Lecture 14 - Arson 2 Flashcards
what is an example of an element that has a low ignition temp and produces a very hot fire with lots of smoke
what is the oxidation state of this element
white phosphorus
oxidation state = 0 as it is in its elemental form
when white phosphorus is ignited and extinguished by placing a lid over it, why does it reignite when the lid is removed
all 4 elements of the fire quadrangle are back
the ignition temp of white phosphorus is very low so there is enough residual heat for it to reignite
how could you put out a white phosphorus fire
pour water on it to cool it down this removes the heat elements and it can’t get hot enough to reignite
white phosphorus is pyrophoric - what does this mean
ignites spontaneously in air
very dangerous to work with
when white phosphorus combusts what does it produce
what does this go on to react with
P4O10 - phosphorus pentoxide
can react with moisture (H20) to form phosphoric acid which can cause acid burns and not good to inhale
how is the use of white phosphorus useful for protection in the military
when it produces phosphoric pentoxide it can be used as smoke screens due to the size of the particles and how they scramble light coming through it
cant be used in this way when this goes on to form phosphoric acid by reacting with moisture in the air
what means an oxidation has occurred (in terms of the electrons and oxidation no)
loss of electrons
increase oxidation number
where is white phosphorus commonly used and give three ways in which it is used
military applications in smoke grenades and signalling
tracer fire (to see where things are going)
smokescreens
incendiary (start fires)
where has the use of white phosphorus been prohibited and why
using it on people by the chemical weapons convention
doesn’t mean people might not still use it
why?
very hot
sticks to the skin
hydrolyses to form phosphoric acid = acid burns
if you are exposed to white phosphorus and on fire what is the solution
why is this
use aqueous copper sulphate
white phosphorus can be quenched by copper sulphate giving copper, phosphoric acid and sulphuric acid
what are the products of a combustions
carbon dioxide and water (water as a liquid)
carbon monoxide if incomplete
what is the sign on the delta H of a combustion and why
negative
as energy is given out
exothermic reaction
what does Hess’s Law state
the enthalpy change for a reaction is independent of the route by which the reaction is achieved but depends only on the initial and final stages
what is Hess’s Law based off
the first law of thermodynamics - that energy is conserved
