Chapter 13 Notes States of Matter. Kinetic Theory and Gases Kinetic Energy—Energy that an object...

Chapter 13 NotesStates of Matter

Kinetic Theory and Gases

• Kinetic Energy—Energy that an object has due to motion.

• The Kinetic Theory is that tiny particles form all matter, and they are constantly in motion.

Kinetic energy vs. Potential Energy

Kinetic refers to motion—so kinetic energy is the energy of motion; this is different from potential energy, which is the possible amount of energy stored in something.

-the kinetic theory states that tiny particles form all matter, that are in constant motion.

Kinetic Theory and Gases

1) A gas is composed of particles that are small, hard spheres with insignificant volume and no particle interaction.

Kinetic Theory and Gases

2) Particles in a gas are in constant motion—they travel straight paths unless they collide with another particle or their container.

Kinetic Theory and Gases

3) All collisions are considered elastic—no energy is lost to friction.

Kinetic Theory and Gases

4.) No kinetic energy is lost when gas particles collide

– elastic collisions occur w/ other gas particles or with the wall of the container

– energy can be transferred in collision but the total kinetic energy of the 2 particles does not change

Kinetic Theory and Gases

5.) All gases have same average kinetic energy at the same temperature

• kinetic energy of motion (molecules are always moving)

• Temperature is a measure of the average kinetic energy of the particles in a sample of matter (at a given temp., all gases have the same avg. KE)

Temperature

↓ in temp = ↓ in K.E. (molecules slow down; theoretically, if you could lower the temp enough motion would cease)

Temperature

Kelvin scale is a direct measure of average kinetic energy

(eg. particles at 200 K have 2x as much nrg as at 100 K)

K= oC + 273 (0 oC = 273 K)

Which has more kinetic energy and does most damage to a brick wall - a big pickup truck or a Honda Prelude?

wt. = 15,000 lbs wt. = 3000 lbs

Kinetic Energy = ½ mv2– big molecules move more slowly, lightweights move faster– gases move from hi concentration → lo concentration– rate they move depends on kinetic energy (in other words,

the size and velocity of particles)

Kinetic Theory and Gases

Effusion = gas escapes thru tiny opening

ex: hole in tire, air effuses and tire goes flat

ex: helium in balloon overnight vs air in balloon

Diffusion = gas A mixing with (moving thru) gas B

ex: perfume sprayed in one room, noticed in next rm

ex: rotten egg

Kinetic Theory and Gases

Graham’s law of effusion:

Rate of effusion= 1/(sq. root of molar mass)

Graham’s law of diffusion:

Rate of diffusion= Rate A = (sq. root of molar mass B/ molar mass A)

Rate B

Behavior of Gases

• Kinetic-molecular theory a great deal of space exists between gas particles

– Large amount of empty space between the particles allows compressibility and expansion of gas particles

Gas Pressure

• Kinetic theory explains the existence of gas pressure.

• Gas pressure—the force exerted by a gas per unit surface area.

Gas Pressure

• The force of one molecule hitting an object is relatively small, but the result of billions of particles of air hitting a surface at once is significant.

Gas Pressure

pressure = force / unit area

To increase pressure (force/area):

1. more particles per unit area

a. decrease volume of container (↓ area)

b. add more particles

2. increase temp: ↑ speed of particles causing ↑ collisions

What happens as you increase altitude (climb a mountain)?

Gravity pulls air particles in toward earth.

The air at higher altitudes has less air above pushing down and fewer air molecules in a given space. Atmospheric pressure decreases as you gain altitude. Pilots gauge their altitude by measuring pressure.

Atmospheric Pressure

A barometer measures atmospheric pressure.

The SI unit for pressure is the pascal (Pa). Atmospheric pressure at sea level is about 101.3 kilopascals (kPa). Other units of measurement are atmospheres (atm), mm Hg, and pounds per square inch (psi).

1 atm = 101.3 kPa = 760 mm Hg = 14.7 psi

Comparison of Pressure Units

Units of Pressure (p390)

1 atm = the average atmospheric pressure at sea level

kilopascal 1 atm = 101.3 kPa

Torricelli 1 atm = 760 torr

mm mercury 1 atm = 760 mm Hg

inches mercury 1 atm = 29.9 in Hg

pounds / in2 1 atm = 14.7 psi

Pressure conversion problems

1. Convert 190 mm Hg to atm

2. The pressure at the top of Mt Everest is 4.89 psi. How many mm of Hg is this? in. of Hg? How many atm?

What is an absence of particles called?

• A vacuum!

• No particles = no pressure

• Atmospheric pressure is the amount of pressure from the particles in the atmosphere colliding with objects.

STP

STP = Standard Temperature and Pressure

Since temperature and air pressure may vary form place to place it is necessary to have standard reference conditions for testing purposes

STP is commonly used to define standard conditions for temperature and pressure

0oC or 273K and 1 atm or 760 mm

Dalton’s Law of Partial Pressures

There are mixtures of gases in a container

– each type of gas contributes a fraction of the particles which will supply a similar fraction of the pressure

Dalton’s Law of Partial Pressures

At constant vol. & temp., the total pressure exerted by a mixture of gases is equal to the sum of the partial pressures

Ptotal= P1 + P2 + P3 + ….Pn

Dalton’s Law of Partial Pressures

example:

Air contains oxygen, nitrogen, carbon dioxide and trace amounts of argon and other gases. What is the partial pressure of O2 at 1 atm of pressure if PN2 = 593.4 mm?

PCO2 = 0.3 mm, and Pother = 7.1 mm ?

Dalton’s Law of Partial Pressures

Partial Pressure

* colliding particles → pressure

* more particles → more pressure

# of particles often measured in moles

Does 1 mol O2 contain the same # of molecules as 1 mol H2?

1 mol = 6.02 x 1023 particles = 22.4 L

Does 1 L of O2 contain the same # of molecules as 1 L H2 ?

Liquids and Kinetic Theory

Particles in a liquid still have kinetic energy—the particles vibrate and spin and slide past each other—but not as much as is present in a gas.

One of the differences between the two is that particles in a liquid are attracted to one another.

The attraction brings the particles closer together, and hold it together with other molecules. This also gives rise to surface tension.

Intermolecular forces

• Intermolecular forces- hold together identical particles (drop of water), carbon atoms in graphite, and the cellulose particles in paper

• All intramolecular, or bonding forces are stronger than intermolecular forces

Dispersion Forces

• Dispersion forces weak forces that result from temporary shifts in the density of electrons in electron clouds (weakest intermolecular force)

– Example: Oxygen molecules are nonpolar (b/c e- are evenly distributed); under the right conditions, oxygen molecules can be compressed into a liquid; the force of attraction between oxygen molecules is dispersion or London forces

Dispersion Forces

• Dispersion forces cont:– e- in an e- cloud are in constant motion– When 2 nonpolar molecules are in close contact or

when they collide, the e- cloud of one molecule repels the e- cloud of the other molecule.

– The e- density around each nucleus is, for a moment, greater in one region of each cloud; each molecule forms a temporary dipole

– When temporary dipoles are close together, a weak dispersion force exists between oppositely charged regions of the dipoles

Dispersion Forces

• Recall your Halogen gases (F, Cl, Br, I) all exist as diatomic molecules.– The # of nonvalence e- from fluorine to chlorine

to bromine, to iodine. B/c the larger halogens have more e-, there can be a greater difference between positive and negative dipoles and thus stronger dispersion forces

Dipole - Dipole Forces

• Dipole – Dipole forces attractions between oppositely charged regions of polar molecules since polar molecules contain permanent dipoles

– Neighboring polar molecules orient themselves so that oppositely charged regions line up

Dipole - Dipole Forces

For Example:

When hydrogen chloride gas molecules approach, the partially positive hydrogen atom in one molecule is attracted to the partially negative chlorine atom in another molecule.

Hydrogen Bonds

• Hydrogen Bonds type of dipole-dipole attraction that occurs between molecules containing a hydrogen atom bonded to a small, highly electornegative atom with at least one lone e- pair

Hydrogen Bonds

For example:

– for a hydrogen bond to form, hydrogen must be bonded to either a fluorine, oxygen, or nitrogen atom

– These atoms are electronegative enough to cause a large partial positive charge on the hydrogen atom, yet small enough that their lone pairs of e- can come close to hydrogen atoms

Rank the intermolecular forces in order of increasing strength

Dispersion forces

dipole-dipole forces hydrogen bond

Liquids

• Kinetic-molecular theory predicts the constant motion of the liquid particles

• Individual liquid molecules do not have fixed positions; forces of attraction between liquid molecules limit their range of motion so that the particles remain closely packed in a fixed volume

Liquids

• Like gases, liquids can be compressed• The change in volume is much less than that of

gases b/c liquid particles are already tightly packed together

Liquids

• Fluidity ability to flow– Liquids are less fluid than gases because of

intermolecular attractions

• Viscosity measure of the resistance of a liquid to flow– As temp. increases, viscosity decreases

Solids (least KE)

• The particles in the solid move, but don’t move around. They vibrate around a fixed point.

• Most solids are crystalline—they have definite repeating structure.

• Substances that have more than one crystalline structure are called allotropes.

Obj. 13 cont…Obj. 13 cont…• PLASMAPLASMA……

~ highest KE~ highest KE

~ violent collisions b/n particles~ violent collisions b/n particles

~ no definite shape or volume~ no definite shape or volume

Changes of State

• We have discussed that the state of a substance does not just depend on the temperature of the substance, but also the pressure that it is under.

• -A phase diagram shows the conditions at which a substance exists as a solid, liquid and gas.

Phase changes that require energy

Vaporization = liquid turns to gas (vapor)

Evaporation = vaporization occurring only at the surface (cooling process)

Melting = solid becomes a liquid

Vapor pressure = pressure exerted by a vapor over a liquid

Boiling = vapor pressure equals atmospheric pressure (cooling process)

Sublimation = solid changes directly into a gas

Evaporation

evaporation—conversion of liquid to a gas when the surface of a liquid is not boiling

evaporation is a cooling process –water (or sweat) absorbs heat kinetic energy risessurface water escapes the chaos and takes some of

the kinetic energy (aka temp) with it leaving the cooler (slower moving) molecules behind to absorb more heat. They suck the heat out till they too escape.

• The temperature at which a solid becomes a liquid is the melting point.

• As kinetic energy is added to a solid, eventually the particles have so much energy that they overcome the interaction between particles and vibrate and spin themselves right out of their structure.

Melting PointMelting Point

Boiling Point

the boiling point is the temperature at which the vapor pressure of the liquid is just equal to the external pressure.

-bubbles of vapor form throughout the liquid as the molecules with the highest kinetic energy go from the liquid phase to a gas and escape into the air.

-boiling and evaporation are both cooling processes for a liquid. In each case, the molecules with the highest amounts of kinetic energy are leaving the liquid and entering the gaseous phase.

Phase changes that release energy

Condensation = gas or vapor becomes a liquid (reverse of vaportization; H-bonds form in liquid water and energy is released)

Deposition = gas or vapor becomes a solid without first becoming a liquid (reverse of sublimation)

Freezing = liquid converts to a crystalline solid

Phase Diagram

The most interesting thing on a phase diagram is a triple point—the only set of conditions where a substance can exist as a solid, liquid and gas simultaneously.

-Looking at a phase diagram, you can see that there is usually a point where a substance will go straight from the solid phase to a gas. This is called sublimation.

Phase Diagrams

• Sublimation—going directly from solid to gas

• Triple point—the one set of conditions where a substance can exist as a solid, liquid and gas simultaneously.

Phase diagram for H2O

In Review…

• What is the kinetic theory?• What do we assume about collisions in gases?• What is gas pressure?• What is the phase with the most KE?• What are most solids?• What is the boiling point dependent on?• What is the triple point?• What is sublimation?