Fires 2

Question 1

What are some reasons that people commit arson?

Answer 1

• Malicious destruction of property
• Insurance fraud
• Attempt to hard a person (revenge)
• Juvenile disorder
• Mental health and psychological problems
• In an attempt to destroy fingerprints, fibres, DNA evidence

Question 2

What are some common settings for arson?

Answer 2

• Vehicle fires
• Theft locations
• Dwellings - to destory physical evidence and obscure a victims death

Question 3

What are some reasons that accidental fires will still carry legal liability?

Answer 3

• Infringement of safety legislation and regulations - unlicensed storage of fuel or other flammable substances/storage in unsuitable containers/excessive quantities
• Neglect/failure to exercise an employers ‘duty of care’ - failure to undertake safety inspections
• Fire caused as a consequence of commititing another crime - illegal bypassing of gas meters/use of unqualified gas fitters

Question 4

Define combustion

Answer 4

The process of burning - a chemical change, especially oxidation, accompanied by the production of heat and light

Question 5

Define fire

Answer 5

Flaming combustion:
A continuous series of exothermic, oxidative reactions that involves the fuels present

Question 6

Define a flame

Answer 6

A region in which chemical interaction between gases occurs, accompanied by the evolution of heat and light

Question 7

What is the fire triangle and what makes it up?

Answer 7

The fire triangle shows what components are essential for a fire to occur, these components are oxygen, heat and fuel.
* A fire can be prevented or extinguished by removing one (or more) of the three components of the fire triangle
* There must be enough heat to initiate combustion (ignition source), this is a momentary input of heat energy, not sustained

Question 8

What type of process is combustion and oxidation, and what heat of combustion?

Answer 8

• Combustion is an oxidation process
• Oxidation is an exothermic process
• The heat that is liberated is called heat of combustion - enthalpy is the correct term at constant pressure

Question 9

What is the standard enthalpy of combustion?

Answer 9

Refers to standard conditions, assumes complete combistion so CO2 and H2O is formed as a gas, ∆𝐻𝑐^0 - 0 indicates at standard conditions
* Temperature 298 K
* Pressure 1 atmosphere

Question 10

What are rules for assigning oxidation numbers?

Answer 10

• Pure elements (in the standard state) = 0
• Monatomic ions = ionic charge (NaCl, Na = +1, Cl = -1)
• In covalent bonds, electron pairs are assigned to the more electronegative element
• Halogens are typically -1 in compounds
• Hydrogen is almost always +1, the expection is metal hydrides (MHx) when it is -1
• Alkali metals (group 1) = +1
• Akaline-earth metals (group 2) = +2
• Oxygen is almost always -2 in compound, the expection is peroxides (O-O) when it is -1

Question 11

What are the steps for balancing REDOX equations?

Answer 11

1. Determine the oxidation states of each species
2. Write each half reaction - balance atoms that change oxidation state, determine numbers of elements gained or lost, balance charges by using H+ (in acidic solution) or OH- (in basic solution), balance the remainder of the atoms (Hs and Os) using H2O
3. Balance the number of electrons transferred for each half reaction using the appropriate factor so that the electrons cancel
4. Add the two half reactions together and simplify where appropriate

Question 12

What is OILRIG?

Answer 12

Oxidation is loss of electrons (more positive)
Reduction is gain of electrons (more negative)

Question 13

Which species is oxidised and which is reduced:
𝐹𝑒^(2+) + 𝐶𝑒^(4+) → 𝐹𝑒^(3+) + 𝐶𝑒^(3+)

Answer 13

Oxidised: Fe
Reduced: Ce

Question 14

What are the oxidation numbers for this equation and which is oxidied:
CH4 (g) + 2 O2 (g) –> CO2 (g) + 2 H2O (g)

Answer 14

C = -4 | C = +4
H = +1 | H = +1
O = 0 | O = -2
Methane is oxidised

Question 15

What are the oxidation numbers for this equation and which has been oxidised:
2 𝐴𝑙 (𝑠) + 3⁄2 𝑂2 (𝑔) → 𝐴𝑙2𝑂3 (𝑠)

Answer 15

Al = 0 | Al = +3
O = 0 | O = -2
Al has been oxidised

Question 16

What is the equation for complete combustion?

Answer 16

C (s) + O2 (g) –> CO2 (g)
* CO2 is the indicator that there is complete combustion

Question 17

What is the equation for incomplete combustion?

Answer 17

2C (s) + O2 (g) –> 2CO (g)

Question 18

What is the equation when CO furhter combusts after incomplete combustion?

Answer 18

2CO (g) + O2 –> 2CO2 (g)

Question 19

Sulfur is an impurity in many fuels, its complete combustion is very slow and rare, but what would that product of complete combustion be?

Answer 19

SO3