gases physical characteristics of gases: the kinetic theory (a model for gases): 1. gases consist of...
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GasesPhysical Characteristics of Gases:
The Kinetic Theory (a model for gases): 1. Gases consist of a large number of tiny particles with
insignificant volume2. 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
molecules5. 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
SOLID LIQUID GASHas its own shape
Takes shape of container
Fills container
Highest density
Middle density
Lowest density
Not compressible
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 cm3
• Temperature (T): measure of the average kinetic energy of the gas
• MUST be in Kelvin!• K = ˚C + 273
• 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 105 Pa =101.3 kPa
• STP: standard temperature and
pressure. 1 atm and 0ºC
2cm
Newtons
area
forcepressure
atmosphericpressure
vacuum
mercury
Boyle’s Law2211 VPVP
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?
2
112 V
VPP
mL 55
20mLatm 2.0P2
atm 0.73P2
Charles’s Law
2
2
1
1
T
V
T
V
When P is constant: direct relationship between V and T (one goes up… the other goes up)
2121 V TT V
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
T1 = ˚C + 273
= 23 + 273= 296 K
Rearrange equation
Solve for final temperature
How cold is that?!?!
2
2
1
1
T
V
T
V
2
2
1
1
V
T
V
T 2
1
21 TV
VT
2TL 1.7
L 1.5K 296
K 126T2
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 CO2 at 22ºC and 20 atm. What is the pressure at 30ºC?
2
2
1
1
T
P
T
P
303K27330T
295K27322T
2
1
?P
atm 02P
2
1
2
2
1
1
T
P
T
P 2
1
21 PT
TP
2PK 295
K 303 x atm 20
21atmP2
2121 PTTP
Practice!
• Pages 419-425
• #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?
2
22
1
11
T
VP
T
VP
2
22
1
11
T
VP
T
VP 2
12
121 VTP
VTP
685torrP
770torrP
2
1
298K27325T
296K27323T
2
1
?V
L 20.0V
2
1
L 32K) torr)(296(685
L) K)(20.0 torr)(298(770V2
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 = 8.314 kPa L K-1 mol-1 if
Pressure is in kilopascals(kPa), Volume is in liters(L), Temperature is in Kelvin(K)R = 0.0821 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 0.050 moles of helium gas at 202.6 kPa and 400 K?
• What pressure will be exerted by 20.16 g hydrogen gas in a 7.5 L cylinder at 20ºC?
• A 50 L cylinder is filled with argon gas to a pressure of 10130.0 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 253.25 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
...PPPP 321Total
Gas Pressure (torr)
N2 593.5
O2 159.2
Ar 7.1
CO2 0.23
Total 760
93.4 kPa
3.3 kPa
Gases You Know N2
The most common gas in our atmosphere (78%)
Not reactive O2
20% of the atmosphere Supports combustion
CO2 Greenhouse gas More dense than air Used in fire extinguishers
H2 Very low density Explosive if mixed with O2
TP V
TP V
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.
20oC 80oC
• 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 76oC on Mt. Everest).
• Higher Pressure = higher boiling point (example = pressure cooker – cooks faster because you cook at a higher temperature.)