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Title: GENERAL CHEMISTRY SPRING 2013

1
The Behavior of GasesUnit 9
• GENERAL CHEMISTRYSPRING 2013

2
Air Pressure
• Pressure equation

Newtons (N)
Square meters (m2)
3
Unit for Air Pressure
f P a
• Pascals
• 1 N/m2 1 Pascal (Pa)
• Standard pressure is 101.325 kPa on Earth

1 kPa 1000 Pa!
Blaise Pascal (French philosopher and
mathematician)
Pascals Triangle
4
Air Pressure
f P a
• Without changing the mass (force), how can you
increase pressure?
• Without changing the mass (force), how can you
decrease pressure?

Decrease the area
Increase area
5
Practice Problem
f P a
• The mass of a brick is 2.0 kg (F19.6N), the
sides are 0.05m and 0.03m. What is the pressure
exerted?

19.6 N 13,067 Pa
(0.05 m x 0.03 m) 13,067 Pa
6
Pressure Units
• Ways to represent pressure

Units of Gas Pressure Units of Gas Pressure
Unit Standard Pressure
Atmosphere 1 atm (exactly)
Inches of Hg 29.9 in Hg
cm of Hg 76 cm Hg
mm of Hg 760 mm Hg
Torr 760 torr
Pounds per sq. in. 14.7 psi
Kilopascal 101 kPa
Torr is named after Evangelista Torricelli
7
The Barometer
• Etymology of barometer
• In Greek, baros weight
• Meter measure
• Literally means measure the weight of air or
air pressure.

Bariatric surgeory is weight loss surgeory
5 Footlong
8
The Barometer
• How a barometer works
• Air presses down on an open tray of Hg
• This downward pressure pushes the Hg up into a
tube
• Higher air pressure causes the mercury to go
higher up in the tube (measured as height, mm Hg)

9
Compressibility
• A gas will expand to fill its container
• Compressibility
• A measure of how much the volume of matter
decreases under pressure.
• Gases easily compress because of the space
between the particles

http//www.garagelibrary.com/images/airbag.jpg
10
Kinetic Molecular Theory
• Postulates the theory is based on
• Gases consist of tiny particles (of negligible
mass) with great distances separating them
• Gases are in constant random motion
• Molecular collisions are elastic
• Average kinetic energy is dependent only on
temperature

11
The Kelvin Scale
• As T increases, so does kinetic energy
• Theoretically, kinetic energy can be zero, but it
hasnt been achieved and probably wont ever be
achieved
• Absolute zero- The temperature at which a
substance would have zero kinetic energy
• The Kelvin Scale- a temperature scale directly
related to kinetic energy
• Zero on the Kelvin scale corresponds to zero
kinetic energy

12
The Kelvin Scale
• Units are Kelvins (K), with no degree (o) sign

Kelvin relates temperature to kinetic energy!
13
Temperature Conversions
• Easy to convert between Celsius and Kelvin
• How do you think?
• oC ? K? Add 273
• K ? oC? Substract 273
• 25oC ? K?
• (25273) 298 K
• 310 K ? oC?
• (310-273) 37oC
• Fahrenheit Celsius?
• (oF 32oF) x 5/9 oC
• (oC x 9/5) 32oF oF

14
Factors Affecting Gas Pressure
• Amount of gas
• Volume
• Temperature

http//www.bmumford.com/photo/highspeed/Ted1.jpg
15
Boyles Law
• Boyles inverse
• States that the volume of a gas varies inversely
(opposite) with pressure if temperature and
amount are held constant
• Written
• P1V1 P2V2
• (Demos from website Boyles Law and Scuba
Diving, 400 Marshmallows in a vacuum,
Mythbusters Scuba Crush)

Robert Boyle
16
Boyles Law in motion
http//www.grc.nasa.gov/WWW/K-12/airplane/Animatio
n/gaslab/Images/chprmt.gif
17
Boyles Practice 1
• A tank contains a volume of 3 L and a pressure of
4 atmospheres. What volume would the gas from
this tank fill up at a pressure of 1 atmosphere?
• P1 4atm
• V1 3L
• P2 1 atm
• V2 ?

Substitute into Boyles Law equation and solve
for V2 P1V1 P2V2
(4 atm)(3L) (1atm)(V2) 12 atmL V2
1 atm
V2 12 L
18
Boyles Practice 2
• Find the volume of a cylinder needed if you want
to put 50 atmospheres of pressure with a volume
of 3 L into a cylinder that can hold a pressure
of no greater than 20 atmospheres.
• P1 50 atm
• V1 3 L
• P2 20 atm
• V2 ?

Substitute into Boyles Law equation and solve
for V2 P1V1 P2V2
(50 atm)(3 L) (20 atm)(V2)
7.5 L V2
19
Charles Law
• Charles direct
• The volume of a gas varies directly with
temperature if the pressure and amount remain
constant
• Mathematically

Demos from Website Charles Law
20
Charles Law in Motion
21
Charles Law Problem 1
• Always convert temperature to Kelvin (K)
• A tank contains a volume of 3 L and a temperature
of 100oC. What volume would the gas from this
tank fill up at a temperature of 200oC?
• V1 3 L
• T1 100oC 373K
• V2 ?
• T2 200oC 473K

Substitute into equation and solve for V2
3L V2 373K
473K
(3L)(473K) (V2)(373K)
4L V2
22
Charles Law Problem 2
• A 275 L helium balloon is heated from 20oC to
40oC. Calculate the final volume assuming
pressure remains constant.
• V1 275 L
• T1 20oC (273K) 293K
• V2 ?
• T2 40273 313K

275L V2 293K 313K
(293)(v2) (275L)(313K)
V2 294 L
23
Temperature-Pressure Relationships
• Gay-Lussacs Law
• The pressure of a gas varies directly w/
temperature if volume and amount remain constant.
• Mathematically

Tire inflation. Summer vs. winterhmmm how do
these two differ?
Demos from website Live balloon, Steel drum
implosions, Mythbusters Water Heater Rocket
Joseph Louis Gay-Lussac 1778-1850
24
Gay-Lussac Problem 1
• Temperature in Kelvin (K)
• A tank contains a pressure of 3 atm and a
temperature of 100oC. What pressure would the
gas from this tank be at a temperature of 200oC?
• P1 3 atm
• T1 100oC (273K) 373K
• P2 ?
• T2 200oC 473K

3 atm P2 373 K 473 K
P2 4 atm
25
Gay-Lussac Problem 2
• Find the pressure needed if you wanted to put gas
at 50oC and 75 atm into a vessel that is at 65oC.
• P1 75 atm
• T1 50oC 323 K
• P2 ?
• T2 65oC 338 K

75 atm P2 323 K 338 K
P2 78 atm
26
Combined Gas Law
• Puts together several scientists work on how
gases behave when conditions are change.
• You can change three things about a gas to affect
pressure
• Amount of gas- this stays the same for now
• Temperature
• Volume

STP??Temperature 273KPressure 1 atm
27
CBL Problem 1 (1 p.11)
• A 50 mL sample of hydrogen gas is collected at
772 mm Hg and 21oC. Calculate the volume of
hydrogen at STP.
• P1 772 mm Hg ? atm
• V1 50 mL ? L
• T1 21oC 294 K
• P2 1 atm
• V2 X
• T2 273K

Since your final conditions are at STP, you need
to convert initial conditions to atm and L
28
CBL Problem 1 solution
P1 772 mm Hg x 1 atm/760 mm Hg 1.016 atm V1 50
mL / 1000 mL 0.05 L
(1.016 atm)(0.05L) (1atm)(V2) 294
K 273 K
Cross multiply, then solve for V2
(1.016 atm)(0.05L)(273 K) (1atm)(294K)(X)
0.05 L V2
29
Ideal Gases
• Ideal gases are
• Gases that behave under all conditions as
predicted by the kinetic molecular theory
• Gases are not ideal
• When they dont behave as predicted by kinetic
molecular theory
• Kinetic energy is
• Energy associated with motion

30
The Ideal Gas Law
• piv-nert
• PVnRT ALL UNITS MUST BE IN
• P Pressure (atm)
• V Volume (L)
• n moles (mol)
• R Ideal gas constant 0.0821 (Latm)/(molK)
• T Temperature (ALWAYS Kelvin)

31
IGL Problem 1
• If a container has a volume of 3 L and is at a
temperature of 60oC and a pressure of 6 atm, how
many moles of gas are in the container?
• V 3L
• T 60oC 333K
• P 6 atm
• R 0.0821 Latm/molK
• n ?

32
IGL Problem 1 Solution
• Substitute in PVnRT and solve for n

(6 atm)(3 L) (n)(0.0821)(333 K)
n 0.7 mol
The unit for R is Latm
molK Be sure units match this, and units will
cancel
33
IGL Problem 2
• If a container has 50 moles of O2 and is at a
temperature of 40oC and a pressure of 3 atm, how
many liters are in this container?
• n 50 mol O2
• T 40oC 313 K
• P 3 atm
• R 0.0821 Latm/molK
• V ?

34
IGL Problem 2 Solution
• Substitute in PVnRT and solve for V

(3atm)(V) (50 mol)(0.0821)(313 K)
V 428 L