Lecture 15- Phase Changes

8/3/2019 Lecture 15- Phase Changes

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CHEM 112

General Chemistry

Dr John Carran

3 Weeks


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Solids Liquids Gases

Phase Changes


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States of Matter

Solid (crystalline, glass)

Liquid

Gas (vapour) Plasma (Sort of)


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Phase Changes

Gas

Liquid

Solid

Condensation

FreezingMelting

Vaporization

DepositionSublimation

Energy transfer is required for every phase change


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http://jchemed.chem.wisc.edu/JCESoft/CCA/pirelli/pages/cca3NI3.html

Some transformations take less energy than others!!!!
http://jchemed.chem.wisc.edu/JCESoft/CCA/pirelli/pages/cca3NI3.htmlhttp://jchemed.chem.wisc.edu/JCESoft/CCA/pirelli/pages/cca3NI3.html


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Each of these states has distinct set of physical properties:

Solid water, ice, is rigid and difficult to compress

Liquid water is difficult to compress, but it flows under pressure Gaseous water, water vapour, flows and is easily compressed

If red atom is removed from a solid, liquid or gas, is it easy to tellwhere it belongs? Explain.


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The state of a substance depends on:

distance between particles, and magnitude of the forces between them

Interplay between these two factors is responsible forarrangement of molecules in each state

in study ofgases, conditions in which intermolecular forceswere negligible were sought. Ideal gas equation could beused

at high pressure and low temperature, intermolecular forcescan not be ignored and gas behaviour departed from ideality

when forces become sufficiently strong, gas condenses toliquid


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Knowledge of strength of intermolecular forces, made up oflong-range attractive forces and short-range repulsive forces,helps in understanding of properties of various substances.

Consider property ofmolar volume.


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Molar volume is the volume of one mole of a substance.

Vm= molar mass/density= [Units are (kg/mol)/(kg/L)= L/mol]

At room conditions, molar volume of most solids and liquidsis between 10 100 cm

3.mol

-1

At room conditions, molar volume of gases around 24 000cm3.mol-1. That is, when a substance vaporizes, the volumeincreases by a factor of 2000 3000 times.

Since mole of any substance contains NA Avogadrosnumber of particles, liquids and solids have far more denselypacked molecules than gases


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Even when solid melts to liquid, whereby bonds areweakened and intermolecular distance increases, volumeonly increases by 2 10%. That is, molar volumes o

solids and liquids are quite similar.

In solids and liquids, the intermolecular separations aresignificantly longer than chemical bond distances foundwithin molecules (intramolecular distances)


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For interest only, additional properties are discussed indicating howintermolecular forces influence each property.

Compressibility (interest only)

Compressibility of a substance is defined as fractionaldecrease in volume per kPa of increased appliedpressure. Pillars for bridges and road beds for highways musthave low compressibility.

Compressi-P

V

V

1

where V = original volume

Why is there a negative sign in this expression?

not needed for exam


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Solids and liquids nearly incompressible while gases

highly compressible

Condensed substances (solids and liquids) are made upof particles in contact or nearly in contact becauseintermolecular forces strong

Gases highly compressible because there is spacebetween particles and intermolecular forces negligible

Large expenditures of energy required to compressliquids and solids because of strong repulsive forcesoperating between molecules in close contact


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Thermal Expansion (interest only)

Thermal expansion is fractional increase in the volume of asubstance per degree increase in temperature. In bridgeconstruction, the spans must contain materials with lowcoefficients of thermal expansion.

T

V

V

1

whereV = original volumeolume after pressure applied

Thermal expansion of liquids and solids far less than thermalexpansion of gases.

not needed for exam


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Thermal expansion is closely related to strength ofintermolecular forces

Strong attractive forces must be overcome for expansion tooccur. Because there are strong intermolecular forces inliquids and solids, they expand little when heated.

Gas particles are far apart with weak intermolecular forces. Asmall temperature change adds sufficient energy for asignificant expansion

Other characteristics such as

fluidityof gases and liquids and rigidity of solidssurface tension

diffusion can be explained by understanding intermolecularattractions among molecules.


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Intermolecular Forces

In general,

Physical properties of matter are related to intermoleculaforces, the forces between particles.

Intermolecular forces are generally weaker than covalentchemical bonds, the forces within molecules. Forces within

molecules are intramolecular forces.

Intermolecular forces include:

Ion ion forces

Dipole dipole forces

Ion dipole forces Induced dipole induced dipole or London Dispersion forces

How is type of force and properties of a substance related?

needed


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Ion Ion Forces:

building blocks are positive (cations) and negative (anions)

ions

composed of monatomic (Na+, Ca

2+, Cl

-) or polyatomic ions

(NH4+, SO4

2-). Ionic substance formedwhen elements with

widely varying electronegativity values bond. E.g. when

group IA, IIA elements bond with group VIB, VIIB elements

Main attractions are Coulomb forces like charges repel,unlike attract

Ion ion forces as strong as covalent bonds

These forces decrease less rapidly in strengthas distanceincreases than other types of interactions


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Ion Ion Bonding

Positive

cation

Negative

anion

Coulomb

Forces


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Dipole Dipole Forces

When molecule is formed from two different types of atoms

polar molecule may be formed as one end of molecule takeson negative charge (red in colour) and other end takes onpositive charge (blue in colour)

usually made of non metals.

Polar molecule contains polar bonds formed when electrons incovalent bond shift towards more electronegative atom. This

gives one end of molecule a negative charge because ofsurplus of negative charge density and other end of moleculea positive charge because of deficiency of electron chargedensity.

when a receptor recognizes a drug


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Consider molecules below.

butane (C4H10) is non polar molecule containing polar bonds

which cancel out dipoles

propanal (CH3COCH3) contains polar bonds and is a polamolecule because overall molecule has a dipole (electron rich

and electron poor regions).

propanone/ acetone


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When polar molecules line up in correct direction,dipole dipole interactions occur.


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In liquids and gases, random motions lead to temporary

dipole-dipole orientations. Thermal energy when added to polar molecular liquids

overcomes dipole-dipole attractions, disrupts favorableorientations and liquid vaporizes.

In solids, dipole-dipole interactions remain in place forming lowmelting point, molecular solids.

Attractions between polar molecules decrease much morerapidly than Coulomb forces as distance between molecules

increase.

you have to supply energy to overcome any of these

intermolecular forces of attraction


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The strength of the dipole dipole force depends on orientation ofmolecules.

(a)would result in net attractive force while(b) would result in net repulsive force.


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Hydrogen bonds

When you experimentally determine boiling points of several

related hydrides, following would be noted.


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As period number increases,

molar mass in a series of related compounds tend to increase

But HF, NH3 and H2O exhibit dramatic deviations from thesesystematic trends

this is because of hydrogen bonding.

Hydrogen bondis formed when

hydrogen atom bonds to highly electronegative oxygen,nitrogen or fluorine atoms which attract electron densityaway from hydrogen. very polar bond is formed.


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this in turn allows hydrogen nucleus, a proton, to be attractedto a lone pair of electrons on a highly electronegative atom inneighboring molecule.

hydrogen bonds only possible with hydrogen because allother atoms have inner-shell electrons to shield nuclei fromattraction by lone-pair electrons of nearby atoms


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Hydrogen bonding in water

(a) Each water molecule is linked

to four others through hydrogen

bonds. The arrangement istetrahedral. Each H atom is

situated along a line joining two O

atoms, but closer to one O atom

(100 pm) than to the other (180

pm). (b) For the crystal structure

of ice, H atoms lie between pairs

of O atoms, again closer to one O

atom than to the other.

(Molecules behind the plane of

the page are light blue.) O atoms

are arranged in bent hexagonal

rings arranged in layers. This

characteristic pattern is similar tothe hexagonal shapes of

snowflakes. (c) In the liquid,

water molecules have hydrogen

bonds to only some of their

neighbors. This allows the water

molecules to pack more densely

in the liquid than in the solid.


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Ion Dipole Forces:These forces are most commonly encountered when ionic solid isdissolved in polar solvent. E.g. NaCl is dissolved in H2O.

NaCl Na+ + Cl-

H2O is made up of polar molecules

Positive and negative ions surrounded by polar watermolecules

H2O + CH3COOH CH3COO + H3O

HCl + H2OH3O + CL^-


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Induced Dipole Induced Dipole Forces (London DispersionForces)

Monatomic elements such as found in the inert gases and nonpolar molecules are attracted by Dispersion forces theweakest of all chemical bonds.

Formed when instantaneous and induced dipoles areproduced

Attraction of balloon to surface is example of induction


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They are formed as follows:

Purely by chance, a normally nonpolar species can becomemomentarily polar.

This temporary dipole produces atom or molecule with aslightly positive and slightly negative end

Dipole in one particle induces dipole in an adjacent particle

Interaction of these two instantaneous and induced dipoles

produces ONLY attractive force between two particles

As particles increase in size, temporary dipoles increase instrength producing stronger dispersion forces


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a. non polar molecule with evenly spread charge density

b. in instantaneous condition-purely by chance-instantaneousdipole formed

c. this instantaneous dipole induces an instantaneous dipole inadjacent molecule

The result attraction called a London dispersion force.

Dipole-dipole (including hydrogen bonding) and dispersionforces make up what chemists commonly refer to as van derWaals forces.

the effect of london dispersion forces is more proncounced

when the atom has more electrons, ie. Xe has more of an

effect than hydrogen.


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Chemistry curiosity!!!

The London dispersion forces

which are always attractive

are the weakest and shortest range of all intermolecularattractions

strength of this force depends upon size and how easily

adjacent atoms or molecules are polarized as a result ofelectron fluctuations.

Gecko is tiny lizard with interesting characteristics.

.


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Geckos

Easily climb vertical walls

Can hang from ceiling by a single toe A nine year old child could be supported from the ceiling by aforce equal in magnitude to the dispersion forces generated bya 5 cm gecko

The gecko does not have suction cups for feet. Instead

The feet are covered with millions of tiny hairs containingbillions of tiny pads

When geckos foot fastens onto a surface, the billions of tinypads come into close contact with the surface

Billions of London dispersion forces are formed

These interactions are responsible for geckos amazing grip.


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Applications of the geckos amazing gripping

Geckos amazing sticking power could lead to products suchas super stick tape

Geckos adhesive system never leaves residue, never getsdirty, never wears out

Scientific breakthroughs(http://www.newscientist.com/news/news.jsp?id=ns99993726/)

scientists are now trying to create a material syntheticgecko hairs- that matches the lizards incredible grip

Metin Sitti, an engineer at Carnegie Mellon University inPittsburgh, has created some polymer-based setae which

look promising These setae which measure tens of micrometres across

could enable robots to walk on outside of space vehicles toinspect for damage such as missing heat-shield tiles


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Repulsive Forces:

When atoms or molecules approach too closely, strongrepulsion occurs between core electron clouds (non valence)of neighboring atoms.

Core cloud is made up of all but valence shell electrons.

This repulsion negligible until distance between centresbecomes small. At this point, rapid increase in strength of

repulsive forces.


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The potential energy between a pair of atoms, ions or moleculesat various distances would resemble the following:

Potential

Energy

Potential energydecreases as

attractive forcesdominate

Potential energy

increases asrepulsive forces

dominate

Separation between

particles

Distance where

attractive and

repulsive forcesbalance


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What type of bond is being formed in each one of thecurves indicated below? Are the bonds being formed, strong or

weak?


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Above graph shows potential energy graph for pair of atoms, ionsand molecules:

Potential energy at large separations arbitrarily set at zero As pairs approach, potential energy becomes negative

because attractive forces come into effect (bond stabilityincreasing)

Lowest point occurs at distance where attractive and repulsive

forces exactly balance Relative values of potential energy at lowest points measure

relative bond strengths

If particles approach too closely, potential energy increases

rapidly (bond stability decreasing) as repulsive forces start todominate

remember potential energy

diagrams and the main points of

them

Documents

Lecture 15- Phase Changes