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Gases: Ch 12 12.1 Pressure

Gases: Ch 12 12.1 Pressure. Basic properties of gases Expand to completely fill their container Take the shape of their container Have low density (compared to solids or liquids) Are compressible Always homogeneous as a mixture Fluid (i.e., they flow). Gas Pressure.

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Gases: Ch 12 12.1 Pressure

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  1. Gases: Ch 1212.1 Pressure • Basic properties of gases • Expand to completely fill their container • Take the shape of their container • Have low density (compared to solids or liquids) • Are compressible • Always homogeneous as a mixture • Fluid (i.e., they flow)

  2. Gas Pressure • Pressure = total force applied to a certain area • larger force = larger pressure • smaller area = larger pressure • Gas pressure caused by gas molecules colliding with surface of their container • More forceful collisions or more frequent collisions mean higher gas pressure

  3. Air Pressure • Air is a mixture of gases (O2, N2, CO2, etc.) • Constantly present when air is present • Decreases with altitude • Due to less overlying air pressing down • Varies with weather conditions • Measured using a barometer (fig 12.2) • Column of mercury supported by air pressure • Longer mercury column supported = higher pressure • Force of the air on the surface of the mercury balanced by the pull of gravity on the column of mercury

  4. Units of Gas Pressure • mm Hg • units for the height of a column of mercury the gas pressure can support. • Standard atmosphere (atm) • equals 760.0 mm Hg (at sea level) • torr • named after Evangelista Torricelli who invented the barometer in 1643 • 760.0 mm Hg = 760.0 torr • pascal (Pa) – named for Blaise Pascal, 17th c. scientist • pounds per square inch (psi, lbs./in2) • Conversions: 1.0 atm = 760.0 mm Hg = 29.92 in Hg = 760.0 torr = 101,325 Pa = 101.325 kPa = 14.69 psi

  5. 12.2 Pressure and Volume: Boyle’s Law • Robt. Boyle, 17th c. • Pressure is inversely proportional to Volume • constant T and amount of gas • graph P vs V is curve (fig 12.5) • as P increases, V decreases by the same factor • P x V = constant (table 12.1 – sample of Boyle’s observations) • Therefore, Boyle’s Law is expressed as: PV = k

  6. According to Boyle’s Law, if the initial volume and pressure of a gas is known, the new volume or pressure can be predicted when a volume or pressure change occurs (all other variable held constant): P1 x V1 = P2 x V2 • Example: Calculating new volume A sample of a gas has a volume of 1.50 L at a pressure of 56.0 torr. What is the volume if the pressure is increased to 150.0 torr? • Write down the given amounts P1 = 56.0 torr P2 = 150.0 torr V1 = 1.50 L. V2 = ? L • Convert values of like quantities to the same units (already done) • Solve using rearranged equation:

  7. Example: Calculating new pressure A sample of a gas has a volume of 1.50 L at a pressure of 56.0 torr. What is the pressure when the volume decreases to 0.250 L?

  8. 12.3 Volume and Temperature: Charles’ Law • Named after Jacques Charles, 18th c. • Volume is directly proportional to Temperature • constant P and amount of gas • graph of V vs T is straight line • as T increases, V also increases; as T decreases, V decreases • Temperature measured in kelvins • V/T = b (constant) • V1 = V2 T1 T2

  9. Absolute Zero • The theoretical temperature at which a gas would have zero volume and no pressure • calculated by extrapolation (see fig 12.7) • never actually attained, although have come close • 0 (zero) K = -273.15 °C = -459 °F • Kelvin temp = Celsius temp + 273 • All gas law problems use Kelvin temperature scale!

  10. 12.4 Avogadro’s Law • The volume of a gas sample is directly proportional to the number of moles (n) of gas molecules present. • V = constant x n (moles) • Constant P and T • More gas molecules = larger volume • Count number of gas molecules by moles • One mole of any ideal gas occupies 22.4 L at standard conditions - molar volume • Equal volumes of gases contain equal numbers of molecules, regardless of the gas V1= V2 n1 n2

  11. Gay-Lussac’s Law • The pressure of a sample of gas is directly proportional to its temperature in kelvins.

  12. 12.5 Ideal Gas Law • The relationships between the P, T, V and number of moles (n) of a gas can be combined together into one general equation called the Ideal Gas Law: • R is the universal gas constant – has the value 0.08206 L atm/K mol • Use the ideal gas law when three of the four properties of the gas are known – then solve for the remaining property • Most ideal gases (i.e., those that obey the ideal gas law) obey this law best when pressure is low (at or below 1.0 atm) and temperature is high (above 0°C) PV =nRT

  13. Combined Gas Law: A combination of Boyle’sLaw, Charles’ Law, and Gay-Lussac’s Law. Use in place of Ideal Gas Law when moles remains constant. Compares the same substance under two different sets of conditions. Rearrange the equation to solve for V2, P2, or T2 under changing conditions.

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