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Phlogistonvs

Oxygenthe story of combustion

The Phlogiston Theory

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Georg Stahl: 1660-1734

Materials that burned were thought to contain an essence of fire: “phlogiston”.

Phlogiston was an invisible fluid that was released from objects into the air as they burned.

Materials that burned easily and left little residue, like paper, were thought to be nearly all phlogiston. Materials that burned poorly were thought to contain very little phlogiston.

Burning would cease when the material ran out of phlogiston, OR the air became saturated with it and could hold no more.

Phlogiston Applied1. candle burns in a

sealed jar

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air begins to fill with phlogiston from candle as it burns

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air is saturated with phlogiston - so the candle goes out

the candle “disappears” as phlogiston is released, because wax is mostly phlogiston

Phlogiston Applied2. wood burns to ash

, or metal burns to ashy “calx”

unburned wood/metal contains much phlogiston

air has little phlogiston

burned ash has lostits phlogiston

air is now saturatedwith phlogiston

phlogiston is releasedas wood or metal burns

The Problem of Mass

• Chemists had long known that fuels like wood and wax LOST weight when they were burned, but metals GAINED weight.

• When the phlogiston theory was first proposed, conservation of mass was not yet an accepted idea. So changes in mass were noted but did not need to be explained in terms of conservation.

• But the discrepancy between the behavior of metals and fuels was puzzling. What was happening?

Stahl’s Solution: Levity• Aristotle had believed each of his four elements

(earth, air, fire, and water) had a “natural direction” of movement. Earth and water flowed downward, and fire and air flowed upwards.

• Phlogiston was an essence of fire. So Geog Stahl hypothesized that phlogiston would desire to flow upwards, too. This would give metals full of phlogiston a “buoyancy” that would lessen their weight on a balance. When they burned, they lost that buoyancy and thus weighed less. This explained the weight gain of metals.

• Fuels like wood lost so much of their substance when burning that this buoyancy affect was not noticed.

Lavoisier: A New Perspective

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Antoine Lavoisier 1743-1794

Once Lavoisier had established the law of conservation of mass, he took another look at combustion reactions.

In sealed systems, the total mass of reactants and products remained the same when objects burned.

So why did fuels lose mass and metals gain mass?

Lavoisier had to “think outside the box” and look at combustion in a new way to solve this problem.

Oxygen Addition1. candle burns in a

sealed jar

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OXYGEN from the air combines with wax in the candle as it burns

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air has lost its oxygen - so the candle goes out

the candle “disappears” as the wax + oxygen form carbon dioxide gas

Oxygen Addition2. wood burns to ash

, or metal burns to ashy “calx”

unburned wood/metal can react with oxygen

air contains oxygen

burned wood ashes havelost carbon dioxide gas

air is now depleted of oxygenbut may have CO2

oxygen is combined with the wood or metal to form

an oxide

burned metal forms solid metal oxide

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