Empirical Properties of Gases

Empirical Properties of Gases

• Gases fill and take the shape of their containers
• Gases have neither a shape nor volume of their own
• Gases diffuse and/or move spontaneously throughout space
• Most gases are colourless and odourless
• Gases are highly compressible; when pressure increases in a gas, its volume decreases, and vice-versa
• If temperature of gas ↑ then the volume of gas ↑
• If pressure of gas ↑ then the volume of gas ↓
• Gases have the lowest density of all states of matter
• Gases have the weakest intermolecular forces of all states of matter
• Gases mix completely and uniformly with other gases when confined in the same space if no chemical reaction occurs
• Gases exert pressure on its surroundings

Defining State of a Gaseous Substance

• The state is defined by four variables: chemical amount, pressure, volume, and temperature
• Chemical amount is measured in moles
• Pressure (P) is force per unit area, measured in Pascals(Pa)
• 1 kPa = 1000 Pa
• 1 kPa = 1000 N/m²
• 1 atm = 101,325 kPa = 760 mm Hg (millimeters of mercury)
• 1 atm is also known as 1 atmosphere
• 14.7 psi = 1.01 bar
• Pressure in chemistry is usually expressed in atm, using dimensional analysis (ratios) to convert

Volume

• Volume (V) is the space occupied by a gas
• Volume is measured in Litres (L)

Temperature

• Temperature (T) of gases measured in Kelvin (K)
• To convert from Celsius to Kelvin: T(K) = T(°C) + 273

Standard Conditions

• Scientists define standard conditions:
  • Standard Temperature and Pressure (STP): 0°C (273 K) and 1 atm (101kPa)
  • Standard Ambient Temperature and Pressure (SATP): 25°C = 298K

Boyle's Law

• The law describes the relationship between pressure and volume
• As pressure of a gas increases, volume decreases, inversely proportional
• Temperature and chemical amount must remain constant
• Boyle's Law is written mathematically as P₁V₁ = P₂V₂

Charles' Law

• The law describes the relationship between temperature and volume
• As temperature increases, volume of a gas also increases if pressure and molarity are kept constant
• Charles' Law can be written mathematically as V₁/T₁ = V₂/T₂
• The temperature (T) must always be in Kelvin (K)
• Relationship between volume and temperature is directly proportional

Combined Gas Law

• The combined gas law relates pressure, volume, and temperature
• Product of pressure and volume of a gas is proportional to temperature, (molar amount is constant)
• Combined Gas Law: P₁V₁/T₁ = P₂V₂/T₂

Kinetic Molecular Theory of Gases

• The kinetic energy of gas particles is proportional to temperature
• As temperature increases, so does the kinetic energy of the gas particles
• Explains why gases are highly compressible because gas particles are far apart
• It's possible to force the particles closer together when pressure is applied
• Gas pressure is the total force of the collisions of gas particles distributed over an area of that container
• Boyle's Law: If the volume of the container is reduced, gas particles travel shorter distances to collide with their container, therefore they collide more frequently, leading to an increased pressure on the container
• Charles' Law: An increase in temperature represents an average kinetic energy

Law of Combining Volumes

• When temperature and pressure are constant, the volume of gaseous reactants and products are in simple whole number ratios
• Avogadro's Theory states that at the same temperature and pressure, equal volumes of gases contain equal numbers of molecules

Molar Volume

• Molar Volume refers to the volume one mole of gas occupies at a specific temperature and pressure
• Molar volume is the conversion factor to convert chemical amount of gas to volume
• Volume (V) = amount (n) x Molar Volume (VM)
• 24.8 L/mol = VSATP and 22.4 L/mol at VSTP

Ideal Gas Law

• The law describes a hypothetical gas that perfectly obeys all the gas laws under all conditions
• Gases are far apart relative to their size
• At high pressures, molecules are forced together so the size of particles and their size matters is negligible
• Constant, random, straight-line motion
• Collisions of gas particles are elastic and no NRG is lost
• At relatively low pressure and high temperature (STP and SATP), real gases behave like ideal gases
• Ideal Gas Law: PV = nRT.
• R (the ideal gas constant) = 8.314 kPa

Dalton's Law of Partial Pressure

• Partial Pressure is the pressure exerted by an individual gas in a mixture
• Dalton's Law of Partial Pressures- the total pressure of a mixture is the sum of the partial pressures of each of the gases in the mixture

Gas Stoichiometry

• Gas Stoichiometry predicts or analyzes the quantity of gas involved in a chemical reaction
• Molar volume and molar mass can be used to convert between volume and mass