Some notes The TA walk in lab will be

closed today for grading. You may not want to sit in the

front row today

Unregistered clickers:

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Devotional

Oct 30,

Reactions go at different speeds

Cellulose + O2 CO2 + H2O Dead tree in forest Wood in fire Sawdust in explosion

Why?

What governs speed of a reaction?

Definition: rate that reactants are consumed, or products are produced

Factors: Physical state of reactants

Temp & Pressure Collision rate Energetic requirements Entropic (organization) requirements

Why doesn’t your Why doesn’t your diamond turn into a diamond turn into a lump of coal?lump of coal?Downhill, but slow!Downhill, but slow!

Effect of energy

The net energy released (or consumed) does not affect rates

The energy required to reach the transition state (“activation energy”) has large effect

Transition stateTransition state The critical arrangement of atoms where the reacting system The critical arrangement of atoms where the reacting system

“decides” whether or not to make products“decides” whether or not to make products Typically involves breaking and/or formation of bondsTypically involves breaking and/or formation of bonds

Effect of Entropy

Reactants frequently must have a certain orientation at the transition state, or reaction will not occur

This corresponds to order in the transition state: entropy of activation

Br + CHBr + CH33Cl Cl CH CH33Br + ClBr + Cl

Effect of Entropy

Reactants frequently must have a certain orientation at the transition state, or reaction will not occur

This corresponds to order in the transition state: entropy of activation

Br + CHBr + CH33Cl Cl CH CH33Br + ClBr + Cl

Examples of Non-Reactive Collisions

Catalysts Decrease energy or

increase entropy of activation without themselves being consumed; speed up rates

H2O2 decomposes spontaneously

But, much faster in presence of Br- (or MnO2).

Catalyst makes new, lower energy route possible

Getting Help from Catalysts

Without catalystNeed a spark

With catalystNo spark needed

An example

H2

H2 + O2

H2 + O2 w/ Pd/C catalyst

Adding OAdding O22 allows allows

faster mixingfaster mixing

Pd surface stretches Pd surface stretches the H-H bond, helping the H-H bond, helping it break and lowering it break and lowering transition state energytransition state energy

Equilibrium

Reactions go in both directions:ReactantsReactants ProductsProducts

You’ve already seen this in the H2 + O2 reaction

This dynamic balance is called “chemical equlibrium”

You’ve already seen this in the H2 + O2 reaction

This dynamic balance is called “chemical equlibrium”

Meaning of Equilibrium

When forward and reverse rates are equal, amount of reactant & product no longer changes

Equilibrium is the state of lowest energy, entropy; everything moves toward it

Collision Rates Limit Reactionsthat take place in gases or liquids

Reactants must get close for electron clouds to interact

What controls collision rate? Temperature (because it is a reflection of

molecular speed) Physical state Pressure (for gases) or concentration (for liquids)

Some notes The TA walk in lab will be

closed today for grading.

Unregistered clickers from Monday’s Quiz:

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Devotional

Oct 30,

Chapters 21-22-23

Revisit the hypotheses of Chapter 12:

1: temperature at which change of state takes place reflects the strength of forces holding matter together

2: high temperature changes reflect

strong forces

Wave model of the atom provides explanations for the observations of Chapter 12 and other observations

MetalsNonmetals

Metals

Nonmetals

How do you identify metals using the periodic table?

Quiz:Which elements lose electronseasily (low ionization energy)?

1. Metals

2. Non-metals

Quiz:For a given row of elements, valence electrons of metals are further away from the nucleus.

1. True

2. False

Neon

ArgonKrypton

Xenon

Why do metals melt at high temperatures?

MELTINGTEMPERATURE

C

BOILINGTEMPERATURE

C(under 1 atm pressure)

StateAt

RoomTemperature

Helium doesn’t form solidexcept under high

pressure!

-269 Gas

Hydrogen -259 -253 Gas

Neon -249 -246 Gas

Nitrogen -210 -196 Gas

Water 0 100 Liquid

Ethanol -117 78.5 Liquid

Table salt 801 1413 Solid

Copper 1083 2567 Solid

Gold 1065 2807 Solid

Why are metals dense?

DENSITYg / cm3 

Solid

Helium

Hydrogen 0.078Neon 1.54Nitrogen 1.09Water 0.90 (0ºC)Ethanol 1.3Table salt 2.2Copper 8.9Gold 19.3

What other properties do metals have?

Classified as network-type solids Generally have:

High Melting T High Boiling T High Density

Conduct both heat and electricity Malleable – flatten into thin sheets Opaque – can’t see through even

thin sheets Reflective (Metallic Luster) – shiny

ONE BONDING MODEL EXPLAINS THEM ALL – THE METALLIC BOND

Which electrons form bonds?

1. The outer or valence electrons

2. the inner electrons with lowest energies

3. All electrons participate equally in bonding.

OrbitalsAtomic Molecular

↑ ↑Li2

2s1 2s1↑↓

Look at the progression of molecular orbitalswhen more atomicorbitals become involved.

Notice that in each molecule, there are empty orbitals.

Li3

↑↓

↑

2s1

↑

2s1

↑

2s1

↑

antibonding

bonding

What happens if “metal molecules” get bigger and bigger?

What would be the pattern of orbitals if you had a “molecule” with 1023 atoms?(about 27 g of Al, 200 g of Hg)

Li4

↑↓

↑2s1

↑

2s1

↑2s1

↑2s1 ↑↓

From atoms to molecules to metals

LiN(metal)

↑2s1

↑

2s1

↑2s1

↑2s1

N a very large number

↑2s1

↑

2s1

↑2s1

↑2s1

N atomic orbitals

N molecular orbitalsvery very closely spaced together

=filledlevels

unfilledlevels

Many closely spaced molecular orbitals gives rise to a continuous energy band

Metals have few valence electrons compared to number of orbitals

Electrons want to be in low energy states – “filled” levels

Levels still exist (and easily available) even when there are no electrons in them – “empty” levels

Where are valence electrons in metals?

Molecular orbitals extend through entire piece of metal (we say they aredelocalized)

Lots of available energy levels for electrons

Small amounts of energy can move electrons between orbitals

Electrons not tightly attached to any particular atom – “Sea of Electrons”

The energy band with mobile electrons explains all metallic properties!

Properties that arise from the Energy Band and Mobile Electrons

High melting temperatures – nuclei surrounded by electron seamelting requires the breaking of strong attractive interactions

Electrical conductivity – mobile charge carriers are the electrons

Thermal conductivity – electrons can absorb/give up heat easily; transportit away because of electron mobility

Malleability – electrons serve as lubricant, allowing layers of nuclei toslide past one another

l20_metal masher.swf

Properties arising from the Energy Band and Mobile Electrons

Opacity – Metals can absorb all colors of light animation: l20_photon.swf

Luster and Reflectivity – also tied to existence of many available energy levels.

What is an ALLOY? Why do they form?

Alloy = A combination of two or more metals to make a new metal

Examples: Copper + Zinc → Brass Copper + Tin → Bronze Gold + Nickel (± Palladium, Zinc) → White Gold Iron + Carbon → Steel Iron + Chromium (± others) → Stainless Steel

Alloys form because metal atoms all have: Few valence electrons Low ionization energies

Classification of the Elements

Metals

Size a constraint for making alloys: very similar or very different favored

Are the properties of alloys the same as pure metals?

NOT QUITE Alloys not as good conductors of

electricity and heat Alloys often melt at a lower

temperature Alloys may be less malleable

than pure metalslow-melting solder used to hold copper pipe together is an alloy

What is a Semi-conductor?

Not quite metal or non-metal Have some properties of metals

Conduct electricity under certain conditions Solids with high melting points

Widely used in computers and other electronic devices Particularly, Si and Ge

Higher ionization energies and more valence electrons give rise to change in the energy level structure

The electrical conductivity of semiconductors and metals with Temperature is very different.

Semiconductors arepoor conductors at low temp

They arebetter conductorsat high tempbut still not as good as metals

Resistivity: measure of the resistance to flow of current; a high number means bad conductor

What’s the basis for differences from metals?Si (3s23p2) Ge (4s24p2)

Compared to metals:Semiconductors have a relatively large number of valence electrons,

held with reasonable tightness, relatively close to the nucleus

More electrons, different energies and closer distance to the nucleus…. Atomic orbitals interact differently.

BA

ND

GA

P

BA

ND

BA

ND

Valence Band filled – no electrical conductivity possible at low temperatures

Energy Band Splits in Two NO LEVELS IN GAP!!

MO’s of semiconductors have one major difference.

Electrical Conductivity of Semiconductors at High Temperatures….

BA

ND

Some electrons get kicked upstairs at high temperaturesgaining access to empty levels AND creating “holes” down below

Remember, conductivity requires mobile electrons withempty energy levels to move into.

BA

ND

BA

ND

BA

ND

Light Emitting Diodes – An application of semiconductors

Photon color emitted by diodeIs related to energy of band gap

Electrical energy (from a battery) can also kick electrons upstairs into conduction band.

When electrons fall back downstairs they emit photons!

BA

ND

BA

ND

You have 3 LED’s. One gives off red light, one blue light and one green light. Which energy structure corresponds to the green LED?

1 2 3

33% 33%33%

Answer Now

10

0of5

1. LED1

2. LED2

3. LED3

LED 1 LED 2 LED 3

EN

ER

GY