Phlogiston - dephlogisticated air - Lavoisier

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Phlogiston - dephlogisticated air - Lavoisier
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Why do things burn?
Early Greeks ? element fire
Alchemists ? principle of sulfur
Phlogiston Cavendishs inflammable air?
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Georg Ernest Stahl (1660 1734)
Johann Joachim Bechter(1635 1682)

• Tera pinguis
• principle of inflammability

• Principle is phlogiston
• (Greek to set on fire)

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Phlogiston Theory
The Elements according to Becher (1669) and
Stahl Three Elements and Three Earths
Fire, Air, and Water (as before)
Sub-classified old concept of earth by
different responses to combustion
- Vitreous (inert) earth
- Volatile (distillable) earth
- Combustible earth phlogiston (Stahl)
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Becher Stahl Theory
- Flames extinguished ? air saturated with
phlogiston.
- Mice die in airtight space ? air saturated with
phlogiston
- Charcoal ? little ash after burning (nearly
pure phlogiston)
- Ores smelted into pure metals because
phlogiston transferred from charcoal  to ore
- Heat metals ? phlogiston released into air
leaving calx
- Rusting of metal ? equivalent to burning of wood
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Phlogiston
combined with other substances - condition not
perceivable by the senses
COMBINED PHLOGISTON, or combined fire
phlogiston dislikes freedom always striving to
conceal itself in combustible substance.
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Phlogiston
escaped as substance burned, ? apparent to
senses as fire or flame
FREE phlogiston
Combustible objects rich in phlogiston

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Phlogiston
after burning? original substance minus
phlogiston
To restore original combustible substance ?
heat residue with something that burned easily -
freed phlogiston combines with residue
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Phlogiston
explanation

• more combustible a substance, more phlogiston it
  contained

- free phlogiston sought to combine with suitable
substance

• mix phlogisticating agents, such as charcoal,
  phosphorus, oils, fats, etc., with the residue of
  the original substance

• heat mixture ? phlogiston freed unites with the
  residue.

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Phlogiston
Wood is high in phlogiston

• Ore is low in phlogiston

Charcoal is high in phlogiston

• Calx is low in phlogiston

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Problems with Phlogiston Theory
combustion and rusting ? different versions of
the same phenomenon?
? combustion of wood phlogiston leaves so
rapidly it heats its surroundings and becames
visible as flame
? rusting -- loss of phlogiston slower and no
flame appears
Wood burns ash lighter loss phlogiston
rusty metal (calx) heavier when phlogiston lost?
Positive and negative weight phlogiston?
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No Problems with weight
Not worried about accurate measurements
variation in weight an unimportant "accident"
Weight changes ? only thought about solids
Weight change gases but only worried about
existence
phlogiston light, lightened substance
containing it ? driving it out of metal naturally
left residue heavier.
Stahl ? Weight increase due to air entering
metal to fill vacuum left after phlogiston escaped
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Phlogiston Theory
phlogiston theory was a step towards chemistry
and away from alchemy.
Moved away from idea of a "spirit" in metals
that could not be seen, felt, or appreciated by
any of the senses
substituted principle of phlogiston ? more
tangible than the "spirit,"
- could be seen and felt as free phlogiston
- weighed and measured as combined phlogiston.
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Phlogiston Theory
Universal explanation for all the known chemical
phenomena?
as experiments multiplied, became evident theory
failed to satisfactorily explain certain
laboratory reactions.
modifications introduced giving theory a
flexibility to cover all cases.
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Phlogiston
modifications contradicted each others ?
simple theory too cumbersome from number of
modifications
no satisfactory substitute better than anything
before or could be suggested.
phlogiston theory cannot be said to have
finally succumbed until the opening years of the
nineteenth century.
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Need to think gases
explanation of changes in weight during
combustion due to gases that appeared or
disappeared while the products were forming
but no attempt in Stahl's day to take gases
into account except to note their existence.
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Need to think gases
The Gases Ash was lighter than wood? what about
vapors given off by burning wood? Not considered.
Rust heavier than metal, but had rust gained
anything from the air? Not considered.
Before deficiency could be corrected, chemists
had to grow more familiar with gases and the
problems with confining for study.
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Pneumatic Trough
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Joseph Priestly (1733 1804)
Identified seven distinct airs with mercury
trough
Lack of conclusions concerning his data
Others made use of them in developing their
theories
Produced a brick-red calx when heated mercury
in air
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Priestleys new air

• Discovered another form of air
• Mercury heated in air ?formed a red substance
  the precipitate per se or calx ( mecuric
  oxide)
• When heated with no air - calx changed back into
  mercury and gave off a gas with most unusual
  properties
• Mice were particularly active in the new air

Priestley tried breathing some and found
himself feeling particularly light and easy
for some time afterwards. Who can tell but that,
in time, this pure air may become a fashionable
article in luxury"
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Priestley and dephlogsticated air

• Combustibles burned more brilliantly and rapidly
  in this gas
• A smoldering splint of wood thrust into a
  container of the gas burst into flame
• Priestley explained this phenomenon in terms of
  the phlogiston theory
• Splint of wood must given off phlogiston at a
  rapid pace
• The gas must have little or no phlogiston
  allowing it to accept the new phlogiston so
  easily
• Priestley therefore called his new gas
  "dephlogisticated air"

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Priestley - dephlogisticated air
Component of ordinary air
Responsible for combustion
Made animal respiration possible
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Cavendish burns inflammable air

• One theory ? phlogistication produces fixed air
• But, Cavendish showed
• Inflammable air common air explosion and what
  remained didnt cause precipitation of limewater
  not fixed air
• What was it?

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Cavendish burns inflammable air

• Burned inflammable air common air in closed
  container
• No loss of weight
• Inside of vessel became dewy
• 423 measures inflammable air phlogisticated 1000
  measures of common air (optimal 2 volumes
  inflammable to 1 volume dephlogisticated air)
• Note
• Cavendish still interprets in terms of phlogiston
  theory
• Lavoisier rejects phlogiston theory interprets as
  evidence water not a simple substance but product
  of the combustion of two gases
• Hard blow at Greek theory of the elements

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Cavendish and increase in weight of calx

• Dephlogisticated air (water F)

Dephlogisticated air inflammable air (F)
water or (F waterELEMENTAL)
Calx (earthy base of metal - F ) water ?
weight
Heat calx ? F from water combines with calx
(earthy base metal F) to form metal
water devoid of F becomes dephlogisticated air
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Lavoisier (1743 1794)
Quantification
No Transmutation
No Phlogiston
Conservation of Mass
New Chemical Nomenclature
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Attack on Myth of Transmutation - late 1770s

• Lowering of levels of oceans attributed to a
  transformation of water into earth
• Hypothesis accepted by chemists who boiled and
  found an earthy residue at the bottom of
  container
• Lavoisier boiled water for a hundred days in a
  "pelican
• Showed residue not due to transmutation of water,
  but to dissolving of pelican's inner surface
• Applied quantitative analysis
• Weighing before and after experiment

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Lavoisier and Combustion
Heat metals in closed containers with limited
supply of oxygen
Maximum amount of calx formed (air saturated
with phlogiston??)
Calx known to weigh more than metal
But, metal, calx, air, flask weighed same
before and after
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Lavoisier and Combustion
If metal-calx gained mass something must have
lost mass
Must be the air ? partial vacuum must exist
Opened flask ? air rushed in
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Lavoisiers Explanations
Ores metal and gas ? heating with charcoal
releases gas forming CO2 leaving metal
Isnt this what Stahl said about phlogiston?
Lavoisiers gas-transfer theory fits weight
changes with combustion
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Lavoisiers Explanations
Wood burns through addition of air, but doesnt
gain weight?
CO2 formed a gas which vanishes into air
If burned wood in closed container ? ash
vapors formed air original wood air
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Lavoisiers Burning Lens
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Conservation of Mass

• Lavoisier states mass never created nor destroyed
  but shifted from one substance to another

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Lavoisier and Oxygen Theory (Part I)

• Heated 4 ounces of mercury in a flask with 50
  cubic inches of air
• 8 cubic inches of air used up (weighing 3 grains)
• remaining mercury and red powder on surface of
  mercury were 3 grains heavier
• new air in flask could not support life or
  burning

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Lavoisier and Oxygen Theory (Part I)

• Removed all the red powder and heated it in
  another flask
• new gas produced volume 8 cubic inches and
  weight of 3 grains
• Originally ? 8 cubic inches of air used up
  (weighing 3 grains)
• new gas supported life and burning
• substances burned in it produced acids therefore
  oxygen acid producer (Greek)

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Test of Phlogiston vs.Oxygen Theories
Phenomena under investigation
combustion
specifically combustion of mercury
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Test of Phlogiston vs.Oxygen Theories
Mercury is heated

• Model 1 Phlogiston Theory

phlogiston is given off when mercury is heated
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Test of Phlogiston vs.Oxygen Theories
Predictions under Phlogiston Model
Water Level?
water level under bell jar should go up / down
Why?
Phlogiston added to air
Weight?
mercury/ash should weight more / less than
original mercury
Why?
Phlogiston driven out
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Test of Phlogiston vs.Oxygen Theories
Mercury is heated

• Model 2 Oxygen Theory

oxygen is taken up when mercury is heated
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Test of Phlogiston vs.Oxygen Theories
Predictions under Oxygen Model
Water Level?
water level under bell jar should go up / down
Why?
Oxygen taken up from air
Weight?
mercury/ash should weight more / less than
original mercury
Why?
Oxygen taken up from air
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Test of Phlogiston Theory
Data
water level under bell jar went up
mercury/ash combination weighed more than
original mercury
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Test of Phlogiston Theory
Data and prediction disagree with phlogiston
theory
Data and prediction agree with oxygen theory
Phlogiston model fails to support combustion of
mercury in this experiment
oxygen model supports combustion of mercury in
this experiment
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What is air?

• Cavendish
• ? 79.167 phlogisticated air
• ? 20.833 dephlogisticated air
• "...whether there are not in reality many
  different substances confounded together by us
  under the name of phlogisticated air"
• Blacks Fixed Air??
• Lord Rayleigh (100 years later) established a
  third gas to be argon

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What is air?
Air mixture of two gases in 1 to 4 ratio
(Lavoisier)
One-fifth was Priestleys dephlogisticated air
- Combined with burning or rusting materials
- Transferred from ore to charcoal
- Essential to life
Remaining four-fifths could not support life ?
azote (Greek meaning no life)
Today
0.9 argon
78 nitrogen
21 oxygen
0.04 carbon dioxide
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Oxygen and Life
Lavoisier life supported by process like
combustion
Breath in air high in O2 but out high in CO2
Measurements dont fit ? some O2 must combine
with Cavendishs hydrogen to make water
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The idea of an element

• Ancients ? elements fundamental types of matter
• Small number of elements ? unification yield
  more complete and powerful explanation
• Ancient Greeks ? four Earthly elements and aether
  in the celestial sphere
• Alchemists added the principles of sulfur and
  mercury to explain the nature of metals -
  Paracelsus later added salt
• These systems of elements or principles persisted
  into 1700s

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The idea of an element - Boyle

• In The Sceptical Chymist ? gold behaves
  chemically as would expect an element to behave
• Gold can be chemical combined with other
  materials, then separated out again into its
  pure form
• Gold has consistent properties - same density,
  color, malleability, etc
• No attempt to identify other materials satisfying
  the criteria he gave ?did not develop a system of
  elements

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The idea of an element

• Lavoisier did develop such a system of elements
• Many of Lavoisiers elements well known in the
  time of Boyle and Newton
• Discovery of various airs instrumental for
  Lavoisier
• trying to make sense of these new observations
• idea of oxygen, hydrogen, and nitrogen as
  elements
• hypothesis water is not an element but composed
  of the elements hydrogen and oxygen

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The idea of an element - Lavoisier

• Altered definition of an element and criteria to
  identify elements
• Boyle and Newton - retained ancient idea of
  elements as a small number of truly fundamental
  types of matter
• Lavoisier ? all we dare venture to affirm of any
  substance is, that it must be considered as
  simple in the present state of our knowledge, and
  so far as chemical analysis has hitherto been
  able to show.
• Table of Simple Substances
• - not eternal unchanging list of elements
• - best list given the current state of chemical
  knowledge
• - contained 33 elements
• - no pretense of high degree of unification

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New Chemical Nomenclature

• Collaboration of Lavoisier and 3 other French
  chemists

• Helped publicize the new anti-phlogistic
  chemistry

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Lavoisiers Textbook
Modern definition of element
Light and caloric
Law of conservation of mass