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GasesGasesChapter 24 Chapter 24
p 371 – 393p 371 – 393
Need calculatorsNeed calculators
PropertiesProperties• Low density• Fill containers• Pressure in all directions• Diffuse quickly• Compressible
Kinetic TheoryKinetic Theory
Image from Casiday and FreyDepartment of Chemistry, Washington University
Kinetic TheoryKinetic Theory1. Constant random motion2. Higher temp → faster motion
(Higher Ek)3. Minimal forces between particles4. Size of particles negligible cf
distance between them5. Particles collide with each other &
with walls with perfectly elastic collisions
QuestionsQuestions• Why does a hot air balloon float in
the air?• Activity 23.1 p 372
Gas PressureGas Pressure• Caused by constant collisions• Force per unit area, P = F/a • Units
– SI: Pascal (1 newton per m2) – very small value! – Chemists prefer kPa.– Meteorologists prefer hpa (=millibar)
Gas PressureGas Pressure• More units
– PSI (pounds force per sq inch) USA– mmHg (mm of mercury in a barometer)– Atmosphere (atm)– 1 atm = 101.3 kPa = 760 mm Hg
Mercury BarometerMercury Barometer
Mercury barometer for Mercury barometer for measuring pressure in measuring pressure in
a vessela vessel
Units of TemperatureUnits of Temperature• SI – degrees Celsius
– Based on 0ºC = freezing pt of water– 100ºC = boiling temp of water at sea
level– Celsius sometimes called centigrade
• USA – degrees Fahrenheit– Water freezes at 32ºF and boils at 212ºF– Complex origins!
Units of VolumeUnits of Volume• SI – cubic metre (very big)• Chemists prefer L and mL• 1 m3 = 1000 L • 1 cm3 = 1 mL• 1 L = 1000 cm3
Behaviour of Behaviour of GasesGases
RelationshipsRelationships
Pressure and VolumePressure and Volume
• Experiment results p 375
Image from Wikipedia
Boyle’s LawBoyle’s Law• Boyle – first to publish a formula –
1662• Fixed mass of gas (sealed)• Fixed temperature• Pressure x volume is constant• PV = k• P1V1 = P2V2 = P3V3 etc• Qs p 376
Temperature and volumeTemperature and volume
Animation from Wikipedia
Charles’ LawCharles’ Law• Discovered around 1787• Volume increases with temperature
BUT …
Charles’ LawCharles’ Law
0 Temp ºC100 200
1
2
Volume (L)
273 Temp K373 4730
-273
Charles’ LawCharles’ Law• Fixed mass of gas (sealed)• Fixed pressure• Volume ÷ temperature is constant• NB: Must use temperature in Kelvin• V/ T = k• V1= V2
T1 T2
• Qs p 380
QuestionQuestion• PV = k
V/ T = kDoes that mean PV = V/ T?
• NO!• k means a constant for that
experiment but depends on the mass you choose etc.
Gay-Lussac’ LawGay-Lussac’ Law• Discovered around 1809• Pressure increases with temperature
BUT …• Same issue as Charles’ law
Gau-Lussac’s LawGau-Lussac’s Law
0 Temp ºC100 200
100
200
Pressure
273 Temp K373 4730
-273
Gay-Lussac’ LawGay-Lussac’ Law• Fixed mass of gas (sealed)• Fixed volume• Pressure ÷ temperature is constant• NB: Must use temperature in Kelvin
• P/ T = k
• P1= P2
T1 T2
Combined Gas LawCombined Gas Law• Previous laws - mass and one other
measure fixed• Fixed mass of gas (sealed)• Combine
• P1 V1 = P2 V2
T1 T2
• Qs p 381
STPSTP• Standard temperature and pressure• 0ºC which is 273 K)• 1 atm which is
– 101.3 kPa or – 760 mm Hg
Dalton’s Law of Partial Dalton’s Law of Partial PressurePressure
• Mixture of non-reacting gases• Constant T & V• Pressure exerted by each gas:
“partial pressure”
• PT = P1 + P2 + P3 etc
• Gases act independently
Collecting GasCollecting Gas
• Convenient to collect over water
• Calculate volume of the gas & Volume of water vapour
• Use Dalton’s law• Qs p 382 & 384
Avagadro’s HypothesisAvagadro’s Hypothesis• Equal volumes of gases
(at same T & P) …• Same number of particles
(molecules)• 1 mole of gas at STP - 22.4L• n = V/22.4 (at STP, vol in L)• Avagadro’s number?
6 x 1023
SLCSLC• Standard laboratory conditions• 25ºC ie 298 K• 1 atm ie 101.3 kPa• Calculate molar volume at SLC
(to check answer see bottom of p 385)
• Sample questions + Q 9 & 10 p 385
VariablesVariables• Combined gas law
– Fixed amount of gas– Other 3 variables P, V & T can vary
• What about a variable amount of gas?
• Remember Avagadro’s hypothesisNever mind what kind of gas …
Combined Gas LawCombined Gas Law• P1 V1 = P2 V2
T1 T2
• PV = k (constant)
T
• Consider 1 mole of gas at STP
• 101.3 x 22.4 = k (constant)
273
• What is the constant?
General Gas EquationGeneral Gas Equation• For one mole of gas
• PV = 8.31
T
• For 2 moles of gas or more?
• PV = 8.31 x 2
T
• PV = 8.31 x n
T
General Gas EquationGeneral Gas Equation• PV = n x 8.31
T• Call 8.31 R – universal gas constant• Only if we use Kelvin, litres & kPa or
Kelvin, m3 and Pa• Can use other units but need different R • Rearrange:
PV= nRT• What is temperature changes?• What if pressure changes?
Use?Use?• PV= nRT• Any pure sample of gas
– known P, V & T – we can calculate n
• Any sample of gas collected over water– known P, V & T + pp of H2O
Use Dalton’s law of partial pressure)– we can calculate n
QuestionsQuestions• Examine sample problem p 386/7• Do Q 11 p 387• Review questions: do even numbers
from 2 to 32.
Stoichiometry & GasesStoichiometry & Gases• Last year:equation such as
• H2 + ½ O2 → H2Ocalculate mole ratio
• Used for mass & concentration to calculate n
• Now use PV = nRT to calculate n• Still use mass ratio as before
QuestionsQuestions• Sample problems p 387 & 388.• Q 12 p 388• Sample problems p 389 & 390
Real GasesReal Gases
• Consider sample of O2
• Cool it more, and more, and more …• What happens at particle level?
Charles’ LawCharles’ Law
1
2
Volume (L)
273 Temp K373 4730
Assumptions of Kinetic Assumptions of Kinetic TheoryTheory
1. Constant random motion2. Higher temp → faster motion
(Higher Ek)3. Minimal forces between particles4. Size of particles negligible cf
distance between them5. Particles collide with each other &
with walls with perfectly elastic collisions
Ideal and Real GasesIdeal and Real Gases• Real gases – laws work reasonably well for
limited range of conditions– Room temp and above– Low pressure (atm & less)
• Break down badly as we approach condensation conditions
• Ideal gas – one for which gas laws work for all conditions – no such gas.
• Ideal gas – often used for calculations
Review QuestionsReview Questions• All the rest!