Kinetic Molecular Theory

• A theory that envisions molecules in motion

• Best describes properties and behaviors of gases

• Can be applied to all states of matter

Solid - Molecules are held close to each other by their attractions of charge. They will bend and/or vibrate, but will stay in close proximity.

Liquid - Molecules will flow or glide over one another, but stay toward the bottom of the container. Motion is a bit more random than that of a solid.

Gas - Molecules are in continual straightline motion. The kinetic energy of the molecule is greater than the attractive force between them, thus they are much farther apart and move freely of each other.

6 Basic Principles of KMT 1. Gases consist of tiny (submicroscopic)

particles.

2. Gas particles are very far apart; the volume occupied by a gas consists mostly of empty space (typically about 99.9% empty).

3. Gas particles are not attracted to each other.

4. Gas particles move randomly in all directions, traveling in straight lines.* Gas molecules travel at very high speeds, about 6000 km/hr (~2700 mi/hr).

5. The higher the average speed of the particles, the higher the temperature of that substance.

Absolute temperature: T based on average particle speed; measured in Kelvin (K)

Absolute zero (0 K): particles are not moving; there is no T below 0 K.K = °C + 273

6. Gas particles collide with each other and with the walls of the container without losing energy.

These are calledElastic collisions.There is no lossof energy.

Pressure = the collision of gas particles with a surface; force per unit area

Nature of GasesExpandable – no definite shape or volume; completely fill their containerCompressible – Particles very far apart are able to be crowded close together; volume can be decreased as the pressure increasesFluid – having insignificant attractive forces, gas particles easily glide past each other. Like a liquid, a gas has the ability to flow.

Low density – gas molecules are very far apart which lead to their low density (1/1000 the density of the same substance as a liquid or solid.)Diffuse - spontaneous mixing of two gases or liquids because particles move randomlyEffuse - process by which gas molecules pass through a small opening from a container at higher pressure to one oflower pressure (a bike tire with a hole)

Kinetic Molecular applies to Ideal Gases but real gases do not behave completely according to the KMT.Very high pressures and very low temperatures, gases will be closer together and attractive forces will apply. (Remember, with the KMT there are no attractive forces.)

Liquids and the KMTProperties of a liquid Definite volume; takes the shape of its

container (fluid) More ordered than a gas du to attractive

forces Attractive forces are intermolecular

Dipole-dipole London dispersion forces Hydrogen bonding

Con’tParticles not bound together in fixed positions but are in constant motion

*interesting fact: helium can flow uphill at temperatures close to absolute zero

Relatively High DensityHigher density due to closer arrangement of molecules as compared to a gas

Relatively IncompressibleLiquids behave more like a solid than a gas regarding compressibility due to the closeness of the moleculesLiquids behave more like a gas in that they are able to transmit pressure equally in all directions.

DiffusionAble to diffuse due to constant random motion of the particles but it is slower than the diffusion of a gasIncrease the temperature…..increase KE ….increase the rate of diffusion

Surface tension•Force that tends to pull adjacent part’s of a liquid’s surface together thereby decreasing the surface area•Results from attractive forces

Physical properties Water:

Is clear, colorless, odorless, and tasteless* Colors, tastes and odors are caused by

substances dissolved in the water.Boils at 100°CFreezes at 0°CDensity = 1.0 g/mL (at 4°C)

Evaporation:The fastest molecules break the H bonds and escape from the surface and become a gas; can occur at any T.* What is left behind is cooler than before because

a high energy (i.e. hot!) molecule escaped, thus removing heat; ex: sweating

Vaporization

BoilingBoiling:: Fastest molecules Fastest molecules

overcome overcome H bondsH bonds and and change phase change phase beneath beneath the surface.the surface.

* The gas rises because it * The gas rises because it is less dense (about is less dense (about 1000 times less dense) 1000 times less dense) than the liquid.than the liquid.

Gaseous matter - steam simulation

http://www.visionlearning.com/library/science/chemistry-1/CHE1.1-matter.htm

Boiling Point: overcome attractive forces and escape the liquid

Altitude and Boiling Point

13.2

SublimationWhy does it bypass the liquid state?

Tends to occur in substances that have very low attractions between molecules. Ex: dry ice (solid CO2) and iodine (I2)

http://library.thinkquest.org/2690/exper/exp19.htm

http://cwx.prenhall.com/petrucci/medialib/media_portfolio/13.html

Cohesion & AdhesionCohesion: attractions within a liquidAdhesion: attractions between a liquid and

something else For water, adhesion is stronger than cohesion

MeniscusMeniscus:: curve at the surface of a liquid curve at the surface of a liquid If cohesion is stronger than adhesion, If cohesion is stronger than adhesion,

meniscus is convex (like mercury)meniscus is convex (like mercury)

Mercury Water

Capillary actionAttraction of the surface of a liquid to the surface of a solidClosely related to surface tensionA liquid will rise quite high in a very narrow tube and will wet the tube

http://kingfish.coastal.edu/biology/sgilman/770lecwatersalt.htm

Freezing: removal of heatDensity of ice (at 0°C) is 0.917 g/mL, so ice floats in water.

http://www.anglianwater.co.uk/schools/noscript/f1.htm

Most liquids become Most liquids become more dense as you cool more dense as you cool them. them. However, when However, when water freezes, a large water freezes, a large expansion occurs.expansion occurs.

Water “organizing” with

H bonds as it freezes.

Forms hexagon shapes.

* Where have you see H2O in repeating

hexagonal patterns before?

Six Phase Changes

SOLID

LIQUID

GAS

B.P.

Temp (°C)

M.P.

C.P.

F.P.

Solids and the KMTAttractive forces hold solids together in fixed positions allowing only vibrational motionVery orderedTwo types Crystalline solids: composed of crystals

with an orderly, geometric, repeating pattern

Amorphous solid: random arrangement of particles

Properties of a solidDefinite volume and shapeDefinite melting point Crystalline structures break out of position(Amorphous solids do not have a definite

melting point, rather they flow over a range of temperatures (glass and plastics). These can be supercooled – can have liquid properties at temps where they appear solid.)

High density and IncompressibilityVery Low rate of diffusion

Crystalline solidsArrangement of particles in the system called a lattice

Smallest portion of a crystal lattice that shows the 3D pattern is a unit cell

Simple cubic Body-centered cubic

Face-centered cubic

Crystalline solidsIonic crystals: positive and negative ions arranged in a regular patternCovalent network crystals: Each atom covalently bonded to its neighboring atom; very large number of atomsMetallic crystals: metal cations surrounded by a sea of electrons which are free to flowCovalent molecular structure: crystal structure of a covalent molecular substance; covalently bonded molecules are held together by intermolecular forces. Ex: water

Review of the Six Phase Changes

SOLID

LIQUID

GAS

B.P.

Temp (°C)

M.P.

C.P.

F.P.

Equilibrium Dynamic Equilibrium

(involves movement) Two opposing changes

occur at equal rates in a closed system

Equilibrium Vapor Pressure of a Liquid Pressure exerted by

a vapor in equilibrium with its corresponding liquid at a given temperature

Volatile liquids evaporate quickly

Boiling PointTemperature at which the equilibrium vapor pressure of the liquid equals the atmospheric pressure.

Molar Enthalpy of VaporizationAmount of energy as heat that is needed to vaporize one mole of liquid at the liquid’s boiling point. Note: temperature is constant

Freezing pointTemperature at which the solid and liquid are in equilibrium at 1 atm

Molar Enthalpy of fusion Amount of energy as heat required to

melt one mole of solid at the solid’s melting point

As you increase the temperature of a liquid, you increase the vapor pressure.

Phase DiagramsThe conditions of pressure and temperature at which two phases exist in equilibrium are indicated on a phase diagram by a line separating the phases.

Triple point is where all 3 phases exist at equilibrium

Critical point indicates the critical temperature and critical pressure

Critical temperature is the temperature above which the substance cannot exist in the liquid state