No Slide Title · Pressure The force exerted on an object divided by the surface area of the...
Transcript of No Slide Title · Pressure The force exerted on an object divided by the surface area of the...
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Gases
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Properties of Gases
assume the volume and shape of their containersmost compressible of the states of mattermix evenly and completely with other gasesmuch lower density than other forms of matter
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Substances that Exist as Gases
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Elements that exist as gases at 25˚C and 1 atm. The Noble gases (the Group 8A elements) are monatomic species; the other elements exist as diatomic molecules. Ozone (O3) is also a gas.
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Some substances found as gases at 1 atm and 25 ºC
Elements H2, N2, O2, O3, F2, Cl2 He, Ne, Ar, Kr, Xe, Rn
Compounds HF, HCl, HBr, Hl, CO, CO2, NH3, NO, NO2, N2O, SO2, H2S, HCN
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Pressure of a Gas
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Pressure
The force exerted on an object divided by the surface area of the object;
P = FA
Any gas confined to a container is found to exert a pressure on the container .
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Gases
© 2012 Pearson Education, Inc.
Atmosospheric Pressure
• is the weight of air per unit of area.
P = FA
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SI Units of Pressure
customary units 1.00 standard atm = 760 mm Hg SI units pressure = force/area pressure = Newton/m2 = Pascal 1 standard atmosphere = 101325 Pa
= 101.325 kPa
= 760 torr
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vacuum
mercury
A Torricellian Barometer
atmospheric pressure
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vacuum
mercury
760 mm Hg(1 standardAtmosphere)
A Torricellian Barometer
atmospheric pressure
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The Gas Laws
Boyle’s LawCharles’ Law
Avogadro’s LawThe Ideal Gas Law
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The Gas Laws and the Scientific Method
ObservationsLaws
TheoryHypothesis
Experiment
BoyleCharles
AvogadroIdeal Gas
Kinetic-molecular theory
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Boyle’s Law
Robert Boyle 1626 – 1691
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Gases
© 2012 Pearson Education, Inc.
Boyle’s Law The volume of a fixed quantity of gas at
constant temperature is inversely proportional to the pressure.
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Pressure(mm Hg)
Volume(arbitrary
units)
724
869951
998
1230
1893
2250
1.50
1.331.22
1.160.94
0.610.51
Boyle’sData
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1.2
1.0
0.8
0.6
0.4
0.2
00 1 2 3 4 5
Pressure(atm)
Volume (L)
P/2
2V
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Pressure(mm Hg)
Volume(arbitrary
units)
724
869951
998
1230
1893
2250
1.50
1.331.22
1.160.94
0.610.51
Boyle’sData
PV
1090
11601160
1160
1200
12001100
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Boyle’s Law
at constant temperature, the volume of a constant amount of gas is inversely proportional to the pressure
constant n, constant T
PV = k
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Plot of P versus V is a hyperbola
PV = k
P = (1/V ) k
is of the form y = mx + b, which is the equation for a straight line
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P
0.6 at m
0.3 at m
2L 4L V
P
1 V
(a) (b)
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Charles’s Law
Jacques Charles 1746 – 1823
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Charles’s Law
at constant pressure, the volume of a constant amount of gas is directly proportional to the absolute temperature
constant n, constant P
V = kT
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50
40
30
20
10
0–300 –200 –100 0 100 200 300 400
He
CH4
H2
N2O
t(ºC)
V(m
L) –273.15 º C
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Celsius scale Kelvin scaleTemperature atthe surface of
the sun
Melting point of gold
Boiling point of water
Body temperature
Melting point of ice
Boiling point of N2
Boiling point of He
Lowest temperature(absolute zero)
5727˚ C
1064˚ C
100˚ C
37˚ C
0˚ C
–196˚ C
–268.95˚ C
–273.15˚ C
6000 K
1337 K
373.15 K
310 K
273.15 K
77K
4.2 K
0 K
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Avogadro’s Law
Amadeo Avogadro (1776 – 1856)
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Gases
© 2012 Pearson Education, Inc.
Avogadro’s Law• The volume of a gas at constant temperature
and pressure is directly proportional to the number of moles of the gas.
• Mathematically, this means V = kn
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The Ideal-Gas Equation
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The Ideal Gas Law
PV = nRT
constant n, constant T (Boyle’s Law)
constant n, constant P (Charles’ Law)
constant P, constant T (Avogadro’s Law)
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The Ideal-Gas Law
PV = nRT
P is pressure in atmospheresV is volume in litersn is number of molesT is absolute temperature in KelvinsR is called the gas constant
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An ideal gas is a hypothetical gas whose pressure-volume-temperature behavior can be completely accounted for by the ideal-gas equation.
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Standard temperature and pressure
PV = nRT
standard conditions defined
P = 1 atmT = 0ºC (273 K) STP
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Standard Molar Volume
The volume occupied by 1 mole of a gas at STP is 22.414 L
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The Gas Constant R
The volume occupied by 1 mole of a gas at STP is 22.414 L
PV = nRT
R = 0.082057 (L atm/mol K)
R = PVnT
R = (1 atm) (22.414 L)(1 mol) (273.15 K)
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Gases
© 2012 Pearson Education, Inc.
The constant of proportionality is known as R, the gas constant.
The Gas Constant R
0.0821 Latm/molK
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Example
What is the volume occupied by 49.8 g of HCl at STP?
V = 30.6 L
P = 1 atmT = 273 K
(1.366 mol) (0.0821 L atm/mol K) (273K)1 atm
= 1.366 mol49.8 g x 1mol36.46 g
nRTV =
P
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Example
What is the volume occupied by 49.8 g of HCl at STP? (alternative solution)
V = 30.6 L
P = 1 atmT = 273 K
n = 49.8 g36.46 g/mol = 1.366 mol
V = x 1.366 mol22.4 L1 mol
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Example
A compound has the empirical formula BH3. At 27 º C, 74.3 mL of the gas exerted a pressure of 1.12 atm. If the mass of the gas was 0.0934 g, what is its molecular formula?
PV = nRT n = PVRT
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Example (cont.)A compound has the empirical formula BH3. At 27 º C, 74.3 mL of the gas exerted a pressure of 1.12 atm. If the mass of the gas was 0.0934 g, what is its molecular formula?
B2H6
n = 0.00338 mol0.0934 g
0.00338 mol= 27.6 g/mol
BH3 = 13.8 g / empirical formula
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The ideal gas law is often used to calculate the changes that will occur when the
conditions of a gas are changed
PV = nRT
If nRT are constant
PV = nRT
If nRP are constant
nRTV
P =1( ) nR
PV = T( )
= P2V2nRTPV =( ) V2
T2
=nRP
VT
=( )
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The ideal gas law is often used to calculate the changes that will occur when the
conditions of a gas are changed
PV = nRT
If PRT are constant
PV = nRT
If nRV are constant
PRT n V =( )
V 2n 2
=Vn
= RTP
( )V
nRP = T( )P2
T2
=nRV
PT
=( )
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Example
A sample of oxygen gas initially at 0.97 atm is cooled from 21 º C to -68 º C at constant volume. What is its final pressure.
PV = nRT
P1 P2
T1 T2
=
P2
T2
=nRV
PT
=( )
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Example
A sample of oxygen gas initially at 0.97 atm is cooled from 21 º C to -68 º C at constant volume. What is its final pressure.
(0.97 atm) 292 K
P2
205 K= P2 = 0.68 atm
P1 P2
T1 T2
=
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Density Calculations
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Density Calculations
n=
V RT
P mass=
Molar mass(V) RT
P
mass
Vdensity =
Therefore: density =RT
P (Molar mass)
mass
Molar massn =
g
g/mol=
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Example
What is the density of UF6 gas at 62 º C and 779mmHg ?
= 13.1g/L
n=
V RT
P
n
V
779mmHg
760mmHg/atm
(0.0821 L atm/mol K) (335 K)=
=L
0.0373 mol 352.03g
1 molx
A common unit for gasses
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Example
Cyanogen, empirical formula CN, is a gas with a density of 2.335 g/L at 0ºC and 1 atm. What is its molecular formula?
= 52 g/mol Cyanogen
n =V
RT
P
= 0.0446 mol
2.335g
n(1atm)
(0.0821 L atm/mol K) (273K)=
(1 L)
26g /mol (CN)= 2
C2N2