Chapter 14 – Gases
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Transcript of Chapter 14 – Gases
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Chapter 14 – Gases
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Kinetic Molecular Theory (KMT)
Defn – describes the behavior of gases in terms of particle motionMakes assumptions of size, motion, &
energy of gas particles
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KMT and Gases
Ideal Gases – imaginary gas that conforms to assumptions of KMT
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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
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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
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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
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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
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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
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Assumptions of KMT
***KEEP IN MIND IDEAL GASES DO NOT EXIST!!!!!!!***
APPLIES AT ALL TEMPERATURES AND ALL PRESSURE
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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
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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
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Characteristics of Gases
1) expansion2) fluidity3) low density4) compressibility5) diffusion
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4 measurable variables of gases
PressureTemperatureVolumeAmount
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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
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Sample problem #1
Convert 2.3 atm to torr
2.3 atm
1 atm
760 torr= 1748 torr
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Sample problem #2
Convert 450 mm Hg to kPa
450 mmHg
760 mm Hg
101.3 kPa= 60 kPa
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Measuring air pressure
What device is used to measure air pressure?????
BAROMETER
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Standard Temp and Pressure (STP)
P = 1 atmT = 0°C
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Temperature
Unit Kelvin (K)
Kelvin = °C + 273
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Temperature
Convert 45°C to Kelvin
45°C + 273= 318 K
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Temperature
Absolute zero – 0 KMolecules stop moving
Question: what is absolute zero temperature in Celsius?
-273°C
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Volume
Units:mLL
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Amount
Unitsgramsmoles
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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
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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
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Boyle’s Law
FormulaP1V1 = P2V2
P1 & V1 are initial conditions
P2 & V2 are final conditions
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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
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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
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Charles’ Law
Formula
V1 & T1 are initial conditionsV2 & T2 are final conditions
2
2
1
1
T
V
T
V
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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
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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
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Gay-Lussac’s Law
Formula
P1 & T1 are initial conditionsP2 & T2 are final conditions
2
2
1
1
T
P
T
P
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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
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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
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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
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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
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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
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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
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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
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Ideal Gas Law
Defn – relationship between P,V,T and # of moles, n
FormulaPV = nRT
ideal gas constant
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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
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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
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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
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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
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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