Lec 4: Revolutions in Chemistry, Phlogiston Flashcards
Explain Phlogiston Theory
Emerged in 18th C, competed with three theories:
- > Aristotelian-Paracelsian
- > Newtonian
- > Georg Stahl’s theory, stating that properties aren’t just the sum of substances; modification in combination. Mixts make something new; aggregates are separable.
- > “Phlogiston theory”
Phlogiston theory = “burnt” theory
- > combustion & calcination of metals
- > the “matter of fire”; a subtle material
- > only detected when it leaves a material
- > takes form of fire/light/heat
Explains world as five chem. principles which carry chemical qualities into reactions
- > air (not part of rxn, gives “gaseousness”
- > water (gives rxn fluidity)
- > vitreous earth (gives fusibility)
- > inflammable earth (inflammability)
- > metallic earth (shine)
Phlogiston Explanations for Combustion, Calcination… ?
Phlogiston explains combustion
- > in rxn, phlog flows (like osmosis)
- > air is devoid of phlogiston; readily absorbs
- > so anything with phlog burns in air
- > air is devoid of phlogiston; readily absorbs
- > wood burns leaving ash; much phlog
- > charcoal little physical residue; pure phlog
- > burn charcoal to extract metal from ore
- > metal absorbs phlogiston, causing separation
Phlogiston explains calcination
- > when heated, metals leave a residue called “calx” (aka rust)
- > heating metal makes it give phlogiston
- > metal - phlogiston = calx
Phlogiston is loose/flexible, explains much
- > everything explainable in phlog rxns
- > identify how phlog flows
- > respiration
- > putrefaction
- > fermentation
- > exothermic processes
- > rusting, oxidation
- > combustion and calcination (the major categories of chem rxn) had one explanation
Problem with Phlogiston?
METALS INCREASE WEIGHT IN CALCINATION
- > most chemists didn’t work quantitatively
- > qualitative explanation enough
- > unifying theory for all processes
Pneumatic Chemistry - roots?
By dawn of 18th C, little or nothing known about air/gases
- > changed with Hooke & Boyle’s pump
- > it had role in life & combustion
Air was seen as an environment for reactions, never having a role in them
- > no easy way to examine/test air
- > changed with Hales’ pneumatic trough
Discuss the Pneumatic Trough
Reverend Stephen Hales (1677-1761)
Made it possible to handle air
- > caught and held air
- > can be contained
- > let it be subject to tests
- > physical and chemical
But Hales thought there was one single kind of “air”
Trough became a staple of labs
Joseph Black (1728-1799) and airs
Realized chemical role of air in rxns
- > demonstrated there were two kinds of air
- > created one by heating magnesia alba
- > then treated limestone with mineral acids
Called airs “atmospheric” and “fixed”
- > fixed because it had been trapped inside solid magnesium carbonate, and he freed it
- > fixed air also produced in respiration, fermentation, combustion
Discovered normal air contained fixed air
- > part of it but separate
- > SHOWED: reactions involving air can be followed with a balance, and measure weight gain/loss
Oxygen
Joseph Priestley (1733-1804) was Britain’s OG chemist, and Henry Cavendish (1731-1810)
- > noticed fixed air produced in fermentation
- > studied gases over mercury
- > let them study those soluble in water
Priestley studied many kinds of airs, e.g.
- > nitrous air (nitric oxide)
- > phlogistcated air (nitrogen)
- > nitrous vaper (nitrogen dioxide)
- > acid air (HCl collected over mercury)
But most importantly, Oxygen
- > heated mercury with focused sunlight
- > like nitrous oxide, but insoluble in H2O
- > and made candle burn
- > made mouse live 2x long
- > lightness in breathing himself
Oxygen and Phlogiston?
In phlog, O2 should be dephlogisticated air
- > burning gives off phlogiston
- > O2 must have very little to make things burn so readily
Cavendish’s Experiment
Eudiometer - combines/mixes two gasses, and runs a spark through mixture
He uncovered water using it:
- > measuring volume before/after, could determine amount of O2 in ir
- > combining Hydrogen with dephlog air, he made water, with equal weight of the gases combined
- > 1784, knew water H, O, and proportion
However Cavendish was a phlogistonist
- > explained as phlog exchange
- > H was “overdephlogisticated water”
- > O was “dephlogisticated water”
- > when combined, both end up with just enough phlog to become water
Emergence of Modern Chemistry
Antoine Laurent Lavoisier (1743 -1794)
- > attacked phlog theory
- > weights during chemical change
Foundations of modern chemistry
- > based on the new element (and compound)
- > explained burning/rusting with oxygen
- > emission/absorption of heat included in chemical system
- > changed direction of chemistry; made it the science we know today
- > couldn’t kill phlog; rebirthed as Caloric
Lavoisier made chem more like Newton
- > only frame hyps grounded in nature
- > approached something like CoM
Reinterpreted results for other people
- > completed Black, Priestley, Cavendish work
- > showed Priestley’s mercury air weight gained = lost
- > and could recombine
- > from Cavendish, figured out H20 compound
- > Oxygen + inflammable air (=== hydrogen)
- > constant, accurate chemical balance throughout experiments
- > e.g. white residue in distillation
- > showed not water to earth, but matterial dissolved out
