balances the force of gravity.
Then the drop stands still. The charge on each drop always is found to be an exact
multiple of 1.6022 X 10-19 coulomb.
Geiger and Marsden beamed aα particles at thin gold foil. Most of the
particles passed straight through, but some were deflected, a few by a great angle.
Rutherford interpreted these data as evidence for a dense, positive mass center
for the atom surrounded by lots of space, a cloud that contains electrons.
Dalton’s atomic theory doesn’t account for subatomic particles. It is more based on
macroscopic information than the microscopic world of the atom.
6. What are the distinction between atomic number, mass number or atomic mass, and the
average atomic mass of an element? How is this different from the mole and Avogadro’s
constant?
The average atomic mass of an element is the decimal number appearing in the
elements block in the periodic table. The individual atoms of an element are of
different masses; the average atomic mass accounts for all the stable isotopes
present in the element. The atomic mass is the sum of the protons and neutrons in
the atom. The atomic number is the number of protons in the nucleus; each atom
has a unique number of protons. The mole is the counting unit of chemistry. It is
the unit given to represent the number equal to the number of carbon atoms in
exactly 12 grams of pure 12C. One mole of something is 6.022 X 1023 anything.
7. Each of the statements given below is true, but Dalton might have had trouble explaining
some of them with his atomic theory. Give explanations for the following statements.
Ethyl alcohol and dimethyl ether have the same composition by mass (52% carbon, 13%
hydrogen and 35% oxygen) yet the two have different melting points, boiling points, and
solubilities in water.
Ethyl alcohol and dimethyl ether have the same empirical and molecular formula but
different structural formulas. The structure of a molecule will also affect its
chemical and physical behavior.
Burning wood leaves an ash that is only a small fraction of the mass of the original wood.
The system was not closed. Matter could escape in the form of carbon dioxide and
water, the products of combustion. Since they are invisible……
Atoms can be broken down into smaller particles.
Dalton just didn’t have the tools to break down an atom and study the particles.
8. State and apply the laws of conservation of mass and definite composition
Mass is neither created nor destroyed and a compound has the same composition no
matter what its source.
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9. State the basic assumptions of Dalton’s atomic theory
Elements are composed of indestructible atoms. Atoms of a given element are alike;
atoms differ between elements. Atoms unite in small whole number ratios to form
compounds.
10. State and apply the law of multiple proportions.
The law of multiple proportions states that the ratio of the masses of one element
combined with a constant mass of another element is a small whole number.
Compounds are made up of atoms combining and all atoms having the same or at least
the same average atomic mass.
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