Chapter 5 Gases Part I
Chapter 5 ­ Gases
5.1 ­ Pressure
A barometer is a device that measures atmospheric pressure.
How it works!
Pressure from the atmoshpere pushes down on the pool of mercury.
The higher the pressure or the more the atm.pushed down, the higher the mercury rises in the tube!
Standard pressure @ sea level ( a typical day) 1 atm of pressure will make the Hg in the column rise to a reading of 760 mmHg.
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Pressure (P) ‐ the force per unit area on a substance’s surface
Pressure (P) = Force Area Pressure (P) = Force (N)
Area (m2)
Force (weight) = mass x gravity
Force is the same, mass and gravity are the same for both bricks (weight).
The area over which the force is applied is different.
Less surface area in contact with ground, so more pressure!
Pressure and area are inversely related!
Pressure and force are directly related
Units of Pressure
Pressures of confined gases (in a container) are often measured with a similar device called a manometer.
Pgas= Patm ­ h
Pgas= Patm + h
pressure of gas is less than atm thus, atm pushes Hg down
toward flask
pressure of gas is greater than atm thus, pushes Hg up towards atm
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Chapter 5 Gases Part I
There are several ways of measuring pressure in chemistry: 1. Pascals (Pa) or kilopascals (kPa): 1 Pascal = 1 N/m
2. atmospheres (atm): 1 atmosphere = normal air pressure at sea level. 3. mm Hg: Barometric pressure 4. torr: Named after Torriceli, the inventor of the barometer Most problems report gas pressure in atmospheres or mm Hg. = 760 mmHg
STP (standard Temperature and Pressure): Since temperature and pressure both affect the volume of a gas, it is important to specify them both in gas problems. = 760 torr
In the metric system, standard temperature = 273 K (or 0°C).
= 101.325 Pa
Standard pressure = 1 atmosphere = 101.3 kPa = 760 mm Hg = 760 torr. Useful Chart for Conversions
1 atm
= 29.92 in Hg
= 14.7 lb/in2
memorize
1 torr = 1 mmHg
Useful Chart for Conversions
When solving gas equations, we must always use the absolute temperature scale, Kelvins. This solves a number of problems Convert 771 torr to atm
1 atm = 760 mmHg
= 760 torr
= 101.325 Pa
= 29.92 in Hg
because there are no negative temperatures or 0 on the Kelvin scale. = 14.7 lb/in2
1 torr = 1 mmHg
To convert Celsius to Kelvins, simply add 273 to the Celsius temperature.
Convert 99.3 kPa to mm Hg
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Chapter 5 Gases Part I
5.2 ­ The Gas Laws of Boyle, Charles, and Avogadro
MEMORIZE THIS EQUATION!!!
Combined Gas Law Equation
P2 V2
P1 V1
=
n1T1
n2T2
Boyle's Law = Pressure and Volume (Temp Constant)
P2 V2
P1 V1
= nT
2 2
n1T1
n and T are constant
P1 V1 = P2 V2
As P , V
Boyle's Law
In the three centuries since Boyle produced these results, measurement techniques have improved tremendously. These results (PV = constant value) only hold @ low pressures. Measurements @ higher pressures reveal PV is not constant, but varies.
Charles's Law = Volume and Temperature
1 L
O
O
O
­273 C
O C
273 C
O
­273 C is an unattainable temperature = absolute zero.
Abolute zero ­ all movement stops, volume of a gas is zero, gases do not exist @ this unattainable temp. (@ least not yet!)
• Kelvin Scale ­ SI base unit for measuring temp. No negatives
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Chapter 5 Gases Part I
Charles' Law
P2V 2
P1 V1
= nT
2 2
n1T1
V1
T1
V 2
= T
2
As V , T and vice versa, n and P are constant
Kelvin Scale
K = Celsius + 273
C = Kelvin ­ 273
When doing calculations with gases, all temperatures must be in Kelvin.
Avogadro's Law
P2V 2
P1 V1
= nT
2 2
n1T1
must be at the same T and P, or constant
V1
n1
V
= 2
n2
­ equal volumes of gases at the same temp and pressure have the same numbers of particles.
Ideal Gas Law
An ideal gas is hypothetical. • It's a real gas that approaches high temps and low pressures.
• Obeys all points of the Kinetic­Molecular theory.
• Real gases act mostly ideal so it is ok for us to assume they are ideal.
PV = nRT P = pressure, V = Volume, n = moles of gas, R = constant, and T = temperature
R = 0.0821 L * atm
Note: the value of R changes depending on units of K * mol
pressure, we will primarily use
equation of state ­ the state of a system at a given time.
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Chapter 5 Gases Part I
Different Values of R
Again, The gas constant “R” may have different values depending upon which unit of pressure is used. R is easily calculated by rearranging the equation to solve for R and then substituting in the values for 1 mole of a gas at STP. R = PV/nT = 1 atm x 22.4 L
= 0.0821 L atm
1 mol x 273 K mol K If pressure is in kPa (101.3 kPa), then R = 8.31L kPa/mol K If pressure is in mm Hg, then R = 62.4 L mmHg/mol K
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