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Question 1

Colloids

Answer 1

• include surfactants, polymers, and particles.
• underpin key West Australian industries in Health, Energy, and Environmental applications including:
  – recovery of metals from ores,
  – explosives,
  – extractions
• etc

state intermediate between a solution and a suspension

Question 2

Solutions

Answer 2

a homogeneous mixture of two or more substances in a single phase.

In a true solution (such as salt or sugar
dissolved in water), no settling of the solute is observed and the solute particles are ions or relatively small molecules.

Question 3

Suspensions

Answer 3

for example, if a handful of fine sand is added to water and shaken vigorously.
Sand particles are still visible and gradually settle to the bottom.

Question 4

Colloid classification: Sol

Answer 4

a dispersion of a solid in a liquid (such as clusters of gold atoms in water) or of a solid in a solid (such as ruby glass, which is a gold-inglass sol, and achieves its colour by scattering).

Question 5

Colloid classification: Aerosol

Answer 5

a dispersion of a liquid in a gas (like
fog and many sprays) and of a solid in a gas (such as smoke).

Question 6

Colloid classification: Emulsion

Answer 6

a dispersion of a liquid in a liquid (such as milk and some paints).

Question 7

Colloid classification: Gel

Answer 7

a system in which at least one component has a low rigidity (such as a cross-linked polymer or a lipid bilayer) and at least one component has a high mobility (the solvent).

Question 8

Types of colloids: hydrophobic

Answer 8

• repel water from the surfaces of the colloidal particles, and attact each other via hydrophobic interactions.

EXAMPLES: emulsion droplets,
hydrophobic polymers

Question 9

Types of colloids: hydrophilic

Answer 9

strongly attracted to water molecules, often have functional groups on
their surfaces (such as –OH and – NH2) which form strong hydrogen bonds to water.

EXAMPLES: mineral oxides like
silica, proteins and starch

Question 10

Preparation of Colloids

Answer 10

Requires an input of energy. (Not spontaneous)

Aerosols are formed when a spray of liquid is torn apart by a jet of gas. Sneezing produces an aerosol.

Material (for example, quartz) may be ground in the presence of the dispersion medium.

Passing a electric current through a cell can lead crumbling of an electrode into colloidal particles.

Chemical precipitation sometimes results in a colloid.

A precipitate (e.g. AgI) may be converted to a colloid by the addition of a peptizing agent, a substance that disperses a colloid.

Clays may be peptized by alkalis, the OH− ion being the active agent.

Question 11

Thermodynamic Instability

Answer 11

a thermodynamic tendency to reduce their surface area (like a liquid).

Stability is a consequence of kinetic effects.

Disperse systems are kinetically nonlabile (high activation energy for collapse)

At first sight, though, the kinetic argument seems to fail: colloidal particles attract one another over large distances.

There is a long-range force tending to collapse them down into a single blob – van der Waals forces.

Question 12

Colloidal Stability

Answer 12

Colloidal particles may adsorb ions
from solution and repulsion
between surface charges prevents
coagulation.
* A hydrophobic colloid is stabilized by
positive ions absorbed onto each
particle and a secondary layer of
negative ions.
* Because the particles bear similar
charges, they repel one another and
precipitation is prevented –
sometimes indefinitely

Question 13

Surfactants = surface active agent

Answer 13

a species that accumulates at the interface of two phases or substances (one of which may be air) and modifies the properties of the surface.

can be hydrophilic and hydrophobic

Question 13

CLASSIFICATION OF SURFACTANTS

Answer 13

• the anionics and cationics (collectively ionics) which dissociate in water into two oppositely charged species (the surfactant ion and its counterion),
• the non-ionics, which include a highly polar (non charged) moiety, such as polyoxyethylene (−OCH2CH2O−) or polyol groups,
• the zwitterionics (or amphoterics), which combine both a positive and a negative group

Question 14

Micelles

Answer 14

colloidsized clusters of molecules, for their hydrophobic tails tend to congregate, and their hydrophilic
heads provide protection.

form only above when the conc of surfactant is equal to or greater than a value called the critical micelle concentration (CMC).

Question 14

The Cleaning Action of Soap

Answer 14

• Soap molecules interact with water through the charged, hydrophilic end of the molecule.
• The long, hydrocarbon end of the molecule can bind through dispersion forces with hydrocarbons and other non-polar substances in grease and oil.

Question 14

Alkynes

Answer 14

compounds with carbon–carbon triple bonds.

Question 14

Micelle Size and Shapes

Answer 14

Ionic surfactant micelles:
* Coulomb repulsions between head groups
* limiting the size ~50 to 150 monomers.

Nonionic surfactants micelles:
* cluster together in swarms of 1000 or more.
* Elongated structures

The shapes of the micelles vary with conc

more commonly flattened spheres close to the CMC, and rod-like at higher concentrations.

The interior of a micelle is like a droplet of oil, the hydrocarbon tails are mobile.

Question 14

Kinetic Stability - emulsions

Answer 14

A fat can be emulsified by a soap because the long hydrocarbon tails penetrate the oil droplet but the – COO- groups (or other hydrophilic
groups in detergents) surround the surface, and form hydrogen
bonds with water.

This produces a shell of negative charge that repels a approach from
another similarly charged droplet, like
particles in water.

Question 14

Alkenes

Answer 14

(e.g., ethene) contain carbon–carbon double bonds and can react with many other alkenes to form polymers.

Question 15

Emulsions: dispersion of two immiscible liquids (ie water and oil)

Answer 15

Prepared by shaking the two components together.
An emulsifying agent stabilizes the product. - ie casein with milk and fats

This emulsifier may be a soap (a long chain fatty acid), a surfactant, or a lyophilic sol that forms a protective film around the dispersed phase.

Question 15

Polymers

Answer 15

made from unsaturated hydrocarbons.

Question 16

Polyethylene

Answer 16

At high temperatures and pressures (in the presence of a catalyst), ethylene is converted to polyethylene polymers with molecular weights up to several million.

Question 17

Free Radical Addition to Alkenes:
Polymerization

Answer 17

Repeating polymers: monomers

built by repeating alkenes (ethylene, propylene vinyl chloride, styrene)

happens in high-temps and high-pressure environments and with a suitable catalyst

Question 18

Steps for Polymerisation

Answer 18

1. Addition
   - Reaction begins when a few radicals are generated by the catalyst. Breaks C = C bone for step 2
2. Propagation
   - Polymerization occurs when a C atom radical formed in step 1 adds to another ethylene molecule. Repetition of this step for hundreds or thousands of times builds the polymer chain.
   - Termination: combination of two chains by chance meeting is a possible chain-terminating reaction.

Question 19

Polyynes and Polyenynes

Answer 19

Naturally occurring compounds containing conjugated carbon-carbon triple bonds have been isolated from plants, fungi, and marine sponges.

Some have antibacterial, pesticidal, and antifungal properties.

The compound containing an ene-triyne (one double bond conjugated with three triple bonds) has been isolated from a daisy plant, and found to be toxic to mosquito and black fly
larvae, and adult nematodes.

Used as part of a global effort to synthesize new compounds with potential to check the spread of mosquito-borne diseases such as malaria and West Nile virus.

Question 20

Molecular Orbital Theory

Answer 20

Atomic orbitals from different atoms in a molecule are mathematically combined to form molecular orbitals spanning two or more atoms.

The number of MOs is equal to the number of atomic orbitals combined.

Electrons “occupy” the lower-energy, bonding MOs.

The molecule is more stable than the separated atoms from
which it is made.

Question 21

Metallic Bonding with MO Theory

Answer 21

consider all the atoms in a sample.
e.g. 1 mol of lithium atoms, considering only the 2s and 2p orbitals, there are 4 X (6 X 1023) atomic orbitals.

This means 24 X 1023 molecular orbitals can be created which span all the atoms in the crystal.

A mole of lithium has 1 mol of valence electrons, and these electrons occupy the lower-energy bonding orbitals.

The bonding is delocalized; the valence electrons are associated with all the atoms and not with particular bonds between pairs of atoms.

Question 22

Band Theory

Answer 22

• Molecular orbitals are constructed
  from the valence orbitals on each atom
  and are delocalized over all atoms.
• An energy level diagram of bonding in
  metals would show MOs so close
  together that they are indistinguishable from one another.
• The band is composed of as many
  molecular orbitals as there are
  contributing atomic orbitals, and each
  molecular orbital can accommodate
  two electrons of opposite spin.

Question 23

Fermi Level

Answer 23

In metals the band gap is small.
At temperatures above 0 K, the thermal energy causes some of the electrons to occupy higher-energy orbitals.

For each electron promoted, two singly occupied levels result:

1. a negative electron in an orbital above the Fermi level
2. A positive “hole“ from the absence of an electron below the Fermi level.

Question 24

Electrical conductivity of a metals

Answer 24

From the movement of electrons and holes in singly occupied states in the presence of an applied electric field.

• When an electric field is applied, negative electrons move toward
  the positive side, and the positive “holes” move to the negative
  side. (Positive holes “move” because an electron from a filled orbital can move into the hole, thereby leaving behind
  a new “hole.”)

Question 25

Alloys

Answer 25

Most metallic objects we use

Mixtures of metal with one or more other metals or a non-metal

The macroscopic properties of an alloy vary depending on the ratio of the elements in the mixture.
For example, “stainless steel” is highly resistant to corrosion and is roughly five times stronger than carbon and low-alloy steels.

Question 26

Alloy catagories

Answer 26

1. solid solutions,
2. heterogeneous mixtures - regions of dif composition can be seen
3. or intermetallic compounds - substances witha definite stoichiometry and formula, formed when the two metals have different electronegativities (CuAl2, Mg2Pb, etc)

in solid solutions:
1. one element is considered a “solvent”
2. one as “solute.”

Solute atoms are randomly dispersed through the solid in such a way that the bulk structure is unperturbed.

Question 27

Semiconductors

Answer 27

materials that do not normally conduct electricity, but can be made to by the input of energy.

• This means devices made from semi- conductors can have “on” and “off’ states, which form the basis of the binary logic used in computers.
• The properties of semiconductor materials can be changed by adjusting the band gap around 10-240 kJ mol-1

Can conduct because thermal energy is sufficient to promote electrons from the valence (occupied) band to the conduction (unoccupied) band.
* Semiconductors can be classified as intrinsic or extrinsic.

Question 28

Insulators

Answer 28

• the band of filled levels (the valence band) is:
• separated from the band of empty levels (the conduction band)
• by a significant energy
  difference, called the “band gap.”
• In insulators, the size of the
  band gap is large eg -580kJ mol-1 in diamonds

Question 29

Intrinsic Semiconductors

Answer 29

The conductivity of intrinsic semiconductors is governed by the temperature and magnitude of the band gap (EXAMPLES: Si and Ge).

Why?

These control the number of electrons in the conduction band.

The smaller the band gap is, the
smaller the energy required to
promote a significant number of
electrons.
As the temperature increases, more electrons are promoted into the
conduction band and a higher conductivity results.

Question 30

Extrinsic Semiconductors

Answer 30

The conductivity of extrinsic
semiconductors is controlled
by adding a small amount of
different elements called dopants.

These can be either p-type and
n-type.

Suppose phosphorus atoms (or
atoms of some other Group 15
element such as arsenic) are
incorporated into the silicon
lattice.

Question 31

N-type Extrinsic Semiconductors

Answer 31

• now has extra electrons because
  phosphorus has one more
  valence electron than silicon.

Semiconductors doped in this manner have a discrete, partially filled donor level that resides just lower in energy
than the conduction band.

Electrons are promoted readily to the conduction band from this donor band, and electrons in the conduction band carry the charge.

Such a material, consisting of negative charge carriers, is called an ntype semiconductor.

Question 32

P-type Extrinsic Semiconductors

Answer 32

Suppose a few silicon atoms in the silicon lattice are replaced by aluminum atoms (or atoms of some other Group 13 element).

Aluminum has only three valence electrons whereas silicon has four.

Four Si-Al bonds are created per aluminum atom in the lattice, but these bonds must be deficient
in electrons.

According to band theory, the Si-Al bonds form a discrete band at an energy level higher than the valence band.

This level is referred to as an acceptor level because it can accept electrons.

The gap between the valence band and the acceptor level is usually quite small, so electrons can be promoted readily to the acceptor level.

The positive holes created in the valence band are able to move about under the influence of an electric potential, so electrical conduction results from the hole mobility.

Because positive holes are created in an aluminum-doped semiconductor, this is called a p-type semiconductor.

Question 33

Element Combinations

Answer 33

• 13–15 semiconductors combine elements from Group 13 and Group 15—e.g., GaAs.
• 12–16 semiconductors combine elements from Group 12 and Group 16—e.g., CdS.
• The band gap in GaAs(s) is 140 kJ
  mol-1, whereas it is 232 kJ mo1-1
  in CdS(s).
• Semiconductors can be further modified by substituting other atoms into the structure.
• By adjusting the stoichiometry it is
  possible to control the size of the
  band gap—e.g., Ga1xAlxAs.

Question 34

Applications of Semiconductors:
Diodes, LEDs, and Transistors

Answer 34

A diode is p-type on one half and n-type on the other.

LEDs (or light-emitting diodes) are materials that convert between light and electricity, made by combining elements.

Transistors are sandwich
structure of either p-n-p or
n-p-n composition.

Question 35

Mechanism for the emission of light from an LED constructed from n- and p-type semiconductors

Answer 35

When p- and n-type semiconductors are joined, the energy levels adjust so that the Fermi levels (EF) are equal.

This causes the energy levels of
the conduction (EC) and valence
(EV) bands to “bend.”

Holes flow from the p side to the n
side, and electrons flow from n to p until equilibrium is reached.

No more charge will flow until a voltage is applied.

When an electric field is applied,
occasionally electrons in the conduction band will move across the band gap and combine with holes in the valence band.

Energy is evolved as light.

The energy of the emitted light is ~equal to the band gap.

Therefore, by adjusting the band
gap, the colour of the emitted light
can be altered.

Question 36

Transistors

Answer 36

• the p- and n-type semiconductor materials constructed into a sandwich structure of either p-n-p or n-p-n composition.
• amplifies an electrical signal,
  making it ideal for powering loudspeakers,
• also be used for processing and storing information, a critical function for computer chips.

By combining thousands of these transistors and diodes, an integrated circuit can be made to computer chips

Question 37

MEMS

Answer 37

size: 50-90 nm across
from polycrystalline silicon

Question 38

Ceramics

Answer 38

solid inorganic compounds that combine metal and non-metal atoms.

• Ceramics are generally hard, and inflexible.
• Most ceramics are good thermal insulators
• Some can be electrically conductive.
• Glass, can be optically transparent, whereas other ceramics are opaque.

Question 39

Glasses

Answer 39

• An amorphous solid structure

The best-known glasses are silicate
glasses (SiO2).
Glasses can be modified using alkali
metal oxides to adjust melting point, colour, opacity, and strength.

Pyrex glass is boric oxide.

The boric oxide raises the softening temperature and minimizes thermal
expansion.

This enables the glass to better withstand temperature changes.

Question 40

Molecular-level representation of glass structure.

Answer 40

(a) Silica glass (SiO2) is somewhat ordered over short distance

(b) Much less order over a larger distance.

(c) By adding metal oxides, lower melting temperature.

Oxide ions are incorporated into the silicate structure.

The negative charge is balanced by the interstitial metal cations.

Question 41

Glasses are transparent and reflective

Answer 41

The combination of transparency and reflectivity of a material is determines its refractive index (n).

Refractive index is the ratio of the velocity of light in a vacuum to the velocity of light in the material.

The refractive index of water is 1.333.
Silicate glasses are in the range of 1.5 to 1.9.

Question 42

Optical Fiber

Answer 42

Use glass transmission and reflection characteristics.

Reflection is accompanied by partial refraction.

Light is refracted from a high refractive
index medium to a low refractive index
medium has:
Angle of refraction > angle of incidence.

As the angle of incidence increases it
approaches the critical angle:
– refraction approaches 90°.
At angles greater than the critical angle:
– no refraction ray,
– total internal reflection.

Glass fibers transmit light using
total internal reflection.

The light that enters at one end of the fibre stays within the fibre.

Total internal reflection in fibres by controlling the refractive indices in the fibre’s core and outside surface.

Chemically, the refractive index is controlled by adjusting the quantity and type of cationic network modifiers (e.g. F doped).

Question 43

Cements, Clays and Refractories

Answer 43

Cements, clays, and refractories are processed by shaping, drying, and then firing.

• Cements are extremely strong; used as structural materials.
• Clays have varying combinations of layers and cations.
  – become plastic when water is added (hydroplasticity).
  – Water in the interlayer, allowing layers to slide.

Question 44

Refractories

Answer 44

resistant to heat.

They can withstand very high temperatures without deforming, up to 1650 °C.

Thermally insulating because of their porous structure.

Porosity also weakens the material.
So, refractories are not as strong as cements.

Refractory bricks are used for furnace linings.

Question 45

Aerogel, a Networked Matrix of SiO2

Answer 45

Aerogel is 99.8% air.
39 times more insulating than fiberglass insulation.

1000 times less dense than glass.
Aerogel was used on a NASA mission to collect the particles in comet dust.

Question 46

Piezoelectricity

Answer 46

mechanical distortion induces an electrical current; and an electrical current to causes a material distortion.

Piezoelectric devices are used:
– Ignitors
– Wristwatches: convert electric impulses to vibrations.

Question 47

Superconductivity

Answer 47

a materials electrical resistivity is almost zero at the critical temperature.

Superconductors:
* Some elements
* alloys (eg niobium-titanium),
* ceramics (eg magnesium diboride),
* superconducting pictides (like fluorine-doped LaOFeAs)
* organic superconductors (fullerenes and carbon nanotubes).

At low temperature, magnetization causes a repulsive force strong enough to counter the force of gravity.

Reaches a superconducting state when cooled using liquid nitrogen (Tc > 77K)

Some ceramics are superconductive near 100K
e.g. YBa2Cu3O7, with TC = 92 K,
HgBa2Ca2Cu2O3, with TC = 153 K.

Question 48

Biomaterials

Answer 48

Biomaterials research examines living systems and:
– Examines
– Understands
– copies
* Biomaterials research searches for substances and processes that can be mimicked in the laboratory.
* E.g collagen-based polymer composites and new adhesives.

Question 49

Gecko Tape

Answer 49

similar structure to the toepads of the lizards.

Created using nanotechnology.

1. A polyimide film silicon wafer.
2. An aluminium mask is created through electron beam lithography.
3. This mask transferred to the film.
4. Ions are bombarded against the metal to remove the mask.
5. This produces polyimide hairs.
6. Material is then removed from the wafer and attached to a flexible base.

Question 50

Nanotechnology

Answer 50

Structures with nanometer
dimensions carry out functions.
* Atoms and molecules can be made to self-assemble
* They arrange themselves in specific ways for functions.

Question 51

Quantum dots

Answer 51

Quantum are used as biological markers by attachment to cells.
* The dots fluoresce in different colours when light shines on them.
* This allows the cells to be imaged.