Gas Chemistry Feel the Pressure?

Nature of Gases Expansion: no definite shape/volume particles travel in all directions to fill space particles travel in all directions to fill space Fluidity: particles easily “slide” past each other (both liquids and gases are referred to as fluids) Low Density: 1/1000 th density of solid/liquid

Nature of Gases (cont.) Compressible: particles can be forced together, reducing empty space in between Diffusion: spontaneous mixing of gases due to random motion rate depends on speed, diameter, attractive forces rate depends on speed, diameter, attractive forces Effusion: pass through small opening under pressure

Variables Describing a Gas (P) Pressure (V) Volume (T) Temperature (n) Amount of gas in moles

Brain Starter!! 1. Convert 3 moles of hydrogen into liters. 2. What is -32 degrees Celsius in Kelvin? 3. Convert 2.5 atm into mm Hg.

Kinetic Molecular Theory (KMT) 1.Gases are tiny particles separated by vast empty space (assumed zero volume) 2.Gas particles move is rapid, straight line motion (possess kinetic energy) 3.Gas particle collisions are perfectly elastic (no net loss of kinetic energy) 4.No attractive or repulsive forces between gas particles 5.Average kinetic energy is directly proportional to the Kelvin temperature.

Ideal Gas Ideal Gas - imaginary gas that perfectly conforms to all assumptions of the KMT Real gases behave like ideal gases at lower pressures and higher temperatures. Deviations: High pressure or Low temp. – attractive forces cause condensation to liquids High pressure or Low temp. – attractive forces cause condensation to liquids Polar molecules behave less ideal than nonpolar molecules Polar molecules behave less ideal than nonpolar molecules

Charles’ Law Observed that raising the temp. 1 ºC increased the volume 1/273 of original volume Observed that raising the temp. 10 ºC increased the volume 10/273 of original volume Decreasing temp caused same decrease in volume

Kelvin Scale and Absolute Zero Charles noted that volume and temperature are only directly proportional if using the Kelvin temp. scale Absolute zero ( 0 K) is the lowest possible temperature 0 K means no kinetic energy (no atom/molecule motion) 0 K means no kinetic energy (no atom/molecule motion) theoretically: zero gas volume at zero Kelvin theoretically: zero gas volume at zero Kelvin Why won’t gases disappear at zero Kelvin?

Temperature Units Degrees Celsius 0°C = Freezing point 0°C = Freezing point 100°C = Boiling point 100°C = Boiling pointConversions: K (Kelvin) = °C K (Kelvin) = °C °F (Fahrenheit) = (9/5)°C + 32 °F (Fahrenheit) = (9/5)°C + 32 °C = K – 273 or °C = (°F – 32) x (5/9) °C = K – 273 or °C = (°F – 32) x (5/9)

Pressure Pressure = force / area Gas particle collisions with a surface cause pressure Atmospheric pressure (sea level): 1 atmosphere (atm)

Air Pressure Atmospheric Pressure- force per unit area exerted against a surface by the weight of air above that surface. Higher altitude = lower pressure because of less air! Breathing issues?

How it affects you! Boiling water: Water boils when Water boils when vapor pressure meets/ exceeds air pressure. If lower pressure, If lower pressure, water boils at lower temperature. Issues?? Issues??

OOPS!!! How could this happen?

Wow, really !?! 1.Tanker was steam cleaned on the inside 2.The Tanker was sealed and allowed to cool 3.Internal pressure was lower than external pressure. 4.Atmospheric pressure collapsed the tanker car.

Pressure Units 1 atm (atmosphere) = average P at sea level 1atm = 760 mm Hg = 760 torr = 760 torr = kPa (kilo Pascal) = kPa (kilo Pascal) = 14.7 psi (pounds per square inch) = 14.7 psi (pounds per square inch)

STP STP = standard temperature and pressure exactly 1 atm and 0 ºC Volume of a gas depends upon T & P so a standard is required for comparison

Charles’ Law V = kT or V/T = k If V 1 /T 1 = k and V 2 /T 2 = k Then

Charles Law Example A gas sample at 25 ºC and 752 mL is heated to 50 ºC, what is the new volume? T1 = 25 ºC = 298 K T2 = 50 ºC = 323 K V1 = 752 mL

Boyle’s Law Observed that doubling volume of sample of gas reduced pressure by ½ Observed that tripling volume of a sample of gas reduced pressure by 1/3 Inverse relationship between P & V

Boyle’s Law Eqns. PV = k or V= k/P k is a constant regardless of gas If P 1 V 1 = k and P 2 V 2 = k then P 1 V 1 = P 2 V 2 Useful version of Boyles Law

Boyle’s Law Example A 1.0 L sample of gas at 1.0 atm is allowed to expand at constant temperature to a volume of 5.0 L, what is the new pressure? (1.0 L)(1.0 atm) = x atm (5.0 L) x = 0.2 atm

Gay-Lussac’s Law P = kT or P/T = k If P 1 /T 1 = k and P 2 /T 2 = k Then

What about moles of gas? (glad you asked)

Dalton’s Law P = kn or P/n = k If P 1 /n 1 = k and P 2 /n 2 = k Then

Avogadro’s Law V = kn or V/n = k If V 1 /n 1 = k and V 2 /n 2 = k Then

Combined Gas Law PV/Tn = k If P 1 V 1 /T 1 n 1 = k and P 2 V 2 /T 2 n 2 = k Then movie clip movie clipmovie clip

Gas Law Simulation Click Here Click Here to launch the gas law simulator Click Here Or Simulations – Heat and Thermo – Gas Properties

Dalton’s Law of Partial Pressures The total pressure of a mixture of gases is equal to the sum of the partial pressures of each individual gas P total = P 1 + P 2 + P 3 … or

Molecular View of Part. Press. Single Gas Molecule – Each collision contributes to total pressure

Molecular View of Part. Press. Many molecules – each collision also contributes to total gas pressure; however, there are more collisions with more gas in the same volume

Gases Collected by Water Displacement Collected gas will contain water vapor P total = P atm = P gas + P water P atm = from barometer P atm = from barometer P water = table of vapor pressures P water = table of vapor pressures P gas = must calculate P gas = must calculate

Molecular View Water molecules are in constant motion. At the surface: Some molecules have enough KE to evaporate Some molecules have enough KE to evaporate Some molecules lose enough KE to condense Some molecules lose enough KE to condense A mixture of gases above water will always contain some water vapor A mixture of gases above water will always contain some water vapor

Water Displacement Example Oxygen was collected by water displacement at 20.0 ºC and mm Hg. What is the pressure of oxygen? Oxygen was collected by water displacement at 20.0 ºC and mm Hg. What is the pressure of oxygen? P atm = P gas + P water P oxygen = mm Hg – 17.5 mm Hg = mm Hg

Vapor Pressure Gas molecules above solution will achieve some temperature dependent pressure (equilibrium) Water Vapor Pressure at 25 ºC is 23.8 mmHg Water Vapor Pressure at 100 ºC is 760 mm Hg

Ever Wonder? Why will a puddle of water evaporate on a sunny day? Doesn’t water evaporate when it boils (100 ºC)? Why won’t the water in a half-filled water bottle that is sealed evaporate?

Forming the Ideal Gas Law Recall that the following equation can be used to describe a 2 state gas system (initial state = 1 and changed state = 2) Since P 1 V 1 /T 1 n 1 = k and P 2 V 2 /T 2 n 2 = k

Forming the Ideal Gas Law Using a single state: Substitute R for k: Rearranged:

R… what the world is R? R = Ideal Gas Constant R is always the same for any ideal gas R depends upon units used to describe a gas

R Values (not for insulation) Common: L*atm/mol*Kvery common L*kPa/mol*KSI units Less Common: 62.4 L*mmHg/mol*K J/mol*K(1L*atm = J) Others

Ideal Gas Law PV = nRT Used to solve for an unknown variable in a single state gaseous system Must know 3 of 4 variables to solve

Ideal Gas Law Example What is the pressure in atm exerted by a mol sample of N 2 in a 10.0L container at 298K? Solve for Pressure: