Gases Physical Characteristics of Gases: The Kinetic Theory (a model for gases): 1. Gases consist of a large number of tiny particles with insignificant volume 2. The particles are in constant, random motion. 3. The collisions between particles and walls are elastic. 4. There are no forces of attraction or repulsion between molecules 5. The average kinetic energy is directly proportional to temperature in Kelvin ( __°C + 273). Diffusion: spontaneous mixing due to random motion (molecules moving from high concentration towards low concentration) Effusion: gas moving through a small hole Real Gas – a gas that does not completely behave according to the kinetic theory Due to: 1. occupy space 2. exert attractive forces on each other

SOLIDLIQUIDGAS Has its own shape Takes shape of container Fills container Highest density Middle density Lowest density Not compressible Compressible Little movement Some movement Rapid movement

Pressure (P): force that a gas exerts on a given area 1 atm = 760 torr = 760 mmHg Volume (V): space occupied by gas 1 L = 1000 mL = 1000 cm 3 Temperature (T): measure of the average kinetic energy of the gas MUST be in Kelvin! K = ˚C Number of moles (n): quantity of gas molecules Properties (P,V,T,n)

What is Pressure? Changing the force or area will change the pressure (shoes!) Atmospheric (air) pressure is measured by a barometer: 1 atm = 760 torr = 760 mmHg =1.013 x 10 5 Pa =101.3 kPa STP: standard temperature and pressure. 1 atm and 0ºC atmospheric pressure vacuum mercury

Boyle’s Law  When T is constant: inverse relationship between P and V (one goes up… the other goes down)  A sample of oxygen gas occupies a volume of 20 mL at 2.0 atm. At what pressure will it occupy 55 mL?

Charles’s Law  When P is constant: direct relationship between V and T (one goes up… the other goes up)

 You get a 1.7 L balloon inside at a temperature of 23˚C. At what temperature will the volume drop to 1.5 L ?  Convert initial temperature to K T 1 = ˚C = = 296 K  Rearrange equation  Solve for final temperature  How cold is that?!?!

Gay-Lussac’s Law  When V is constant: direct relationship between P and T (one goes up… the other goes up)  Higher T: more collisions in same area.  A fire extinguisher has CO 2 at 22ºC and 20 atm. What is the pressure at 30ºC?

Practice! Pages #7-12,15-17 &20

Combined Gas Law True when moles are constant Use to remember –Boyle’s law: –Charles’s law: –Gay-Lussac’s law: A balloon has a volume of 20.0 L at 23ºC and 770 torr. What will its volume be at 685 torr and 25ºC?

Ideal Gas Law PV=nRT R is the gas constant. It will always have the same values. You must know which one to use R = kPa L K -1 mol -1 if Pressure is in kilopascals(kPa), Volume is in liters(L), Temperature is in Kelvin(K) R = L atm K -1 mol -1 if Pressure is in atmospheres(atm), Volume is in litrers(L), Temperature is in Kelvin(K)

Ideal Gas Law What volume is needed to store moles of helium gas at kPa and 400 K? What pressure will be exerted by g hydrogen gas in a 7.5 L cylinder at 20ºC? A 50 L cylinder is filled with argon gas to a pressure of kPa at 30ºC. How many moles of argon gas are in the cylinder? To what temperature does a 250 mL cylinder containing 0.40 g helium gas need to be cooled in order for the pressure to be kPa?

Dalton’s Law of Partial Pressure The total pressure of a gas sample is equal to the sum of the partial pressures of individual gases. Example: Earth’s atmosphere GasPressure (torr) N2N O2O Ar7.1 CO Total760

93.4 kPa 3.3 kPa

Gases You Know  N 2  The most common gas in our atmosphere (78%)  Not reactive  O 2  20% of the atmosphere  Supports combustion  CO 2  Greenhouse gas  More dense than air  Used in fire extinguishers  H 2  Very low density  Explosive if mixed with O 2

TPV TPV Boyle’s Law Charles’s Law

Vapor Pressure and Boiling Vapor Pressure – the pressure exerted by a vapor in equilibrium with its liquid state. Liquid molecules at the surface escape into the gas phase. These gas particles create pressure above the liquid in a closed container.

The condition in which two opposing processes are occurring simultaneously at equal rates is called a dynamic equilibrium. A liquid and its vapor are in equilibrium when evaporation and condensation occur at equal rates. This can only be obtained in a closed container.

Vapor Pressure increases with increasing temperature. 20 o C 80 o C As temperature increases, the amount of vapor generated by a liquid in a closed container increases. This occurs because as the liquid gains kinetic energy, the molecules can overcome the intermolecular forces of attraction that are prevalent in the liquid phase.

Evaporation vs Boiling Evaporation - when a liquid  gas at any temperature Vaporization – When a liquid  gas when heat is applied or at the boiling temperature Boiling – occurs when the vapor pressure above the liquid equals the atmospheric pressure.

Vapor Pressure Curves Graph shows how boiling points change with change in vapor pressure.

Boiling Points change with pressure changes. Less pressure = lower boiling point (example = water boils at lower temperatures at high elevations- water boils at 76 o C on Mt. Everest). Higher Pressure = higher boiling point (example = pressure cooker – cooks faster because you cook at a higher temperature.)