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- Chapter 18
- By Dr. Steve Rapp
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- State the law of conservation of mass.
- Know Antoine Lavoisier’s contribution to science.
- Describe the Phlogiston Theory.
- Be able to use the universal accounting equation to account for mass in
nuclear and chemical environments.
- Review balancing equations.
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- The total amount of matter in a closed system remains constant
- This law holds true in chemical reactions but is modified in cases where
atoms disintegrate and matter is converted to energy or energy is
converted to matter.
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- Lavoisier's experiments were among the first truly quantitative
chemical experiments ever performed. He showed that, although matter
changes its state in a chemical reaction, the quantity of matter is the
same at the end as at the beginning of every chemical reaction. These
experiments provided evidence for the law of the conservation of mass. Lavoisier
also investigated the composition of water, and he named the components
of water oxygen and hydrogen.
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- Some of Lavoisier's most important experiments examined the nature of
combustion, or burning. Through these experiments, he demonstrated that
burning is a process that involves the combination of a substance with
oxygen. He also demonstrated the role of oxygen in animal and plant
respiration. Lavoisier's explanation of combustion replaced the
phlogiston theory, which postulates that materials release a substance
called phlogiston when they burn.
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- Weight loss occurs when combustibles are burned because they lose
phlogiston.
- Fire burns out in an enclosed space because it saturates the air with
phlogiston.
- Charcoal leaves very little residue when burned because it is made
mostly of phlogiston.
- Animals die in an airtight space because the air becomes saturated with
phlogiston.
- Some metal calxes turn to metals when heated with charcoal because the
phlogiston from the charcoal restores the phlogiston in the metal.
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- Since according to the Law of Conservation of mass matter cannot be
created or destroyed, there is no generation or consumption of mass.
- Therefore, the UAE simplifies to:
- Accumulation = Final Amount – Initial Amount and Net Input =
Input-Output
- Therefore, Accumulation = Net Input
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- During nuclear reactions or in the case of particle accelerators,
conservation of mass is violated.
- In nuclear reactions, mass is consumed and converted to mass.
- In particle accelerators, mass is generated from energy.
- Remember E = mc2.
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- In the special case of nuclear reactions the UAE becomes:
- Final Amount – Initial Amount = Input – Output + Generation -
Consumption
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- Fission is the process whereby a heavy element such as uranium-235 is
broken down into smaller fragments. The mass of the smaller fragments is
actually less than the original heavy element.
- 235U + n à
Fission products + 234 n
- E = 3.2 x 10-11 J/atom
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- The most successful and promising fusion confinement device is known as
a tokamak.
- The word tokamak is actually an acronym derived from the Russian words toroid-kamera-magnit-katushka,
meaning "the toroidal chamber and magnetic coil."
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- This donut-shaped configuration is principally characterized by a large
current, up to several million amperes, which flows through the plasma.
- The plasma is heated to temperatures more than a hundred million degrees
Celsius (much hotter than the core of the sun) by high-energy particle
beams or radio-frequency waves.
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- Objective: Investigate the relationship between temperature and plasma
pressure, the relationship between temperature and plasma density, and
the low temperature limit at which a toroidal plasma current is
produced.
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- Accelerators propel protons to almost the speed of light requiring large
amounts of energy to do so.
- When these protons impact stationary objects, or collide with particles
moving in the opposite direction the energy is converted into mass.
Positrons, bosons, fermions, leptons, hadrons, classons, neutrinos, or
muons may be produced.
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- Since mass is not lost or gained in chemical reactions the entire UAE is
used.
- Final Amount – Initial Amount = Input – Output + Generation –
Consumption
- But since mass is not generated or consumed the UAE can become:
- Final Amount – Initial Amount = I + O
- If the reactor is run at steady state we have 0 = Input – Output.
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- Each day, a power plant burns 20,000 tons of coal that contains 1%
sulfur. The plant produces 1000 tons per day of ash containing 2%
sulfur. The power plant is operated in a steady state manner so that no
ash accumulates in the furnace. How much sulfur is emitted up the stack?
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- Remember the law of conservation of mass. No element mass is lost or
gained and the plant is at steady state.
- Input = Output so Mass of S in coal feed = Mass of S in Ash + Mass of S
in stack gas, therefore:
- .01 (20,000 ton) = .02 (1000 ton) + S
- 200 ton = 20 ton + S
- S= 200 ton – 20 ton = 180 ton
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- In order to apply the UAE to a chemical reaction involving chemical
species the chemical equation must be balanced.
- The UAE must be applied to each chemical species as shown in example
18.7, pages 511-513 in your text.
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- Determine the reactants and products.
- Reactants written on the left
- Products written on the right
- Assemble the parts of the chemical equation (write the correct formulas
for the reactants and products).
- Balance the equation by adding coefficients.
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- Balanced equations have the same number and kind of atoms on each side.
- Coefficients, not subscripts, may be changed to balance an equation.
- Never change a subscript to balance an equation.
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- Balance the following: zinc +
hydrochloric acid yields zinc chloride + hydrogen.
- Zn + 2 HCl ZnCl2 + H2
- Remember, never change a subscript; changing the subscript changes the
substance.
- Add only coefficients.
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- Balancing
- Must have same number of atoms of each type on both sides.
- Achieve this by adjusting coefficients in front of formulas.
- Example: combustion of propane
in air to give carbon dioxide and water.
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- C3H8(g) + O2(g) CO2(g) + H2O(l)
- Balance C:
- C3H8(g) +
O2(g) 3CO2(g)
+ H2O(l)
- Balance H:
- C3H8(g) + O2(g) 3CO2(g) + 4H2O(l)
- Balance O:
- C3H8(g) + 5O2(g) 3CO2(g) + 4H2O(l)
- Meaning: 1 mol C3H8
reacts with 5 mol O2 to form 3 mol CO2 and 4 mol
H2O
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- Physical state symbols
- g = gas
- l = liquid
- cr =crystalline solid
- aq=water solution
- s= solid
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