Download Chemistry- The Gas Phase

Chemistry
The Gas Phase
I. Pressure
When gas molecules collide with a surface,
we say that they exert pressure
Pressure: Force exerted on an object per
unit area
Pressure = Force/Area
= 1N /m2
= 1 pascal (Pa)
Atmospheric Pressure
Force of atmosphere/Area is = to weight of
column of air above it
Gas is fluid (like water) so exerts pressure in
all directions on an object
Atmospheric Pressure = Pressure exerted by
Earth’s atmosphere
Column of Air/Column of Mercury
Units for Pressure
1 Pa = 1 N/m2
1000 Pa = 1 kPa
1 atm = 101.3 kPa
= 760 mmHg
= 760 torr
II. Boyle’s Law
(Temp the same) P proportional to 1/V
PV = constant
(inversely proportional)
Boyle’s Law: P1V1= P2V2
III. Charles’s Law
(Kept pressure the same)
Temp is directly proportional to volume
V/T = constant
V1/T1 = V2/T2
Directly proportional means
Linearly (line) proportional
If V1/T1= V2/T2, what can’t T =?
Must use Kelvin for temp (K = oC + 273.15)
IV. The Combined Gas Law
P1V1 = constant
T1
or…
P1V1 = P2V2
T1
T2
Problems
1. A weather balloon is filled with helium.
T = 25 oC P = 1.0 atm V = 4.5 m3
As it rises:
P = 0.30 atm T = 15 oC
?V
2. Gas is stored in 3.00 L container at a
pressure of 456 torr. If T stays constant,
what will P be if V is decreased to 0.75L?
OYO’s
12.1 Gas is stored in metal container (V can’t
change.) If gas is packed in at T=0.00 oC and P=
3,415 kPa, what will P equal when stored at
room temp (25oC)?
12.2 At room temp and normal atm P (1.00 atm),
the avg lungs have a capacity of 5.5 L. Susie
takes a deep breath and dives into a cold lake (T
= 19oC, P=915 torr)
?V
12.3 You fill a balloon with air to V= 2.31L at room
temp. What is V when you stick it in a freezer
(T = -5oC)?
V. Avogadro’s Law
At constant P and T, the V of a gas is directly
proportional to the # of moles (n) of the
gas. V/n = constant
VI. Ideal Gas Law
Combining Boyle’s, Charles’s, and Avogadro’s Laws, we
get:
PV = nRT (memorize)
R = gas constant
= 0.0821 L atm
mole K
= 8.315 J
mole K
Don’t memorize R
Ideal Gas: a hypothetical gas whose pressurevolume-temperature behavior can be completely
accounted for the ideal gas equation. (molecules
or atoms of gas don’t interact with each other at
all)
This happens when:
1. Volume of container is large compared to the
V of the gas (P is low)
2. Molecules of gas don’t repel or attract one
another (they’re moving too fast – higher T)
At Standard Temperature and Pressure (STP)
T = 0 oC
P= 1 atm
Ideal Gas Problems
1. Sulfur hexafluoride is a colorless,
odorless, very unreactive gas. Calculate
the pressure (in atm) exerted by 1.82
moles of the gas in a steel vessel of
volume 5.43L at 45oC.
2. Calculate the volume (in liters) occupied
by 7.40 g of NH3 at STP.
OYO
12.4 A small bubble rises from the bottom of
a lake, where the T is 8oC and the P is 6.4
atm. Calculate the final V (in mL) of the
bubble if its initial V was 2.1 mL.
12.5 A gas in a 12.2 L container at P=13.5
atm and T=25oC. How many moles of gas
are present?
12.6 How many L does a 1.00 mole of an
ideal gas occupy at STP?
VII. Gas Stoichiometry
Steps:
Amount of Reactant→ Moles of Reactant→ Moles of Product→ Amount of Product
(grams or volume)
(grams or volume)
Sodium azide (NaN3) is used in some automobile air bags. The
impact of the collision triggers the decomposition of NaN3 as
follows:
2NaN3(s)→ 2Na(s) + 3N2(g)
The N2 gas produced quickly inflates the bag between the driver
and the windshield and dashboard. Calculate the V of N2
generated at 80oC and 823 mmHg by the decomposition of
60.0 g NaN3.
One way H2 gas can be produced commercially
is to react a strong acid with a metal:
2HCl(aq) + Zn(s) → H2(g) + ZnCl2(aq)
If 512 g of HCl are reacted with excess Zn at
T=24 oC and P=0.95 atm, what volume of H2
gas will be produced?
OYO’s
12.7 The equation for the metabolic
breakdown of glucose (C6H12O6) is the
same as the equation for the combustion
of glucose in air:
C6H12O6(s) + 6O2(g)→ 6CO2(g) + 6H2O(l)
Calculate the volume of CO2 produced at 37oC
and 1.00 atm when 5.60 g of glucose are used
up in the reaction.
12.8 In a car engine, gasoline (mostly octane, C8H18) is
burned to make CO2(g) and H2O(g). If a gallon of
octane is burned in excess O2 at P=1.00 atm and
T=180oC, how many L of CO2 are produced? 1 gallon
of octane = 3.2 x 103 g.
12.9 Methanol (wood alcohol) can be produced as
follows:
CO(g) + 2H2(g) → CH4O(l)
If 500.0 L of H2(g) are reacted with excess CO at STP,
how many grams of CH4O can be produced?
VIII. Dalton’s Law of Partial
Pressures
Definition: When 2 or more gases mix
together, the total pressure of the mixture
equals the sum of the individual pressures
PT = P1 + P2 + P3 + …..
The pressure of an ideal gas does not
depend on the identity of the gas, only the
amount of the gas.
One mole of gas (at STP) takes up
about the same space – regardless
of the mass of the gas!
Dalton’s Law of Partial Pressures
Dalton’s Law of Partial Pressures is especially useful when
you collect a gas “over water.”
But, first we need to learn about
Vapor Pressure:
The pressure exerted by the vapor which sits on top of
any liquid
Dependent on:
1. The identity of the liquid
- water's vapor pressure is low (molecules
escape slowly)
- gasoline's vapor pressure is high (molecules
escape quickly)
2. Temperature
- vapor pressure increases with temperature
- when vapor pressure equals atmospheric
pressure, boiling occurs
Boiling Point: The temperature at which the vapor pressure
of a liquid is equal to normal atmospheric pressure
At what temperature would you expect the vapor pressure
of water to equal 1.0 atm or 760 torr?
100oC
Would the boiling point for water be higher or lower in
Denver?
Denver!
Would it take longer to hard-boil an egg in Troy or Denver?
Denver!
Relating vapor pressure to Dalton's law:
-- When a gas is "collected over water,"
part of the gas pressure is due to water
vapor pressure
- higher temperatures result in an
increase in water vapor pressure and a
decrease in the collected gas pressure
-- Can calculate how much of each using
Dalton's law and Table 12.1 (Vapor
Pressure of Water)
Example: O2 gas generated by the
decomposition of KClO4 is collected.
V=128 mL, P= 762 mmHg. Calculate the
mass (in grams) of O2. The pressure of the
water vapor at 24oC is 22.4 mmHg.
Example:
A chemist produces hydrogen gas in an
experiment and collects that gas in the
presence of water. If the total pressure of
gas collected was 1.115 atm and the
temperature was 26 oC, what pressure of
hydrogen gas was collected?
OYO
12.10 A chemist collects carbon dioxide
over water. The pressure of gas in the
collection container was 961 torr. If
T=22oC, what was the pressure of the
CO2? Water vapor pressure at 22oC =19.8
torr.
Mole Fractions – Another way of
using Dalton’s Law
A mole of gas takes up approximately the
same amount of space- regardless of its
identity (22.4 L)
Mole Fraction (x): # of moles of
component/total # moles in mixture
no units (dimensionless quantity)
Example 12.3
A mixture of gases contains 3.0 moles of oxygen
gas, 5.0 moles of nitrogen gas, and 1.0 mole of
neon gas. What is the mole fraction of each
component in the mixture?
Total # of moles = 3.0 + 5.0 + 1.0 = 9.0 moles
Mole fraction = # moles of component/ total #
moles
xO2 = 3.0 moles/9.0 moles = 0.33
xN2 = 5.0 moles/9.0 moles = 0.56
xNe = 1.0 moles/9.0 moles = 0.11
OYO
12.11
A tank of compressed air contains 672.0 g
of O2, 2,184.0 g of N2, and 40.0 g of Ar.
What is the mole fraction of each
component in this mixture?
Restating:
PT = P1 + P2 + P3 + …..
P1 = x1 PT , where x1 is the mole fraction
of a particular gas
Example A: A gas mixture contains 40.0 grams of
carbon monoxide and 10.0 grams of nitrogen
dioxide. If the total pressure of this gas mixture
is 1.2 atms, what is the partial pressure of each
individual gas?
Example B
A gas mixture contains 456 torr of oxygen,
112 torr of CO2 and 501 torr of nitrogen.
What is the mole fraction of each
component in the mixture?
OYO’s
12.12 A sample of artificially made air contains 78
g of N2 and 21 g of O2. If the pressure of the air
sample is 760 torr, what is the partial pressure of
each gas?
12.13 A chemist produces H2 gas in chemical
reaction and collects it over water at 23oC. If the
pressure of the gas inside the collection vessel
is 781 torr, what is the mole fraction of the H2
gas in the vessel?