Exam 2 PART 1 Forensics CHEM 1110 R01

Atom

Atom

small particle that retains its identity during chemical reaction; The building blocks from which all matter is constructed are called

Matter

anything that has mass and occupies space

Element

substance that cannot be separated (broken down) into simpler substances by chemical means; 92 naturally occurring and 17 man made; substance whose atoms all have the same atomic #

Compounds

Substances are composed of two or more elements in fixed proportions; can be separated by chemical means to yield elements they were constructed from

Protons

The atomic number indicates the number of _______ in the nucleus. Have positive charge

Electrons

In a neutral atom, the number of protons is equal to the number of ______; have a negative charge

Neutrons

have a neutral charge

Atomic number

determined by number of protons in the nucleus; equals number of electrons

Atomic Mass

equal to the sum of protons and neutrons

Isotopes

2 or more forms of the same element (same # of protons); differ in number of neutrons and mass

Periodic table

divided into 7 rows (periods) and 18 columns (groups); elements within a group have similar physical and chemical properties

Metals

good conductors, lustrous (shiny), ductile, malleable, SOLIDS

Nonmetals

poor conductors, dull, many are GASEOUS; no clear delineation of periodic table from metal to nonmetals

Semimetals or Metalloids

share properties with both classes (metals and nonmetals)

Molecule

two or more atoms bonded together; smallest unit of a pure substance that can exist and still retain its physical and chemical properties

Spectrophotometry

method used to measure how much a chemical substance absorbs light by measuring the intensity of light as a beam of light passes through a solution,

Continuous Spectrum

containing all visible colors in a smooth unbroken band; Produced by a white light from an incandescent light bulb when passed through a prism

Line Spectrum

a series of individual color lines separated by black spaces; An element that is placed in a flame emits light that, when passed through a prism, produces a(n) Line spectrum.

Ground State

where electrons start; the lowest energy level

Excited State

produces a line spectrum when electrons move up from the ground state when GIVEN ENOUGH ENERGY

Atomic absorption

atoms absorb light from a lamp and unabsorbed light reaches the detector; electrons of the sample are excited by the flame and the light absorbed by the electron is detected; can only detect one element at a time and must be calibrated with knowns

Atomic Emission

light is emitted by excited atoms in the flame

Atomic Spectroscopy

metal ions in solution are transformed into gaseous atoms by flame, furnace, or plasma that operates at temperatures between 2000C and 11000C

Plasma

mixture of gases that conduct electricity b/c it contains significant concentrations of cation (+ charged ions) and electrons

ICP-OES

Inductively Coupled Plasma Optical Emission Spectroscopy; can determine concentrations of as many as 70 elements simultaneously in some

X-Ray Fluorescence Spectrometry

Used to measure the elemental composition of materials; non-destructive method that can identify a large # of elements simultaneously

Scanning Electron Microscopy Energy Dispersive X-Ray

decreases costs of instruments; SEM operates on sample principles as a light microscope but uses electrons rather than light; hitting the surface w/ electrons also causes X-Rays to be emitted which are elem

Gun Shot Residue

GSR; un-burnt powder contains lead, barium, and antimony and trace amounts of aluminum, sulfur, tin, calcium, potassium, chlorine, and silicone; easily removed with normal washing; if taken right away it can determine who shot the gun and from what distance

GSR Analysis

Neutron Activation Analysis: expensive and no longer used Inductively Coupled Plasma: same principles as covered for glass Atomic Absorption Spectrophotometry: most commonly used technique SEM-EDX: more expensive than AAS but yields results ~25% more conclus

Electronic Configuration

shows the arrangement of electrons around the nucleus of an atom

Chemical Bonding

attractive force that holds atoms together; atoms bond in the octet

Octet Rule

atoms attempt to acquire an outer orbital of 8 electrons either by sharing or donating electrons

Chemical Bond Types

covalent (polar and non-polar), ionic, and metallic

Covalent Bonds

result when atoms SHARE electrons to fill their outer shell; generally formed between two nonmetals

Ionic Bonds

formed when atoms transfer electrons to achieve an octet (filled outer shell); generally formed between metal and nonmetal; metals give up electrons--> become POSITIVE ions; nonmetals receive electrons--> become NEGATIVE ion; different charges

Metallic "Bond"

formed in solid metals; make up 80% of all elements; all metal ions share all electrons in a "sea" of free moving charges (responsible for metal conducting ability

Electronegativity

A measure of an atom's attraction for the electrons it shares in a covalent bond

Absolute Differences in Electronegativity

1.7 or greater --> ionic bond between .5 and 1.7 --> polar covalent bond .5 or less --> covalent bond

Polar Covalent Bond

covalent bond in which the electrons are not equally shared, but rather displaced toward the more electronegative atom

Partial Charge

Slight negative charge; difference in electronegativities of atoms in a bond creates Polar Bond

Chemical Reactions

characterized by the rearrangement of atoms when reactants are transformed into products

Law of Conversation of Mass

number of atoms on each side of the arrow must be equal

Combustion

chemical reaction of a substance with oxygen, usually from the air, that gives out heat

Activation Barrier

in order for a reaction to take place atoms must collide with enough energy to overcome the ______

Spontaneous Reactions

occur when the products are more stable than the reactants; most are exothermic and some are endothermic

Exothermic

give off heat

Endothermic

absorb heat

Chemistry of Fire

Fire is the result of an oxidation reaction; oxygen source may be air (~20% O2) or may be a substance that contains oxygen; Rusting = slow/controlled oxidation Fire = fast oxidation Explosions = extremely fast/uncontrolled oxidation

Oxidation Reaction

reaction of a substance with oxygen

Intensity of a Fire

fire is a chemical reaction; rate of reaction increases so does the _______ of the fire

Ways to increase/decrease rate of a chemical reaction

Manipulate the temperature; change the concentration of the reactants; add a catalyst

Effect of Temperature

a fuel will only start if enough of it is in the gaseous state; liquids do not burn, it is the vapor located just above the surface that is on fire; must be sufficient oxygen to allow the reaction to take place

Flash Point

minimum temperature at which a liquid fluid will produce enough vapor to burn

Ignition Temperature

Temperature at which vapor will burn (always much higher than its flash point)

Flammable Liquids

have flash points <100F; gasoline and acetone

Combustible Liquids

has flash points >100F; kerosene and home heating oil

accelerants

material used to start or sustain a fire; can range from gasoline to wad of paper; arsonists use a signature recipe; aka ignitable liquids; determining which accelerant was used and how is vital to

Pyrolysis

breakdown of solids by heat; when a solid is ______, a variety of gaseous products are released that combine with oxygen to produce a fire

Smoldering

combustion at a surface that occurs without a visible flame; characteristic of solids w/ relatively large surface area (charcoal and heavy fabrics)

Effect of Concentration

Fuel and air must mix in gaseous state to ignite RATIO IS CRITICAL; if ratio is too low (lean) or high (rich) the mixture will not burn; reaction cant take place unless a collision occurs; rate of reaction increases as temperature does (more collisions)

Flammable range

concentration of gaseous fuel that will support combustion

Lower explosive limit

LEL; lowest concentration that will burn

Upper explosive limit

UEL; highest concentration that will burn

rate of oxidation

DOUBLES for every 10C increase; explains why fires spread so rapidly; fire only stops when it runs out of fuel or oxygen

Fire Triangle

Oxygen on the left and blue; heat on the right and red; fuel on the bottom and brown

Stages of a Fire

typically advance through three steps; Incipient (Growth) stage Free-burning (development) stage Smoldering (decay) stage

Incipient Stage

initially the fire is contained due to limited fuel source, as it grows the fuel source

Free Burning Stage

combustion is now a chain reaction; there is enough heat to produce more flammable gas which produces more reactions

Flashover

occurs at ~1100F; hot enough to ignite ALL fuels

Smoldering Stage

no fuel is left to burn and no open flames exist, but there are a lot of combustible gases remaining, if enough oxygen is introduced a backdraft will occure

Hydrocarbons

molecules composed of only carbon and hydrogen; basic ones as known as ALKANES; most accelerants are not composed of a single hydrocarbon but rather a mixture of them (gasoline, kerosene, Paint thinner)

Structural Isomers

contain same number of C's and H's but have different structure

Point of Origin

place where a fire starts; fires tend to burn longer at or near _______; typically has greatest amount of

How to Identify POI

Burn patterns; melting of materials; discoloration of metals

Burn Patterns

physical marks and char that remain after a fire; char depth can be used to determine fire's duration

Burn Pattern Geometry

indicate how arsonists handled the accelerant; conical means accelerant was placed in a bin or inside of something hourglass--> accelerant was placed close to the wall puddles on floor --> accelerant was poured drag marks --> accelerant was brought in from another room or to another room

Melting of Materials

damage to various materials can give estimate of temperature of the fire; plastics melt between 200-750F Glass melts between 1100-2600F;

Discolored Metals

Metals change color when heated to dif. temps. and retain that color once cooled (color can be basis of estimating temperature of fire); melted metals in fire debris usually indicate that the fire was extremely hot

Headspace Sampling

sample is heated under controlled conditions, and the vapor emitted by the sample is controlled and analyzed by gas chromatography, flame ionization detection, or mass spectrometric detection

Passive headspace diffusion

Charcoal sampling; Captures the vapor in the headsp

Solid-phase micro extraction

A variation on the charcoal sampling technique

Gas Chromatography

chemical fingerprint of the sample is compared with the fingerprint of known compounds and identified based on the time of its elution from the GC column or by mass spectrum

Most Common Indicators of Arson

Evidence of flammable liquid, objects out of place, incendiary device, unusual damage, evidence of a crime committed to prior fire, multiple POIs, tampered thermostat

Bombs

can be highly sophisticated mixture of chemicals and electronics or gun powder in a pipe

Explosion

release of mechanical or chemical energy in a violent manner that generates heat and the release of large quantities of gas; any substance capable of producing an explosion

Explosions

explosives are held inside a container until that container fails; resulting destruction of container produces shrapnel

Deflagration

chemical explosion in which the reaction moves through the explosive at less than the speed of sound

Condensed Explosives

liquids, solids, or gel

Dispersed Explosives

gases or aerosols; dust explosions can occur from fine material in high enough concentration

Detonation

chemical explosion in which the reaction front moves through the explosive at a greater speed than the speed of sound

Types of Explosives

2 categories--> Primary- sensitive to shock and heat; will detonate powerfully Secondary --> very stable; burn rather than detonate if not enclosed

Nitroglycerin

first major commercial explosive used during 1850's; highly unstable; prone to shock induced detonation (i.e. walking by it); freezing material reduces sensitivity but accidents are common when thawed

Dynamite

NG aDsorbed onto an inert material, like clay; created by Alfred Nobel in 1866; adding Ethylene Glycol Dinitrate prevents it form freezing; can be used in colder climates

Ammonium Nitrate/Fuel Oil (ANFO)

ammonium nitrate is farmers fertilizer; when combined with fuel oil (home heating/diesel fuel) highly effective explosive is made; most common explosive; blasting agent; best in large quantities

TNT

2, 4, 6-trinitrotoluene; Military Explosive; used mainly during WWI and WWII; low melting point (makes it moldable); stable and must be initiated with another explosive

RDX aka C4

most powerful and brisant (shattering capability) explosive; extremely stable (must be combined with other compounds to be useful; can be made into putty; pop

HMX

High melting point explosive or Octogen; 30% more powerful than TNT; highly shock sensitive; usually mixed with TNT to reduce its shock sensitivity (stabilize)

IED

Low explosive ____ = most common among domestic terrorists; 2 necessary components (container and fuse/primer to detonate); 2 types 1. use commercially available products modified to act as explosives 2. use combo of chemicals

Low Explosive IEDs

made from houshold chemicals and components (road flares, matches, sugar, chlorine tablets, aluminum, nails, pressure cookers); kids make MacGyver bombs out of toilet cleaner and tin foil in soda bottles`

High Explosive IEDs

common among international terrorists; homemade with acetone, hydrogen peroxide, and acid; extremely unstable (blows during production); contains NO nitrogen and cannot be detected by most pre/post analysis equipment