Unit 6 Gases, Phase Changes and Introduction to Thermochemistry
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Transcript of Unit 6 Gases, Phase Changes and Introduction to Thermochemistry
Unit 6Gases, Phase Changes and Introduction to Thermochemistry
Characteristics of Gases
Pressure
Kinetic-Molecular Theory
The Gas Laws
Partial Pressures
Effusion and Diffusion
Real Gases
Part I: Gases
Properties of Gases
Three phases of matter
solid
liquid
gas
Definite shape and volumeDefinite volume, shape of container
Shape and volume of container
Properties of Gases
A gas is a collection of molecules that are very far apart on average. In air, gas molecules occupy only 0.1%
of the total volume. In liquids, molecules occupy ~ 70% of
the total space.
Properties of Gases
Gases are highly compressible. Volume decreases when pressure is
applied.
Gases form homogeneous mixtures with each other regardless of the identities or relative proportions of the different gases. Water and gasoline = heterogeneous
mixture. Water vapor and gasoline vapor =
homogeneous mixture.
Properties of Gases
Properties of gases vary depending on their composition. Air: ~ 78% N2 and ~ 21% O2
CO2: colorless, odorless CO: colorless, odorless, highly toxic NO2: toxic, red-brown, irritant N2O: colorless, sweet odor (laughing
gas)
Pressure
Four quantities are commonly needed to describe a gas: amount of gas (n) Temperature (T) Volume (V) Pressure (P)
Pressure
Gases exert pressure on the objects in their surroundings.
Pressure is caused by collisions between the gas molecules and objects with which they are in contact.
Pressure: the force exerted on a unit area
P = FA
Pressure
Atmospheric pressure: the pressure exerted by gas molecules in the air on all objects exposed to the atmosphere Atmospheric pressure varies with
altitude.Altitude (ft above sea level)
Atmospheric Pressure
in. Hg Torr psi
0 29.92 760 14.7
5000 24.9 632.5 12.23
10,000 20.58 522.7 10.1
Pressure
Gravity decreases Density of gas decreases Fewer gas molecules
Fewer collisionsLower pressure
Why does atmospheric pressure decrease with increasing altitude?
Pressure
Many different units used to report pressure.
millimeters of Hg (mm Hg) inches of Hg (in. Hg) pounds per square inch (psi) atmosphere (atm) torr (torr) pascal (Pa) = SI base unit kilopascal (kPa)
Must know units and
abbreviations!!
Pressure
Relationships between different pressure units:
1 atm = 760 mm Hg= 760 torr= 29.92 in. Hg= 14.7 psi= 1.01325 x 105 Pa
Must be able to
interconvert between
units.Memorize the ones in red…I’ll
give you the others.
You must know that 1 kPa = 1000 Pa
Pressure
Example: The measured pressure inside the eye of a hurricane was 669 torr. What was the pressure in atm?
Pressure
Example: On a nice sunny day in Chicago the barometric pressure was 30.45 in. Hg. What was the pressure in Pa?
Pressure
Example: On Titan, the largest moon of Saturn, the atmospheric pressure is 1.631 Pa. What is the pressure in atm?
Kinetic Molecular Theory
The behavior of gases can be described and explained using kinetic molecular theory.
the “theory of moving molecules”
You must know the basic ideas that are part of kinetic molecular theory.
Kinetic Molecular Theory
Gases consist of large numbers of molecules that are in continuous, random motion.
The combined volume of all the molecules of the gas is negligible compared to the total volume in which the gas is contained. i.e. the molecules are very far apart
on average
Kinetic Molecular Theory
Attractive and repulsive forces between gas molecules are negligible.
Energy can be transferred between molecules during collisions, but the average kinetic energy of the molecules does not change as long as the temperature remains constant. Collisions are perfectly elastic.
Kinetic Molecular Theory
The average kinetic energy of the molecules is proportional to the absolute temperature.
At any given temperature all molecules of a gas have the same average kinetic energy.
As T (in K) increases, KE increases.
Gas Laws
Four variables are needed to define the physical condition or state of any gas: Temperature (T) Pressure (P) Volume (V) Amount of gas (moles: n)
Equations relating these variables are known as the gas laws.
P
Gas Laws
Consider a fixed amount of gas that is confined to a container with a certain volume.
At a specific temperature, the gas sample will exert a certain pressure on the container..
Gas Laws
VolumeVolumedecreasesdecreases
PP
What will happen to the pressure if the volume is decreased?
Gas Laws
As the volume of a fixed quantity of gas decreases, the pressure increases because: gas molecules are more tightly packed
togetheri.e. denser
more collisions between gas molecules and the container
greater pressure
Gas Laws
Boyle’s Law: The volume of a fixed quantity of gas
maintained at constant temperature is inversely proportional to the pressure.
Mathematically,
V = k x 1 or PV = k or P1V1 = P2V2
Pat constant temperature and quantity of
gas
Gas Laws
Charles’ Law: The volume of a fixed amount of gas
maintained at constant pressure is directly proportional to its absolute temperature.
V = k x T or V = k or V1 = V2
T T1 T2
At constant pressure and quantity of gas
Remember: T must be in Kelvin
Gas Laws
On a molecular level, as the temperature of a gas maintained at constant pressure decreases, KE decreases fewer collisions between gas
molecules and the environment (i.e. container)
volume decreases in order to maintain constant pressure
Gas Laws
What happens when you “blow up” a balloon?
– the number of moles of gas (n) increases
and– the volume of the
gas (balloon) increases
Gas Laws
Avogadro’s Law: The volume of a gas maintained at
constant temperature and constant pressure is directly proportional to the number of moles of the gas.
Mathematically,
V = constant x nAt constant temperature and pressure
Gas Laws
At any given temperature and pressure, as the amount of gas increases, the number of gas molecules increases the number of collisions between gas molecules and
the environment (container) increases the volume must increase in order to maintain
constant pressure
Gas Laws
In a chemical reaction, we use the coefficients to tell us how many moles or molecules are used or produced in a chemical reaction.
N2 (g) + 3 H2 (g) 2 NH3 (g)
1 mole of nitrogen reacts with 3 moles of hydrogen to produce 2 moles of ammonia
Gas Laws
Since the volume of a gas is directly proportional to the number of moles of gas at constant temperature and pressure, we can also use the coefficients to represent the volume of a gas involved in a reaction. (Avogadro’s Hypothesis)
N2 (g) + 3 H2 (g) 2 NH3 (g)
1 liter of nitrogen reacts with 3 liters of hydrogen to produce 2 liters of ammonia
Gas Laws
Boyle’s Law, Charles’ Law, and Avogadro’s Law can be combined to make a more general gas law:
Ideal Gas Law:
PV = nRT
where P = pressureV = volume n = moles T = temperature (K) R = gas constant