Alchemy Unit 8: Gas Laws

Evangelista Torricelli (1608-1647)

- 1643: fills up a glass tube (sealed at one end) with mercury. The tube is inverted and lowered into a dish that also contained mercury.

- Observation: The mercury did not flow out (neither did it stay completely filled).

- The level of the mercury in the tube hovered consistently 30 inches (760 mm) above the level of the mercury in the dish.

- Invented the barometer

Barometer

Instrument for measuring atmospheric pressure

- Air does have mass and so can cause a force to be exerted towards the earth's center.

- Weight of mercury is balanced by atmospheric pressure to a constant value (depending on elevation).

- Boyle would refer to this as the "spring of the air."

Boyle's Law

- Pressure of a gas and its volume are inversely proportional.

- P ∝ 1/V or V ∝ 1/P

P•V = constant

- P1V1 = P2V2

Joseph & Etienne Montgolfier (1740-1810 and 1745-1799)

- Nov. 1783: launched a hot air balloon made from linen from Paris.

- Rose 3000 feet, floated for about 25 minutes, and landed 5 miles away.

Jacques Alexandre Cesar Charles (1746-1823)

- Launched a balloon made from India rubber.

- Used a recently isolated explosive gas, instead of hot air, called "inflammable air" by its discoverer Henry Cavendish.

- Generated inflammable air using 1000 lbs of iron filings and 500 lbs of sulfuric acid.

- This balloon rose 2000 ft and drifted 27 miles away over 2 hours.

- Ben Franklin was standing in the crowd.

Charles' Law

V ∝ T or T ∝ V

V/T = constant

At constant pressure = V1 / T1 = V2 / T2

Guillaume Amontons (1663 - 1705) - Paris

- Instrument-maker and physicist (Paris)

- Developed barometers, hygrometers, thermometers.

- Considered the idea of absolute zero.

- Pioneered study of friction.

- Recognized the directly proportional relationship between pressure and temperature (if volume is constant).

Joseph Louis Gay-Lussac (1778-1850) - Aquitaine/Paris

- French Professor of Chemistry at Ècole Polytechnique and later of Physics (Sorbonne).

- Assistant and successor to Claude Berthollet.

- Recognized the directly proportional relationship between pressure and temperature (if volume is constant).

Amonton's Law (Gay-Lussac's Law)

P ∝ T or T ∝ P

P/T = constant

P1 / T1 = P2 / T2 (At constant volume)

Amedeo Avogadro (1776-1856) - Piedmont

- Responds to work of Gay-Lussac (and others) about the combination of gases ("Gases combine in whole number volumes").

- 1811: speculates upon a relationship between mass, volume, and the number of particles of the gas.

- Proposes that equal volumes of gases contain equal numbers of gas particles (molecules).

- Hypothesis neglected by his contemporaries. The idea of molecules had not yet gained traction.

Avogadro's Law

V ∝ n or n ∝ V

V/n = constant

V1 / n1 = V2 / n2 (At constant T and P)

Stanislao Cannizzaro (1826-1910) - Sicilian

- First chemical congress took place in Karlsruhe, Germany in 1860.

- Campaigned for recognition of Avogadro's theory. Soon afterwards, chemistry community actively accepts Avogadro's Law.

- Volume of a gas is directly proportional to amount (at same temperature and pressure).

Ideal gas equation

PV = nRT

R = 0.08206 l•atm/mol•K

Kinetic Molecular Theory

Explanation for the behavior of gases, including the historical gas laws. Contains 3 postulates.

Postulate 1

A gas consists of a large collection of individual particles with empty space between them. The volume of each particle is considered negligible compared to the volume of the sample.

Postulate 2

Gas particles are in constant, random, straight-line motion, except when they undergo collisions.

Postulate 3

Collisions are elastic. The total kinetic energy is constant.

Third law of thermodynamics

- The entropy of a pure crystalline solid at absolute zero (0 K) is zero.

- Temperature is directly proportional to motion (velocity).

Kinetic Molecular Theory (2)

- Pressure comes from the combined impact (force) of gas particles on the container walls.

- As the number of molecules increases, so will collisions with each other and the walls of the container.

- The force of the impact increases with increased velocity (F = ma) that increases with temperature.

- External and internal pressure will equalize if the container is expandable.

John Dalton - (1766-1844) English

Dalton's Law of Partial Pressure: In a mixture of unreacting gases, the total pressure is the sum of the partial pressures of each of the individual gases.

Ptotal = P1 + P2 + P3 + ...