CHM 1020c Exam 3 FSU Dillon

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122 Terms

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Fire is a chemical chain reaction which takes place with the evolution of

heat and light

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In order for a fire to take place there are 3 main ingredients that must be present

Oxygen, Heat and Fuel.

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In chemistry we call the type of reaction that produces fire a

combustion reaction

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Combustion is a

high-temperature exothermic (heat releasing) redox (oxygen adding) chemical reaction between a fuel and an oxidant, usually atmospheric oxygen, that produces oxidized, often gaseous products, in a mixture termed as smoke.

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Whenever we complete a combustion reaction a hydrocarbon (compound of C and H) there are generally the same products formed:

CO2 and H2O

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Combustion cannot take place in an atmosphere devoid of

oxygen

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When heat is produced in the process of a chemical reaction this is known as an

Exothermic Reaction

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When heat is absorbed from the reacting substances this is known as an

Endothermic Reaction

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There are 4 main methods for stopping a combustion reaction (putting out a fire):

Smothering

Starvation

Cooling

Breaking the Chain Reaction

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Smothering

placing an object over a fire that allows no oxygen to enter the combustion area, the fire will quickly run out of fuel and die.

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Starving

removing any flammable materials from its path

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Cooling

essentially extinguishes the fire by removing the "spark" necessary to keep the reaction going.

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Breaking the Chain Reaction

chemicals in a fire extinguisher work by creating a non-flammable coating in the surface of the area on fire.

another way of starving the fire since the coating essentially removes the fuel from the path of the fire

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Arson is

the criminal setting of a fire to commit at least vandalism and at worst murder or even mass murder.

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Arson is difficult to investigate for three main reasons:

The arsonist can plan out the arson well in advance and bring all the tools needed to commit the act with him/her.

The arsonist does not need to be present at the time of the act.

The fire itself destroys evidence tying the arsonist to the crime.

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Looking for accelerants is the

first step to determining if a fire is arson

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The presence of residues in the soot left by petroleum based accelerants can be a dead giveaway that

an arson has been committed

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The search of the fire scene must focus on finding the

fire's origin

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How the fire started gives the best indication as to whether the fire was

accidental or intended

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Some common signs of arson include:

Evidence of multiple sites of ignition

Lines of accelerant residue indicating it was poured from space to space in the structure

The majority of the burning taking place at the floor rather than the ceiling. (Heat rises so naturally fire does too but if there is a lot of accelerant on the floor the majority of the burning will take place there)

The presence of unburned combustible liquids (these are rarely completely consumed in the fire)

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Starting at the location suspectedto be the origin of the fire, the ash, soot and any other porous materials should be collected and stored in

airtight containers

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A vapor detector (sniffer) can be used to

collect and identify vapors from the burned areas of the fire.

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Finally, the investigators look for obvious ignition devices like

matches, electronic ignitors or even the glass of a "Molotov cocktail".

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Once collected the most common method for identifying the accelerants at a fire is by the use of a

GC (Gas Chromatograph) or GCMS (Gas Chromatography Mass Spectrometer).

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The gas chromatograph is the most

sensitive and reliable instrument for detecting and characterizing flammable residues

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Most arsons are started by accelerants such as

gasoline and kerosene

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Gas chromatography can separate the hydrocarbon components and

assign a unique patterned graphic for each product type.

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The pressure exerted by the vapor of the liquid at any given temperature is called the

Vapor Pressure

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Liquids with high vapor pressures have lower boiling points and therefore

lower activation energies

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The boiling point is the temperature at which the vapor pressure equals

atmospheric pressure

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This means that liquids with lower boiling points and higher vapor pressures are easier to

burn

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The Flash Point of a liquid is

the minimum temperature at which a liquid gives off vapor within a test vessel in sufficient concentration to form an ignitable mixture with air near the surface of the liquid

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flash point is dependent on

the boiling point and consequent vapor pressure of the liquid.

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any liquid with a flashpoint less than 100°F is considered to be a

flammable liquid

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Any liquid with a flashpoint between 100°F - 200°F is considered

combustible

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the relative hazard of a flammable liquid increases as the flashpoint

decreases

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Explosives are substances that undergo a rapid oxidation reaction with the production of large quantities of

gases

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TNT is explosive for two reasons:

1. TNT is composed of the elements carbon, oxygen and nitrogen. When TNT explodes it forms several covalent gases: CO, CO2 and N2 that are very stable. The production of these very low energy (stable) bonds means that a great deal of energy is released. It should be noted that most explosives contain these same elements.

2. TNT itself is high energy and unstable. The structure of TNT (above) shows three fairly large nitro groups (NO2) bound to Toluene. Because these groups are fairly large and in close proximity to each other they cause strain on the structure of the Toluene. Remember that groups of electrons repulse each other. Other compounds under similar conformational strain are also explosive for this same reason.

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There are 3 major classes of explosives:

Low

Primary High

Secondary High

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Low explosives are those that burn only at their

surface

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Gun powder and fireworks are the most common

low explosives

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primary explosives are those that

ultrasensitive to heat, shock, or friction and provide the major ingredients found in blasting caps or primers used to detonate other explosives.

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Secondary high explosives are those chemicals that

do not have to be contained to explode and are relatively stable and safe to handle

require an electrical spark, fuse, intense heat, or sharp blow to initiate their explosion.

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The process for examining evidence at the site of an explosion:

Search systematically the entire scene and try to locate the epicenter (origin) of the explosion.

Objects located at or near the origin of the explosion must be collected for laboratory examination. Each object should be placed individually into a sealed airtight metal canister. Some explosive residues are known to be able to seep through plastic containment and contaminate other evidence.

Often a crater is located at the origin and loose soil and other debris must be preserved from its interior for laboratory analysis.

One approach for screening objects for the presence of explosive residues in the field or laboratory is the ion mobility spectrometer (IMS).

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An Ion-mobility spectrometer (IMS) is an instrument that detects and separates ions in the

gas phase

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Chemical Kinetics is the study of

reaction rates, how reaction rates change under varying conditions and by which mechanism the reaction proceeds.

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There are five general properties that can affect the rate of a reaction:

The concentration of the reactants. The more concentrated the faster the rate.

Temperature. Usually reactions speed up with increasing temperature.

Physical state of reactants. Powders react faster than blocks - greater surface area and since the reaction occurs at the surface we get a faster rate.

The presence (and concentration/physical form) of a catalyst (or inhibitor). A catalyst speeds up a reaction, an inhibitor slows it down.

Light. Light of a particular wavelength may also speed up a reaction

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Increasing temperature also means the molecules are

moving around faster and will therefore "bump" into each other more often.

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Catalysts speed up

chemical reactions

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increasing the concentration of the reactants will increase

the frequency of collisions between the two reactants.

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Higher concentrations mean more

collisions and more opportunities for reaction.

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By increasing the pressure,

you squeeze the molecules together so you will increase the frequency of collisions between them.

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the larger the surface area of the solid,

the faster the reaction will be

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rate=

change in concentration/change in time

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There are a couple of rules to writing rate expressions:

Expressions for reactants are given a negative sign. This is because the reactant is being used up or decreasing.

Expressions for products are positive. This is because they are increasing.

All of the rate expressions for the various reactants and products must equal each other to be correct. (This means that the stoichiometry of the reaction must be compensated for in the expression)

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There are two general methods by which investigators determine the time of death:

The Rate Method: in this method the time of death is estimated by evaluating the presence/absence of an indicator in a deceased in conjunction with the known behavior of such indicators.

The Concurrence Method: in this method the time of death is estimated by evaluating events which happen at or near the time of death, or offer information suggesting a time period for the death event.

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The first steps in determining time of death are

1) collection of the body

, 2) collection of any evidence around the body and

3) collection of information about the deceased.

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As an aqueous fluid, blood will dry at a predictable rate if we know the environmental parameters:

temperature, humidity, airflow, surface area etc.

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Rigor Mortis

If the body feels warm and no rigor is present, death occurred under 3 hours before.

if the body feels warm and stiff, death occurred 3-8 hours earlier.

If the body feels cold and stiff, death occurred 8-36 hours earlier.

If the body is cold and not stiff, death occurred more than 36 hours earlier.

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There are also several factors that can severely impact the onset and timeline of Rigor:

temperature

illness

activity before death

physical conditions where the body are found

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Livor Mortis (Lividity) is

the settling of blood in body due to gravity

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Livor Mortis starts to develop

2-4 hours after death, becomes non-fixed or blanchable up to 8-12 hours after death and fixed or non-blanchable after 8-12 hours from the time of death.

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Blanching is what occurs when you

press your finger on your skin and you see a white spot for a few seconds

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The pooling of blood is a

physical process based on the loss of blood pressure when the heart stops beating and will therefore occur at the same rate whether the temperature is cold or not, so it is less susceptible to atmosphere than rigor

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Algor Mortis is

The cooling of the body after death

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There are two main ways the body decomposes:

Autolysis and Putrefaction

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Autolysis

is the process by which digestive enzymes within the body cells break down carbohydrates and proteins.

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Putrefaction

is the predominant cause of tissue degradation and is due to bacterial activity. Putrefaction starts 4 to 10 days after death.

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Most of the appearance of a dead body over time is due to putrefaction:

Bloating

Green discoloration of abdomen

Marbling along blood vessels-a brown black discoloration in blood vessels caused by hydrogen sulfide gas

Blisters and skin slippage

Loss of hair and nails

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There are four general stages of putrefaction:

Putrefaction (4-10 days after death) - Autolysis occurs and gases (odor) and discoloration starts.

Black putrefaction (10-20 days after death) - exposed skin turns black, bloating collapses and fluids are released from the body.

Butyric fermentation (20-50 days after death) - the remaining flesh is removed, butyric acid is formed "fermenting" the remains and the body begins to mold if in contact with the ground.

Dry Decay (50-365 days after death) - decay is very slow now due to lack of fluids, hair and fingernails fall out.

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The food found in a victim's stomach can give approximate time of death based on

the degree of digestion that has taken place

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One method for approximating time of death not listed in the table is the determination of

Vitreous (eye fluid) Potassium concentration.

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In a human being, the signs of death are:

No breathing

No heartbeat

No pupillary response to light

No response to pain stimuli

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This process of blood seeping into the peripheral tissues of the body is called

livor mortis

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Between 3 to 6 hours after the heart stops, a chemical in the body called

ATP (Adenosine Triphosphate) runs out.

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ATP requires

the intake of oxygen to be produced so its production ceases with death.

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When ATP is no longer available, the chemical myosin becomes irreversibly locked onto the muscle tissue and the muscle "locks" into place. This is called

rigor mortis

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As bacteria inside the body continue to multiply and consume tissue, the product gases of their digestion: hydrogen sulfide, carbon dioxide and methane (H2S, CO2, CH4) cause two very well-known features of death:

The smell and the swelling.

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The smell is predominantly caused by

Putrescine, or tetramethylenediamine, and Cadaverine or 1,5-pentanediaminethe

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The bloating or swelling of the body is due to the pressure

(CO2, H2S) build up as gases are released from the autolysis of cells as well as the bacterial digestion of tissues.

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Time line of Decomposition

2 days

Cells autolysis

Greenish purple staining occurs, blood decomposing

Skin takes on Marbled Appearance

4 days

Skin blisters

Abdomen swells with carbon dioxide due to bacteria in intestines

6-10 days

Corpse bloats with CO2

Corpse eventually bursts

Fluid begins leaking from openings as cell membranes rupture

Eyeballs liquefy

Skin sloughs off

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The simplest and most efficient way to stop decomposition is

freezing, called cryopreservation

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The material that composes most weapons is

metal

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physical properties of metals include

shininess, malleability, ductility, and conductivity.

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A malleable (mal ee uh bul) material is one

that can be hammered or rolled into flat sheets and other shapes.

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A ductile material is one that

can be pulled out, or drawn, into a long wire

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Metal atoms are arranged around each other into what is called a

cubic cell

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There are three main types of packing in the transition metals:

Face-Centred Cubic (FCC),

Body-Centred Cubic (BCC) and Hexagonal Close Packed (HCP).

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Grains are

regions of irregular structure that exist in the overall crystalline structure of the metal.

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Grain size can be controlled by the time in which the metal is allowed to

cool

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The faster the cooling process the smaller the grains that form and

the harder the metal becomes.

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Another common material used to make weapons is

wood

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Wood is a mixture of compounds:

cellulose, hemicelluloses, lignin, and extractives

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The cellulose in wood makes it strong and resistant to

breakage

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the lignin in wood makes it resistant to

compression

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Polymers are

long chains of interlinked molecules

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A ligature is

a piece of rope or wire that is used to suffocate the victim

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Guns are dangerous because they

fire projectiles at a high rate of speed that cause cellular damage when they strike the body.

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Bullets are cylinders that contain a packing of black powder and a primer to create a spark to start the

reaction

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The rate at which a projectile leaves a gun is called its

Muzzle Velocity.