Post on 11-Jan-2016
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Chapter 14 – Gases
Kinetic Molecular Theory (KMT)
Defn – describes the behavior of gases in terms of particle motionMakes assumptions of size, motion, &
energy of gas particles
KMT and Gases
Ideal Gases – imaginary gas that conforms to assumptions of KMT
Assumptions of KMT
A) gas particles do not attract or repel each other
no intermolecular forces occurring. gases are free to move in their containers without interference from other particles
Assumptions of KMT
B) gas particles have NO volume
almost all of the volume of a gas is empty space. The particles are insignificant in size compared to all the space
Assumptions of KMT
C) gas particles are in constant motion
they move in straight lines until they collide with each other or the walls of container
Assumptions of KMT
D) no kinetic energy is lost when gas particles collide with each other or walls
aka elastic collision. There is no loss in speed of the particles
Assumptions of KMT
E) ALL gases have same average kinetic energy at a given temperature
as temperature increases, more energy. As temp decreases, less energy
Assumptions of KMT
***KEEP IN MIND IDEAL GASES DO NOT EXIST!!!!!!!***
APPLIES AT ALL TEMPERATURES AND ALL PRESSURE
Real Gases
Defn – do not behave according to assumptions of KMT
Characteristicsi) real gas particles have volumeii) real gas particles exert attractive
forces on each other
Real Gases
When do real gases act like ideal gases?
at very low pressure and very high temp- at low P, molecules far from each
other- at high temp, molecules move too fast to have intermolecular interactions
Characteristics of Gases
1) expansion2) fluidity3) low density4) compressibility5) diffusion
4 measurable variables of gases
PressureTemperatureVolumeAmount
Pressure
Defn – force/area; pressure caused by collisions of molecules
Units: atmosphere (atm), millimeter of mercury (mm Hg), torr, kiloPascal (kPa)
Conversions:1 atm = 760 mm Hg = 760 torr = 101.3 kPa
Sample problem #1
Convert 2.3 atm to torr
2.3 atm
1 atm
760 torr= 1748 torr
Sample problem #2
Convert 450 mm Hg to kPa
450 mmHg
760 mm Hg
101.3 kPa= 60 kPa
Measuring air pressure
What device is used to measure air pressure?????
BAROMETER
Standard Temp and Pressure (STP)
P = 1 atmT = 0°C
Temperature
Unit Kelvin (K)
Kelvin = °C + 273
Temperature
Convert 45°C to Kelvin
45°C + 273= 318 K
Temperature
Absolute zero – 0 KMolecules stop moving
Question: what is absolute zero temperature in Celsius?
-273°C
Volume
Units:mLL
Amount
Unitsgramsmoles
Gas Laws
Each gas law relates the 4 variables:PressureTemperatureVolumeAmount
For the next 4 gas laws, amount is not a factor
We will only pay attention to P, T, & V
Boyles’ Law: P/V relationship
Defn – at constant T, volume of a fixed amount of gas varies inversely with pressureinversely – as one variable increases,
other variable decreases
P
V
Boyle’s Law
FormulaP1V1 = P2V2
P1 & V1 are initial conditions
P2 & V2 are final conditions
Boyle’s Law
A balloon filled with helium gas has a volume of 500 mL at 1.00 atm. When it rises to a higher altitude, the pressure is reduced to 0.50 atm. If the temperature is constant, what is the volume of the balloon?
1000 mL
Charles’ Law: V/T relationship
Defn – at constant P, volume of fixed amt of gas varies directly with Kelvin tempDirectly – as one variable increase, the
other increases
V
T
Charles’ Law
Formula
V1 & T1 are initial conditionsV2 & T2 are final conditions
2
2
1
1
T
V
T
V
Charles’ Law
A helium balloon has a volume of 2.75 L at 20°C. On a cold day, the balloon is placed outside where it shrinks to 2.46 L. What is the temperature outside?
262 K
Gay-Lussac’s Law: P/T relationship
Defn – at constant V, pressure of fixed amt of gas varies directly with Kelvin tempDirectly – as one variable increase, the
other increases
P
T
Gay-Lussac’s Law
Formula
P1 & T1 are initial conditionsP2 & T2 are final conditions
2
2
1
1
T
P
T
P
Gay-Lussac’s Law
Before a trip from Sugar Land to South Padre, the pressure in an automobile tire reads 2.1 atm at 27°C. Upon arriving in Padre, the gauge reads 2.3 atm. What is the temperature in South Padre?
329 K
Combined Gas Law: P/T/V
Defn – relationship between P,V, & T of fixed amount of gas
Formula
2
22
1
11
T
VP
T
VP
Combined Gas Law
The volume of a gas is 27.5 mL at 22.0°C and 740 mm Hg. What will be its volume at 15°C and 755 mm Hg?
26 mL
Avogadro’s Principle
Defn – equal volumes of gases at same temp and pressure contain equal number of molecules- the size of the molecule does not influence the
volume a gas occupies
- for example, 1000 relatively large Kr gas molecules occupies the same volume as 1000 smaller He gas molecules
Molar Volume of a Gas
Molar Volume – volume that one mole of a gas occupies at STP (0°C, 1 atm)
1 mole = 22.4 L
Molar Volume of a Gas
Ex prob #1: calculate the volume 0.881 mol H2 will occupy at STP.
0.881 mol H2
0.881 mol H2 x 22.4 L H2
1 mol H2
= 19.7 L H2
Molar Volume of a Gas
What is the mass of 0.0752 L of oxygen gas at STP? 0.0752 L O2
0.0752 L O2 x 1 mol O2
22.4 L O2
0.0752 L O2 x 1 mol O2 x 32 g O2
22.4 L O2 1 mol O2
= 0.107 g O2
Ideal Gas Law
Defn – relationship between P,V,T and # of moles, n
FormulaPV = nRT
ideal gas constant
Ideal Gas Constant
3 different R constants
R = 0.0821 L·atm
mol·K = 62.4 L· mm Hg
mol·K = 8.314 L·kPa
mol·K
pay attention to what unit of pressure is given in problem then use the appropriate R
Ideal Gas Law
Ex prob 1: what is the volume of 0.250 moles of oxygen gas at 20°C and 740 mm Hg?
which R do we use?
6.18 L O2
Ideal Gas Law
Ex prob 2: calculate the pressure, in atm, of 4.75 L NO2 containing 0.86 mol at 27°C.
4.46 atm
Ideal Gas Law
Ex prob 3: calculate the grams of N2 present in a 0.600 L sample kept at 1.0 atm and 22°C.first determine # of moles
0.025 mol N2
convert to grams0.70 g N2
4 gas laws vs. ideal gas law
The differences between ideal gas law and the other 4 are:
1) Ideal gas law utilizes an amount (moles), other 4 don’t
2) The 4 gas laws have a change in condition; ideal gas law does not