QCHEM Chapter 13: Gases

Gas

• A substance that Uniformly fills any container.

• Mixes completely with any other gas.

• Exerts pressure on its surroundings.

• Atmospheric gases should be generally colorless, odorless, and tasteless.

Pressure

Pressure=force/area

SI UNIT is Pascal (Pa), which is equivalent to one Newton per square meter.

Barometer

• Device used to measure atmospheric pressure.

• Mercury flows out of the tube until the pressure of the column of mercury standing on the surface of the mercury in the dish is equalto the pressure of the air on the rest of the surface of the mercury in the dish.

• In 1644, Italian scientist, Torricelli, described the first barometer.

• On a typical day at sea level, the atmosphere will support a column of mercury 760 mm high.

• Mercury is 13.6 times denser than water. (water will float on top of it)

Manometer

• Device used for measuring the pressure of a gas in a U-shape container of liquid in which there is a difference in the PRESSURES acting in the two arms of the tube (this causes the liquid to reach different heights in the two arms).

Boyle’s Law

• Boyle studied the elasticity of gases in a J-tube

• By adding Mercury to the open end of the tube, he trapped a small volume of air in the sealed (hook) end.

• Boyle studied what happened to the volume of the gas in the sealed end of the tube as he added mercury to the open end.

• He found that the higher the pressure, the lower the volume and vice versa.

• Pressure and volume are inversely related.

  • when temp and number of moles (n) is constant

P1xV1=P2xV2

Charles’ Law

• French chemists Jacques Charles ands Joseph Gay-Lussac studied the changes in gas volume caused by temperature at constant pressure

• They worked with Lord Kelvin

• Charles’ Law states that the volume of a gas is directly proportional to its temperature when pressure is held constant. As temperature increases, volume increases, and vice versa.

• It is also known as the law of volumes. It is an experimental gas law that describes how gases tend to expand when heated.

• Gas liquefaction involves cooling gas to a temp below its boiling point so it can be stored and transported in its liquid phase.

• The Kelvin scale MUST BE USED because zero on the Kelvin scale means a complete stop of molecular motion—> absolute zero (lowest temp that is theoretically possible at which the motion of particles that constitute heat would be minimal).

V1/T1=V2/T2

Gay-Lussac’s Law

• Gay-Lussac’s Law states that the pressure of a gas is directly proportional to its absolute temperature when the volume is held constant. This relationship indicates that as temperature increases, pressure increases, and vice versa.

• When scuba diving, pressure increases (starting at 1 atm at surface level) by 1 atm for every 10 meters in depth through which the diver descends.

• As temp decreases, the kinetic energy of gas particles decreases—> they hit the walls of the container less often and with less force, resulting in lower pressure.

  • Pressure is higher in warmer temperatures, since molecules move faster

  • Pressure is lower in cooler temperatures, since molecules move slower.

Avogadro’s Law

• Volume and number of moles are directly related (with constant temp and pressure).

• As volume increases, number of moles increase

• Must use mole-mole ration, so you should balance the equation

V1/n1=V2/n2

Combined Gas Law

• The Combined Gas Law relates the pressure, volume, and temperature of a fixed amount of gas. It combines Boyle's, Charles’, and Gay-Lussac's Laws, allowing calculations when gas conditions change. The formula is P1V1/T1 = P2V2/T2.

The Ideal Gas Law

• An ideal gas law is defined as one in which all collisions between atoms/molecules are perfectly elastic (meaning that when they collide with each other or with the walls of their container, no kinetic energy is lost—they bounce off without sticking or changing speed due to the collision) and in which there are no intermolecular forces. In such a gas, all the INTERNAL ENERGY is in the form of kinetic energy and any change in internal energy is accompanied by a change in temp.

• An ideal gas can be characterized by 3 state variables: absolute pressure (P), volume (V), and absolute temperature (T). PV = nRT.

  • pressure in atm

  • temp in kelvins

  • R is the universal law constant—0.08206

  • volume in liters

• Used to manipulate into all the other laws

Dalton’s Law of Partial Pressures

• For a mixture of gases in a container, Ptotal=P1+P2+P3…

• The TOTAL PRESSURE exerted is the sum of the pressures that each gas would exert if it were alone.

• The PARTIAL PRESSURE of a gas is the pressure that a gas would exert if it were alone in the container.

• P=n(total)RT/V

• The pressure of the gas is affected by the number of particles

• The pressure is independent of the type of gas (nature of the particles).

  • When volume and temp is the constant

The Kinetic Molecular Theory of Gases—POSTUALTES

1) Gases consist of tiny particles (atoms or molecules).

2) The particles are so small, compared with the distances between them, that the volume (size of the individual particles can be assumed to be negligible (zero).

3) The particles are in constant random motion, colliding with the walls of the container. These collisions with the walls cause the pressure exerted by the gas.

4) The particles are assumed not to attract or to repel each other.

5) The average kinetic energy of the gas particles is directly proportional to the kelvin temperature of the gas

Meaning of temperature

Kelvin temperature is directly proportional to the average kinetic energy of the gas particles.

Relationship between Pressure and Temperature

Gas pressure increases as the temperature increases because the particles speed up. As temperature rises, gas particles gain kinetic energy and collide with the container walls more forcefully, leading to increased pressure.

Relationship between Volume and Temperature

Volume of a gas increases with temperature because the particles speed up. This leads to greater movement and collisions of the particles against the walls of the container, resulting in an increased volume.

STP conditions

STP=standard temperature and pressure

0 degrees C (273K) and 1 atm (101.325kPa)

Therefore, the molar volume is 22.42 L at sTP

1mol=22.4L under STP