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

As liquid nitrogen (-196oC) is poured over a balloon, the volume of the balloon decreases.

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?

Gas Laws

What happens when you “blow up” a balloon?

Gas Laws

What happens when you “blow up” a balloon?

Gas Laws

What happens when you “blow up” a balloon?

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

Gas Laws

The value of the gas constant (R) depends on the units of P, V, n, and T. T must always be in Kelvin n is usually in moles

If P (atm) and V (L), then R = 0.08206 atm.L

mol.K If P (torr) and V (L),

then R = 62.36 L.torrmol.K

I will give you these on the test.