Ch 12 Gases Though the chemical behavior of gases differ, all gases have very similar physical behavior Gases are distinguished from other states of matter: -- gas volume changes greatly with pressure and temperature -- gases have very low viscosity, i.e., they flow easily, as through pipes and small holes -- most gases have relatively low densities under normal conditions ( C and normal atmospheric pressure) -- gases are miscible, they mix easily with each other in any proportion The physical behavior of gases can be described completely by four variables: pressure (P), volume (V), temperature (T), and amount (number of moles, n). 1

2 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)

3 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

4 Air Pressure Air is a mixture of gases (O 2, N 2, CO 2, 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

5 Units of Gas Pressure mm Hg –units for the height of a column of mercury the gas pressure can support. Standard atmosphere (atm) –equals mm Hg (at sea level) torr –named after Evangelista Torricelli who invented the barometer in 1643 –760.0 mm Hg = torr pascal (Pa) – named for Blaise Pascal, 17 th c. scientist pounds per square inch (psi, lbs./in 2 ) Conversions: 1.0 atm = mm Hg = in Hg = torr = 101,325 Pa = kPa = psi

Pressure and Volume: Boyle’s Law Robt. Boyle, 17 th 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

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8  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 L?

Volume and Temperature: Charles’ Law

10 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 = °C = -459 °F Kelvin temp = Celsius temp All gas law problems use Kelvin temperature scale !

12.4 Avogadro’s Law 11

12 Gay-Lussac’s Law

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 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 Though no ideal gas actually exists, most simple gases show nearly ideal behavior at ordinary temperatures and pressures PV = nRT

14 Combined Gas 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 V 2, P 2, or T 2 under changing conditions. A combination of Boyle’sLaw, Charles’ Law, and Gay- Lussac’s Law.