Lecture 7

Question 1

What is combustion ?

Answer 1

Burning (in air or oxygen)

Question 2

combustion must be complete or incomplete. Explain the difference between the two

Answer 2

Complete: plentiful supply of methane and oxygen produces carbon dioxide and water

incomplete: limited supply of air (methane oxygen) it produces: carbon monoxide and water OR carbon water

Question 3

What are the effects of combustion?

Answer 3

-water vapor not toxic
-carbon monoxide is toxic
its colorless, odorless, and tasteless

Question 4

What are examples of fuels?

Answer 4

methane or natural gas

Question 5

Exothermic reactions do what?

Answer 5

gives out heat

Question 6

What is combustion?

Answer 6

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.

Question 7

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

Answer 7

Oxygen, Heat and Fuel (gas, solid, or liquid).

Question 8

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

Answer 8

combustion reaction

Question 9

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

Answer 9

CO2 and H2O

Question 10

Combustion cannot take place in an atmosphere devoid of

Answer 10

oxygen.

BUT it requires energy at first to “jump start” the process.

Question 11

In the video of the whoosh test they used

Answer 11

rubbing alchohol and ethanol

to prove concentration does matter for a faster burn

Question 12

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

Answer 12

Exothermic Reaction.

Question 13

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

Answer 13

endothermic reaction

Question 14

whether endothermic or exothermic, both types of reactions still require an ______ to begin

Answer 14

Activation Energy

Question 15

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

Answer 15

Smothering
Starvation
Cooling
Breaking the Chain Reaction

Question 16

Why do you put a grease fire out by smothering?

Answer 16

The answer to this is in the formula of water itself. Water contains oxygen.

If the fire is hot enough (which grease fires often are), the water, rather than putting the fire out, vaporizes into flammable gases that actually provide fuel in the form of oxygen and hydrogen gas molecules.

Question 17

Explain cooling

Answer 17

In fires that burn at lower temperatures than a grease fire, water in sufficient amounts will extinguish the flame. This process is called cooling and essentially extinguishes the fire by removing the “spark” necessary to keep the reaction going. The cooling of the temperature lowers the overall energy of the reaction until it cannot make it over the activation barrier.

Question 18

T/F Many fire extinguishers use chemical compound powders like baking soda. How do they work ?

Answer 18

These chemicals work by creating a non-flammable coating on the surface of the area on fire and breaking the chain reaction of the fire. This is actually another way of starving the fire since the coating essentially removes the fuel from the path of the fire.

Question 19

What is arson?

Answer 19

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

Question 20

Arson is difficult to investigate for three main reasons:

Answer 20

1. The arsonist can plan out the arson well in advance and bring all the tools needed to commit the act with him/her.
2. The arsonist does not need to be present at the time of the act.
3. The fire itself destroys evidence tying the arsonist to the crime.

Question 21

How to investigate arson

Answer 21

• finding the chemicals left at the scene -identifying them
• reconstructing the path the fire took to find areas of ignition and patterns in the arsonist’s methods
• examining a fire scene for signs of arson as soon as the fire has been extinguished.
• Looking for accelerants is the first step.
• The presence of residues in the soot left by petroleum based accelerants
• fire scene must focus on finding the fire’s origin, which may be most productive in any search for an accelerant or ignition device.

Question 22

T/F How the fire started gives the best indication as to whether the fire was accidental or intended.

Answer 22

True

Question 23

common signs of arson include:

Answer 23

• 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)

Question 24

How do you collect evidence of arson

Answer 24

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. These materials are the most likely to contain left over/unburned accelerant.

A vapor detector (sniffer) can be used to collect and identify vapors from the burned areas of the fire.

Samples of various burned materials should also be collected throughout the burned area for later analysis.

Finally, the investigators look for obvious ignition devices

Question 25

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

Answer 25

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

The gas chromatograph is the most sensitive and reliable instrument for detecting and characterizing flammable residues.

Most arsons are started by accelerants such as gasoline and kerosene.

Question 26

Two primary hazards associated with flammable and combustible liquids are ___ and ____

Answer 26

explosion and fire

Question 27

Why is alcohol flammable?

Answer 27

The hydrogen bonds in water limit its vapor pressure and therefore make it very hard to transition into the gas phase. Molecules that do not have hydrogen bonds or that have very weak forces holding them together are much easier to get into the gas phase.

Question 28

What is vapor pressure?

Answer 28

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

The vapor of the liquid exerts pressure on its container

Question 29

T/F Liquids with high vapor pressures have lower boiling points and therefore lower activation energies.

Answer 29

True

Question 30

What is boiling point?

Answer 30

the temperature at which the vapor pressure equals atmospheric pressure, such that the pressure of the atmosphere can no longer hold the liquid in a liquid state and bubbles begin to form

Question 31

T/F liquids with lower boiling points and higher vapor pressures are easier to burn.

Answer 31

true

Question 32

T/F It is the liquid’s vapor rather than the liquid itself that ignites when mixed in certain proportions with air in the presence of an ignition source.

Answer 32

True

Flammable and combustible liquids vaporize and form flammable mixtures with air when in open containers, when leaks occur, or when heated.

Question 33

What is flash point?

Answer 33

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.

Question 34

The flash point is therefore dependent on the _________ and consequent vapor pressure of the liquid.

Answer 34

boiling point

Question 35

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

Answer 35

flammable liquid.

Question 36

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

Answer 36

combustible

Question 37

What are explosives?

Answer 37

substances that undergo a rapid oxidation reaction with the production of large quantities of gases.

It is the sudden buildup of gas pressure (see products above) that constitutes the nature of an explosion.

Question 38

The speed at which explosives decompose permits their classification as _______.

Answer 38

high or low explosives.

Question 39

The ________ produced by the expanding gases causes much of the damage associated with an explosion.

Answer 39

shock wave

Question 40

TNT is explosive for two reasons:

Answer 40

1. TNT is composed of the elements carbon, oxygen and nitrogen.
2. TNT itself is high energy and unstable.

Question 41

There are 3 major classes of explosives:

Answer 41

Low
Primary High
Secondary High

Question 42

The classification of explosives is based on what?

Answer 42

the speed by which the reaction takes place

Question 43

What are low explosives?

Answer 43

those that burn only at their surface.

The slow reaction allows the gases to build up pressure and smoothly fire the bullet from the gun.

Ex. Gun powder and fireworks

Low explosives only explode when contained.

Question 44

What are primary high explosives?

Answer 44

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

Question 45

What are secondary high explosives?

Answer 45

those chemicals that do not have to be contained to explode and are relatively stable and safe to handle.

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

relatively insensitive to heat, shock, or friction

ex. Dynamite, TNT, Semtex (plastics)

Question 46

How to examine evidence at the site of an explosion

Answer 46

1. Search systematically the entire scene and try to locate the epicenter (origin) of the explosion.
2. 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.
3. Often a crater is located at the origin and loose soil and other debris must be preserved from its interior for laboratory analysis.
4. One approach for screening objects for the presence of explosive residues in the field or laboratory is the ion mobility spectrometer (IMS).

Question 47

An ________ is an instrument that detects and separates ions in the gas phase.

Answer 47

Ion-mobility spectrometer (IMS)