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7/31/2019 Student Ch 1 Gases
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1
GASESChemistry II Chapter 1
Instructor
--
Ravi
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2
THREE STATES OF
MATTER
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3
General Properties
of Gases There is a lot of free
space in a gas.
Gases can be expandedinfinitely.
Gases fill containers
uniformly and completely. Gases diffuse and mix
rapidly.
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4
Properties of Gases
Gas properties can be modeledusing math. Model dependson
V = volume of the gas (L)
T = temperature (K)
ALL temperatures in theentire chapter MUST be in
Kelvin!!! No Exceptions! n = amount (moles)
P = pressure(atmospheres)
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5
PressurePressure of air is
measured with aBAROMETER(developed by
Torricelli in 1643)Hg rises in tube until force of Hg
(down) balances the force ofatmosphere (pushing up). (Justlike a straw in a soft drink)
P of Hg pushing down related to
Hg density
column height
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6PressureColumn height measures
Pressure of atmosphere 1 standard atmosphere
(atm) *
= 760 mm Hg (or torr) *
= 29.92 inches Hg *= 14.7 pounds/in2 (psi)
*HD only
= 101.3 kPa (SI unit isPASCAL) *HD only
= about 34 feet of water!
* Memorize these!
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7
Pressure Conversions
A. What is 475 mm Hg expressed in atm?
1 atm
760 mm Hg
B. The pressure of a tire is measured as 29.4 psi.
What is this pressure in mm Hg?
760 mm Hg
14.7 psi= 1.52 x 103 mm Hg
= 0.625 atm475 mm Hg x
29.4 psi x
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8
Pressure Conversions
A. What is 2 atm expressed in torr?
B. The pressure of a tire is measured as 32.0 psi.
What is this pressure in kPa?
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9
Boyles LawP 1/VThis means Pressure
and Volume areINVERSELY
PROPORTIONAL ifmoles andtemperature areconstant (do notchange). For
example, P goes upas V goes down.
P1V1 = P2 V2
Robert Boyle(1627-1691).Son of Earl ofCork, Ireland.
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Boyles Law and Kinetic
Molecular Theory
P proportional to 1/V
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Boyles Law
A bicycle pump is agood example ofBoyles law.
As the volume of theair trapped in thepump is reduced, itspressure goes up,and air is forced intothe tire.
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Charless
LawIf n and P are
constant,then V T
V and T are directlyproportional.
V1 V2=
T1 T2
If one temperature goes
up, the volume goes up!
Jacques Charles (1746-
1823). Isolated boronand studied gases.Balloonist.
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Charless original balloon
Modern long-distance balloon
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Charless Law
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Gay-Lussacs Law
If n and V areconstant,then P T
P and T are directlyproportional.
P1 P2=
T1 T2
If one temperature goes
up, the pressure goes up!
Joseph Louis Gay-Lussac (1778-1850)
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16
Gas Pressure,Temperature, and Kinetic
Molecular Theory
P proportional to T
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Combined Gas Law
The good news is that you donthave to remember all three gaslaws! Since they are all related toeach other, we can combine theminto a single equation. BE SUREYOU KNOW THIS EQUATION!
P1 V1 P2 V2=
T1 T2
No, its not related to R2D2
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Combined Gas Law Problem
A sample of helium gas has a volume of 0.180
L, a pressure of 0.800 atm and a temperature of
29C. What is the new temperature(C) of thegas at a volume of 90.0 mL and a pressure of
3.20 atm?
Set up Data TableP1 = 0.800 atm V1 = 180 mL T1 = 302 K
P2 = 3.20 atm V2= 90 mL T2 = ??
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Calculation
P1 = 0.800 atm V1 = 180 mL T1 = 302 KP2 = 3.20 atm V2= 90 mL T2 = ??
P1 V1 P2 V2= P1 V1T2 = P2 V2 T1
T1 T2
T2 = P2 V2 T1P1 V1
T2 = 3.20 atm x 90.0 mL x 302 K
0.800 atm x 180.0 mL
T2 = 604 K - 273 = 331 C
= 604 K
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21And now, we pause for this
commercial message from STP
OK, so its really not THIS kind
of STP
STP in chemistry stands for
Standard Temperature andPressure
Standard Pressure =
1 atm (or anequivalent)
StandardTemperature = 0 deg
C (273 K)
STP allows us to
compare amounts ofgases between different
pressures andtemperatures
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Avogadros HypothesisEqual volumes of gases at the same
T and P have the same number ofmolecules.
V = n (RT/P) = knV and n are directly related.
twice as manymolecules
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Avogadros Hypothesis and
Kinetic Molecular Theory
P proportional to n
The gases in thisexperiment are all
measured at thesame T and V.
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24
IDEAL GAS LAW
Brings together gasproperties.
Can be derived from
experiment and theory.BE SURE YOU KNOW
THIS EQUATION!
P V = n R T
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Using PV = nRTP = PressureV = Volume
T = Temperature
N = number of moles
R is a constant, called the Ideal Gas Constant
Instead of learning a different value for R for all thepossible unit combinations, we can just memorize
one value and convert the units to match R.
R = 0.0821L atm
Mol K
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Using PV = nRTHow much N2 is required to fill a small room
with a volume of 960 cubic feet (27,000 L)to 745 mm Hg at 25 oC?
Solution
1. Get all data into proper units
V = 27,000 L
T = 25 oC + 273 = 298 K
P = 745 mm Hg (1 atm/760 mm Hg)= 0.98 atm
And we always know R, 0.0821 L atm / mol K
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Using PV = nRTHow much N2is reqd to fill a small room with a volume of 960
cubic feet (27,000 L) to P = 745 mm Hg at 25 oC?
Solution
2. Now plug in those values and solve for
the unknown.
PV = nRT
n =(0.98 atm)(2.7 x 10
4L)
(0.0821 L atm/K mol)(298 K)
n = 1.1 x 103
mol (or about 30 kg of gas)
RT RT
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Learning Check
Dinitrogen monoxide (N2O), laughinggas, is used by dentists as ananesthetic. If 2.86 mol of gas occupies a
20.0 L tank at 23C, what is the pressure(mm Hg) in the tank in the dentist office?
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30Health NoteWhen a scuba diver is several
hundred feet under water, thehigh pressures cause N2 from
the tank air to dissolve in the
blood. If the diver rises too
fast, the dissolved N2 will formbubbles in the blood, a
dangerous and painful
condition called "the bends".
Helium, which is inert, lessdense, and does not dissolve
in the blood, is mixed with O2
in scuba tanks used for deep
descents.
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31Collecting a gas over water
Gases, since they mix with other gases readily, must
be collected in an environment where mixing can notoccur. The easiest way to do this is under waterbecause water displaces the air. So when a gas iscollected over water, that means the container isfilled with water and the gas is bubbled through the
water into the container. Thus, the pressure inside thecontainer is from the gas AND the water vapor. This iswhere Daltons Law of Partial Pressures becomesuseful.
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GAS DENSITY
Highdensity
Lowdensity
22.4 L of ANY gasAT STP = 1 mole
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