Explosions 3 Flashcards
What are the Kistiakowsky-Wilson rules?
What is the step after this?
predicting the products of explosion
once we know relative proportions of products then can calculate heat (enthalpy) of explosive and volume (moles) of gas produced to calculate the explosive power (QV)
What does oxygen balance dictate?
whether a detonation consumes external oxygen
What is ideal product in detonation?
Why is this only ideal?
CO2
If don’t have enough oxygen will form CO and if even less oxygen will form carbon (soot)
How can oxygen balance be determined from molecular formula?
CaHbNcOd
Take C, H and N quantities to form CO2, H2O and N2:
o C –> CO2 need two O per C = 2a
o H –> H2O need ½ O per H = b/2
o N –> N2 no O needed
o Oxygen available = d
The number of oxygen atoms required is given by (may not be an integer):
OB = d – 2a – b/2
Ω=([d-2a-b⁄2]×16)/M×100
where:
M = molecular weight
What does positive, 0 and negative oxygen balance mean?
positive - oxygen efficient
0 - exactly right amount of oxygen
negative - oxygen deficient
When can you apply Kistiakowsky-Wilson rules?
Why is this used?
when oxygen balance is between 0 and -40%
if oxygen balance less than 0, carbon and hydrogen cannot be fully oxidised so need to use K-W rules to predict what detonation products are and to test how oxygen deficient we will be
What are the K-W rules?
- C atoms to form CO
- H atoms to form H2O
- CO molecules to form CO2
- N atoms to form N2
Is K-W rules for detonation or combustion products?
detonation ONLY
for combustion, uses oxygen from atmosphere
What gaseous product would qualitatively indicate near-neutral or positive oxygen balance? And why?
CO2 – as last step in K-W rules
Will only form CO2 if have oxygen left over and if between 0 and -20%
this implies 𝑑>2𝑎−𝑏⁄2 in C𝑎H𝑏N𝑐O𝑑
How can you calculate explosive power?
explosive power (kJ L kg-2) = heat (kJ kg-1) x volume (L kg-1)
explosive power = QV
(only use magnitude of Q value, not sign)
in kJ mol-1:
enthalpy (heat) of determination = ΔdH = ∑prod ν prod ΔfH prod - ∑react ν react ΔfH react
in kJ kg-1:
ΔdH x (1000/M)
where M = molecular weight
in L mol-1:
volume (moles) of detonation = V = moles of gas x 22.4 L mol -1
in kJ kg-1:
V x (1000/M)
where M = molecular weight
What is typical value for explosive power?
10^5 or 10^6 or 10^7
Do primary or secondary explosives have higher heat of explosions or gas volume values?
secondary explosives have higher:
heat of explosions values
gas volume values
What is power index?
relative scale where picric acid set to 100
Do primary or secondary explosives have higher power index values?
secondary explosives
How does correlation of enthalpy and entropy of detonation change with power indexes?
Does correlation work better for primary or secondary explosives?
correlation is better for higher power index explosives
works better for secondary explosives (higher index powers)
What are mixed explosives used for?
Give two examples
to optimise the performance of each component
aluminium
thermite
Why is aluminium added to torpex?
Why can’t we add too much aluminium?
why is aluminium a good element to use for this?
- aluminium has a high enthalpy of combustion and increases length of blast wave through resonance effect.
- aluminium scavenges oxygen atoms from the gaseous products increasing the enthalpy of detonation
- although it is already oxygen deficient, it is worth it as we get extra heat and shattering power from aluminium.
too much aluminium is counter-productive as it reduces the volume of gas formed - must find a balance
aluminium is cheap and easy to incorporate within explosives
What is thermite?
Where is it used?
Why is it used?
aluminium and iron oxide
incendiary weapons
used because provides extra heat and enthalpy
What is required for mixed explosives?
need lots of rigorous testing to find out optimum mix is
What are the modified K-W rules?
How do these differ from regular K-W rules and why?
When are these used?
- H atoms to form H2O
- C atoms to form CO
- If C atoms remaining (and no O) for solid C
- N atoms to form N2
flipped from normal rules to stop the production of hydrogen gas (which doesn’t happen)
when oxygen balance is <-40%
