Transcript Gases - North Thurston Public Schools

Gases
Characteristics of Gases
• Unlike liquids and solids, they
– Expand to fill their containers.
– Are highly compressible.
– Have extremely low densities.
Composition of the Atmosphere
Atmospheric gases scatter
blue light more than other
wavelengths, giving the Earth
a blue halo when seen from
space.
Nitrogen
78.0842%
Oxygen
20.9463%
Argon
0.93422%
Carbon
dioxide
0.03811%
Water vapor
about 1%
Other
0.002%
Pressure
• Pressure is the
amount of force
applied to an area.
F
P=
A
• Atmospheric
pressure is the
weight of air per
unit of area.
Ideal-Gas Equation
PV = nRT
The Ideal-Gas concept assumes that the
gas molecules behave according to the
conditions in the Kinetic Molecular
Theory, which will be discussed later
in this unit.
Kelvin
• All motion within an atom ceases at zero
Kelvin (K) -- this point is called absolute zero
• 0 K = -273 C
• Absolute zero has never been reached in a lab
• Space is at an average of 2.73K (no joke) due
to big bang explosion!
Oxygen boiling point: 90.15 K (-183.0 °C )
Oxygen melting point: 54.36 K (-218.79 °C, -361.82 °F).
Nitrogen boiling point 77.35 K (-195.8 °C)
Nitrogen melting point 63.14 K (-210 °C or -346 °F)
Sample Data for Hall attendees and absentees
1. Length of Mg 3.00cm
1.422g=100.cm
5a. 20.0ᵒC
5b. 748 torr
6. 30.0mL
Kinetic-Molecular Theory
This is a model that aids
in our understanding of
what happens to gas
particles as
environmental
conditions change.
KMT
• 1. Gases consist of large numbers of molecules that are in
continuous, random motion.
• 2. The combined volume of all the molecules of the gas is
insignificant relative to the total volume in which the gas is
contained. REAL GASES TAKE UP SOME VOLUME
• 3. Attractive and repulsive forces between gas molecules are
insignificant. REAL GASES HAVE SOME ATTRACTIVE FORCES
• 4. Energy can be transferred between molecules during
collisions, but the average kinetic energy of the molecules
does not change with time, as long as the temperature of the
gas remains constant. NOT ALL COLLISIONS ARE ELASTIC
• 5. The average kinetic energy of the molecules is proportional
to the absolute temperature in Kelvin.
Main Tenets of Kinetic-Molecular
Theory
1. Gases consist of large numbers of molecules
that are in continuous, random motion.
Main Tenets of Kinetic-Molecular
Theory
• 2. The combined volume of the gas molecules
is insignificant relative to the total volume in
which the gas is contained.
…REAL GASES TAKE UP SOME VOLUME
• 3. Attractive and repulsive forces between gas
molecules are so small that they are ignored.
…REAL GASES HAVE small ATTRACTIVE and
REPULSIVE FORCES
Main Tenets of Kinetic-Molecular
Theory
4. Collisions between gas molecules are assumed to be
elastic, in that no energy is lost during collisions.
...NOT ALL COLLISIONS ARE ELASTIC
Main Tenets of Kinetic-Molecular
Theory
5. The average kinetic energy of the molecules is
directly proportional to the absolute temperature, in
Kelvin.
Kinetic molecular theory
• Pressure is caused by collisions of molecules
on the walls of the container. High pressure
results from high force of collisions and great
frequency of collisions. (total pressure is the
sum of the forces for all collisions)
• Temperature approximates molecular motion.
With increase temperature there is an
increase in KE and increase of force of
collisions and therefore greater pressure.
• What is the effect of a volume increase at
constant temp?
– Molecules collide less often with other molecules
and walls, so pressure decreases
• Effect of a temp increase at constant volume
– Increase in KE (KE=1/2mv2), and increase of force
of collisions (F=ma), therefore pressure increases
(P=F/A)
Effusion
The escape of gas
molecules
through a tiny
hole into an
evacuated space.
The trend: The
smaller the gas’s
molar mass the
faster the gas
moves and the
faster the effusion
Why is helium losing more gas
through the pores of the ballon
than nitrogen
Diffusion
The spread of one
substance throughout
a space or throughout
a second substance.
Again gases with smaller
molar masses move
faster
Real Gases
In the real world, the
behavior of gases only
conforms to the idealgas equation at
relatively high
temperature and low
pressure.
Why? At high temperatures molecules are moving very fast and any
loss of speed from resulting collisions or intermolecular forces are less
significant. At low pressures it is assumed to have very few molecules
and therefore individual gas molecule’s volume, energy lost during
collisions and intermolecular forces are less significant.
Deviations from Ideal Behavior
• The assumptions made in the kinetic-molecular model
break down at high pressure and/or low temperature.
• We assume gases occupy no space and have no
attractions.
• Real molecules do occupy space and attract one another.
• Usually only seen under high pressure conditions –useful
for industry which moves lots of gases around under high
pressure
R- The Universal Gas Constant
R can be calculated from values of STP and the Ideal
Gas Law equation:
PV=nRT
Ideal Gas Law Problem
Determine the volume in L of a gaseous system with
the following properties: 295K, 2.05moles, 775torr
Ideal Gas Law Problem
Determine the Temperature in Celsius of a gaseous
system with the following properties: 100.0L,
2.50moles, 1.25atm
Ideal Gas Law Problem
Determine the Pressure in torr of a gaseous system
with the following properties: 12.5L, 0.0995moles,
at 125ᵒC
Ideal Gas Law Problem
Determine the mass of a gaseous system containing
only Oxygen with the following properties: 50.0L,
400.K, 105kPa
Ideal Gas Law Problem with Stoich.
Determine the volume of water vapor formed if
2.50moles of C3H8 reacts with 10.0moles of Oxygen
gas at 25.0ᵒC and 767mmHg.
Ideal Gas w/ Stoich.
2.50L of Hydrogen gas at 1.05atm and 290.K reacts
with excess oxygen gas. How many grams of water
vapor could be formed theoretically?
Ideal Gas Stoich
1.50L of hydrogen gas at 752torr and 303K reacts
with 0.250moles of oxygen gas. Determine the
maximum mass of water vapor that can be formed.
Ideal Gas w/ stoich
10.0g of C2H6 reacts with 10.0L of Oxygen at 1.30atm
and 298K. Determine the maximum mass of Carbon
dioxide that can be formed.
Lab Calculations
Data table #3 convert mm Mg ribbon to moles
mmgmol use 1.4266g=1000mm
Data Table #5a Lab Temp convert to Kelvin
celsius + 273 = Kelvin
Data table#7 P(lab)=P(Hydrogen) +P(water vapor)
so… P(hydrogen) = P(lab) – P(water vap)
Look up Pwater as a function of temp in Table 12.2
Mole and Molar Volume Lab
#8 Use the Combined Gas Law to solve for V2 use STP
conditions for T2=273K and P2 =760 torr
P1V1/T1 = P2V2/T2 then V2= (P1V1T2)/(T1P2)
-once
you have solved for V2 convert your answer
from mL to L to use in #9
#9 (Your answer from #8 in L) / (moles of H2)
Moles of H2 is determined from the balanced
equation and moles of Mg that reacted (#3) you
are to assume that Mg is the limiting reactant and
HCl is in excess. Since the mole ratio is 1:1 then
moles of Mg=moles of H2