CHEM 176 Exam 2 Review

Wavelength (λ)

Distance between corresponding points of two consecutive waves, measured from crest to crest or trough to trough.

Frequency (ν)

Number of cycles per second, measured in Hz (s^-1).

Amplitude

Height of the wave, indicating the relative intensity of light.

Speed of light in a vacuum (c)

c = λν = 2.998×108 m/s

Energy of light

E = hν

h = Planck’s constant = 6.626×10-34 J

v = Frequency

Wave-Particle Duality of Light

Concept that light exhibits properties of both waves and particles.

Intensity

Brightness, number of photons

Higher energy photons for blue light

Lower energy photons for red light

Low energy photons for infrared light

Electromagnetic Spectrum

The full range of electromagnetic radiation organized by frequency or wavelength.

Energy →

Frequency →

Wavelength ←

Additive Color Mixing

Combining wavelengths of light to create new colors, using red, green, and blue. (Red, green blue) Ex: projector

Subtractive Color Mixing

Absorb certain wavelengths of light and reflect others, creating colors by subtracting wavelengths. (Yellow, magenta, cyan) Ex: printer

Physiological Perception of Color

Light enters eye -> cones stimulated -> signals to brain -> brain blends signals together

Rod Cells

Light and dark. Primarily responsible for vision in low-light conditions and detect shades of gray. Works best in bright light, concentrated in the center of the retina.

Cone Cells

Responsible for color vision in well-lit conditions and are sensitive to different wavelengths of light. More sensitive than the cones to dim light.

S-cones

Most sensitive to short wavelengths (blue)

M-cones

Most sensitive to medium wavelengths (green)

L-cones

Most sensitive to long wavelengths (red)

Diffraction

Electromag comes in, scattered across electrons in lattice, gives rise to x-ray diffraction, which can measure shapes and sizes of packing materials in gems and minerals

Refraction

light is bent, shifted over (index of refraction)

Important for crystals/gems, when light passes through, it will have a particular index of refraction (how strongly the light is bent) and can distinguish between types of gems with it.

Optical microscopy

Uses visible light and a system of lenses to magnify small object. Cross-sectional analysis, raking light

Grazing/raking light

Visible light is exposed to painting surface at very shallow angles to reveal surface topography, artwork condition, and artist’s techniques and work time. raking light for examining texture of paintings and transmitted light measure color (reflected light).

Ultraviolet (UV) Radiation

Absorbed by varnish layers of paintings, some organic molecules and minerals fluoresce under UV light

Infrared (IR) Radiation

Examining underdrawings of paintings that were done with charcoal, examination vibrational spectra of organic molecules.

Underdrawings are done used carbon-based materials (which absorb IR radiation; gesso does not absorb IR radiation.

X-ray Techniques

X-rays reveal the structural elements of a painting (tears, holes, support). X-rays can also reveal paintings underneath the top layer.

X-ray Radiation Absorption

X-ray absorbance for detecting heavy metal pigments and structural features of paintings and other objects. Pigments containing elements with high atomic numbers (i.e. Pb, Hg) absorb X-rays more strongly than those with low atomic numbers (i.e. Ca, Fe, C). Will see this in X-ray image.

X-ray Fluorescence

X-ray fluorescence can be used to determine elemental compositions of pigments. Uses the interaction of x-rays with a material to determine its elemental composition.

Feast of the Gods

X-ray, infrared, and cross-section data

X-ray and infrared revealed that three figures were clothed, forested background

Cross-sections complement other methods. X-rays and infrared reveal superimposed layers; cross-sections reveal their sequence.

Bellini, Dossi, then Titian

Composition of paints

Paint = pigment + binder + vehicle + additives

Gum arabic

Complex mixture of polysaccharides and glycoproteins, mostly made up of arabinose and galactose

Dyes vs. Pigments

Dyes are soluble that chemically bond with a material.

Pigments are insoluble and are suspended in a medium or binder.

Inorganic Pigments

Metal-containing salt, larger particle size, more opaque, poor color strength/hiding power/flow, less chemically reactive, higher heat stability, more toxic.

Organic Pigments

Carbon-based (but can also contain metal), smaller particle size, more translucent, excellent color strength/hiding power/flower, more chemically reactive, lower heat stability, generally less toxic.

Manmade inorganic pigments

Cobalt blue (CoAl2O4) Smalt

Red lead (Pb3O4) Minium

Vermilion (HgS) Cinnabar

Tyrian purple

Indigo

Beta-caroten

How color is produced from inorganic pigments

Excitation of d-electrons on a metal atom

How color is produced from organic pigments

Excitation of delocalized electrons in aromatic or conjugated systems that have functional groups attached

Oil

pigment + linseed oil, hardens via oxidation/crosslinking

Acrylic

pigment + acrylic polymer + water, versatile

Watercolor

pigment + gum arabic/glycol + water, translucent

Pastel

pigment + gum arabic/methylcellulose, higher pigment = softer pastel

Eggtempera

pigment + egg yolk + water

Why egg yolk?

Holds significant amount of fatty acids (oleic acid, palmitic acid, linoleic acid)

Contains lecithin, natural emulsifier and stabilizer

Proteins (ovalbumin and livetin) form durable, flexible film

Yolk dries through oxidation and polymerization, creating a hard durable paint layer

Doesn’t shrink significantly as it dries

How do oil paints “dry”?

Formation of a triglyceride (fats and oils). Oxidation of oil exposed to air creates free radicals. Free radicals cause polymerization (cross-linking). Makes oil thicker and less fluid. Polymerized oil forms durable and flexible film. Oil changes from liquid to solid, though this process takes several days/weeks. Once fully cured, creates a tough, flexible film.

Cotton

Mostly cellulose, but some hemicellulose, pectin, waxes and fats

Silk

Composed of a central protein fiber called fibroin coated with a waxy substance called siricin

Wool

Twisted Molecular Chains & Helical Coils (Fundamental protein structure)

Microfibrils (Fine strands of keratin inside microfibrils)

Macrofibrils (Bundles of smaller fibrils)

Cortical Cells (Building blocks of fiber)

Wool Fiber (Whole strand)

Rayon

Derived from cellulose (wood pulp or cotton linters).

Cellulose is treated with alkali and carbon disulfide to form viscose solution.

The viscose is extruded through a spinneret into an acid bath, where it solidifies into rayon fibers.

Polyolefin (Polyethylene, Polypropylene)

Made from polymerizing ethylene (PE) or propylene (PP) using catalysts.

Melted and extruded into filaments or stretched into fibers.

Known for strength, moisture resistance, and low cost.

Polyester

Formed by reacting a dicarboxylic acid (like terephthalic acid) with a dialcohol (like ethylene glycol).

This condensation reaction forms a polyester polymer.

Melt-spun into fibers and drawn to improve strength and elasticity.

Polyamides (Nylon)

A dicarboxylic acid (like adipic acid) reacts with a diamine (like hexamethylenediamine).

This forms a long-chain polyamide polymer (nylon).

Melt-spun into fibers, then drawn and oriented for strength.

Urethanes (Spandex/Elastane)

Created by reacting diisocyanates with polyols to form a polyurethane polymer.

The polymer is spun into fibers using a dry spinning process.

Results in high elasticity and stretchability.

Bakelite (Phenol-Formaldehyde Resin)

Formed by reacting phenol with formaldehyde under heat and pressure.

The reaction forms a thermosetting polymer that is placed in a mold to cure into a hard, clear solid.

Used for electrical insulators, kitchenware, and more.

Mordant

Makes the dye adhere to the fabric

Acts as linker between dye molecule and fabric

Usually a metal cation

Charge of metal cation of mordant attracts the lone fairs of the fiber/fabric and the dye molecule(binds to dye and fabric) = dye won’t come out

Forgery or Real Examples

Vinland Map (analysis of parchment dating makes it forgery)

Getty Kouros (stylistic inconsistencies, up in the air)

La Bella Principessa (authenticity through forensic studies)

Condensation Polymer/Step-Growth Polymers

Formed by condensation reaction with the loss of a small molecule (H2O, Ch3OH, HCI), varied backbones are more chemical reactive

Addition Polymer/Chain-Growth Polymers

Formed by an addition reaction without the loss of any atoms or molecules, common with unsaturated monomers (with double bonds)

Physical properties of chemicals depends on

Chain length, cross-linking, functional groups, branching

Polymer

A macromolecule made of small repeating units (monomers) linked together by covalent bonds

Degree of Polymerization (DP)

Number of repeating units (n or i); because most polymer mixtures contain chains of varying lengths, the chain length is often referred to in terms of average DP.

Homopolymer

All monomers are identical (ex: Polyethylene, Polymethylmethacrylate)

Copolymer

Made of different monomers (alternating, random) (ex: Saran, Vinyl Chloride)

Polymer Structures

Linear: uninterrupted straight chain

Branched: occasional branches off longer chain

Dendritic: tree-like molecule

Cyclic: chain with no end

Generally, polymers are shapeless chains (like spaghetti) that are connected to each other chemically or physically (cross-linked) to make one giant unit

Polyolefins

Formed from simple alkenes (e.g., ethylene, propylene) through addition polymerization.

Example: Polyethylene (PE), Polypropylene (PP), Polystyrene (PS)

Functional Groups: None (hydrocarbon-based)

Polyesters

Formed by the condensation reaction between carboxylic acids (-COOH) and alcohols (-OH).

Functional Group: Ester (-COO-)

Polyamides

Formed by the reaction between carboxylic acids (-COOH) and amines (-NH₂).

Functional Group: Amide (-CONH-)

Polyurethanes

Formed by the reaction between isocyanates (-NCO) and alcohols (-OH).

Functional Group: Urethane (carbamate, -NHCOO-)

Proteins (Polymers of Amino Acids)

Amino acids link via peptide bonds (amide bonds) between amine (-NH₂) and carboxyl (-COOH) groups.

Example: Collagen, Keratin, Hemoglobin

Functional Group: Amide (Peptide bond, -CONH-)

Fibers

High tensile strength and elastic modulus in one direction (ex: sugars, silk, cotton, wool, cellulose, starch)

Elastomers

Loosely-crosslinked polymers with “memory”

Plastics

Moldable and shapeable; hybrid of fiber and elastomer

Thermoplastics: remeltable and recyclable, does not cure with little crosslinking, softens on heating and hardens on cooling

Thermosets: will not remelt or recycle, crosslinks on heating and cures into given shape

Composites

Materials made from two or more components, polymers serve as matrix that binds reinforcing fibers (ex: construction, sports equipment, medical devices)

Adhesives

Bonds surfaces together via physical or chemical means

Paints

Helps pigments adhere to surfaces and provide protection (ex: automotive or industrial coatings)

Coatings

Highly viscous, must adhere to surfaces (ex: beetles, candy shells, shellac)

Polyethylene Common Uses

Film wrap, plastic bags (low density); electrical insulation, bottles, toys (high density)

Polypropylene Common Uses

Carpet, upholstery

Cellulose vs. starch vs. glycogen

Cellulose (plant structural component): unbranched chain of glucose molecules in opposite orientations (repeating). The sugar (β-D glucose) molecules are linked when water is eliminated forming a disaccharide.

The cellulose polymer is a polysaccharide.

Starch (plant energy storage): unbranched chain of glucose molecules all oriented in the same direction

Glycogen (animal energy storage): branched chain of glucose molecules resulting in a forked arrangement

Alkanes

Alkenes (denotes presence of one or more double bonds)

Alkynes (C-C triple bonds)

Aromatic ring (benzene C6H6)

Alcohol -OH

Aldehyde -C(O)H, -CHO, or -C(=O)H

Ketone RC(O)R’ or -COOH

Carboxylic acid -CO2H or -COOH

Ester RC(=O)OR’ or RCO2R’

Ether ROR’

Amine

Amide

Nitrile

Nitro group