Gases
General Properties of Gases
Gases have characteristics properties which depend on their molecular arrangements.
Properties of Gases | |
Density | low |
solubility | readily |
Shape and volume | fills container, no fixed volume |
Diffusion | easily |
motion | very freely |
Particle Arrangement | widely separated |
Compressibility | easily |
- gases exist usually as molecules, usually diatomic. Examples of these gases are O2, Cl2, I2, N2, Br2, F2, and H2. However, noble gases, such as Neon, Argon and Helium exist as individual atoms. They are called monatomic gases.
- Gases have no definite shape and volume
- Gases are easily compressed when pressure is applied.
- Gases expand when heated and contract when cooled.
- Gases exert pressure
- The densities of gases are relatively small compared to the densities of solids and liquids.
- The force of intermolecular attraction between gas particles is negligible (no force)
- Gases mix evenly and completely when contained in the same vessel. (solution)
Measurable Properties of Gases
The following are the measurable properties of gases
Pressure – defined as the force per unit area. Pressure is expressed mathematically as:
P = force/ area. Some units for pressure are atm, torr, mmHg, and Pascal.
1 atm = 760 torr = 760 mmHg
1 torr = 1 mmHg
1 atm = 101,325 Pa
1 atm = 14.70 lb/in2
Instrument used to measure air pressure:
1. Barometer – an instrument used to measure atmospheric pressure
2. Manometer – an instrument used to measure air pressure in a closed container
Volume – the volume of a gas is the space it occupies
Temperature – the temperature of a gas is determined using a thermometer. It is usually expressed in °F, °C, and K. The following are the useful formulas in converting one unit of temperature to another.
- converting from °F to °C
°C = 5/9 (°F – 32)
- converting from °C to °F
°F = {9/5(°C)} + 32
- converting from °C to Kelvin
K = °C + 273
- converting from Kelvin to °C
°C = K – 273
Amount of gases – the quantity of gas is being measured is always expressed in moles.
Density – although gases are very light, they still have measurable densities.
To discuss behavior of gases, it is convenient to designate standard temperature and pressure (STP) which is 0°C or 273°K and 1.0 atm (760 mmHg or 101.325 kPa).
Kinetic Molecular Theory
The following are the postulates of the Kinetic Molecular Theory:
a. A gas consists of very small particles, each of which has a mass.
b. The distances separating gas particles are relatively large.
c. Gas particles are in constant, rapid, random motion.
d. Collisions of gas particles with each other or with the walls of the container are perfectly elastic.
e. The average kinetic energy of gas particles depends only on the temperature of the gas.
f. Gas particles exert no force on one another.
Gas Laws
Boyle’s Law
1. Boyle’s Law states that the volume of a given quantity of a gas varies inversely with its pressure when temperature is held constant. This means that if the pressure is increased, volume is decreased and if the volume is increased, pressure will decrease.
2. Boyle’s Law can be expressed using the formula P1V1 = P2V2
3. The property of a gas to decrease in volume when pressure is applied or increased is called compressibility.
Charles’ Law
1. Charles’ law states that at constant pressure, the volume of a fixed amount of gas is directly proportional to its absolute temperature.
2. Charles’ law can be expressed mathematically as V1T2=V2T1
3. Temperature expressed in Kelvin scale is called absolute temperature.
Gay-Lussac’s Law
1. Joseph Louis Gay-Lussac (1778-1850), a French Chemist, pioneered in the study of the relationship between pressure and temperature.
2. Gay-Lussac’s Law states that pressure is directly proportional to the absolute temperature when the volume and the amount of gas are kept constant. If the absolute temperature is doubled, the pressure exerted by the gas also doubles.
3. Gay-Lussac’s Law can be expressed mathematically as: P1T2 = P2T1
Combined Gas Law
1. Kinetic Molecular Theory states that molecules are in constant motion.
2. Combined effects of temperature and pressure on volume of a gas (Gay-Lussac combined gas law) makes use of both Boyle’s and Charles’ Law. These two (2) laws together known as Combined gas laws V1P1T2 = V2P2T1
Dalton’s Law of Partial Pressure
1. Partial pressure refers to the pressure each gas would exert at the same temperature and at the same volume in the absence of other gases.
2. Dalton’s Law of Partial Pressure states that the total pressure of a mixture of gases equals the sum of the partial pressures of each of the gas in the mixture.
3. Dalton’s Law of Partial Pressure can be expressed as: PTotal = PA + PB + PC + … PN
Avogadro’s Law
1. According to Avogadro’s Law, different gases with the same number of molecules at the same condition of pressure and temperature occupy the same volume. Correspondingly, one mole of any ideal gas at standard temperature and pressure occupies a constant volume (molar volume of gas = 22.4 L/mole).
2. Avogadro’s Law can be expressed as: V1n2 = V2n1
Ideal Gas Law
1. Ideal Gas Law considers all the measurable factors that affect the behavior of gases. These include pressure, temperature, volume, and the amount of gas.
2. The Ideal Gas Equation is derived from Boyle, Charles, and Avogadro.
3. The Ideal Gas Law can be expressed as:
(1) PV = nRT
(2) PV M.M = gRT
(3) P M.M = DRT
Where:
P = pressure in atm M.M = molar mass (g/mol)
V = volume in L g = mass in grams
n = no. of moles T = Temperature in Kelvin
R = 0.0821 L atm/ K mol D = density in the formula (g/L)
Graham’s Law of Diffusion
1. Diffusion is the movement of particles from one place to another. The wide distance between gas molecules and their low densities enable them to move faster.
2. Thomas Graham concluded that “the rate of diffusion of gases is inversely proportional to the square roots of their densities.” This statement is known as Graham’s Law of Diffusion.
3. Graham’s Law of Diffusion can be expressed mathematically as:
Rate of Diffusion α
4. The density of gases is directly related to its molar mass. From this relationship, Graham’s Law can be expressed as:
Rate of Diffusion α
5. If we compare the rates of diffusion of two gases, the equation for Graham’s Law of Diffusion will become:
where: r1 = rate of diffusion of gas 1
r2 = rate of diffusion of gas 2
MM1 = Molar Mass of gas 1
MM2 = Molar Mass of gas 2
where: r1 = rate of diffusion of gas 1
r2 = rate of diffusion of gas 2
D1 = density of gas 1
D2 = density of gas 2
6. If we compare the time of diffusion of two gases, the equation for Graham’s Law of Diffusion will become:
where: t1 = time of diffusion of gas 1
t2 = time of diffusion of gas 2
MM1 = Molar Mass of gas 1
MM2 = Molar Mass of gas 2
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