Fires 3 - Thermodynamics Of Fire Flashcards
What is elemental form of phosphorous and its OS
P4 and OS = 0
What is different about white phosphorous?
What is different about the fire it produces?
Why?
it is pyrophoric (it ignites spontaneously in air and only requires a small amount of ignition) to produce a very hot fire and loads of smoke
once a dish is placed over it, one part of fire triangle lost so flame extinguished but when removed again the fire returns (it doesn’t go out like a candle)
ignition temp of white phosphorous is very low (30 degrees) and there is still residual heat/energy from combustion that can restart the combustion process
same with if water was added to dish it limits oxygen but when removed it is still hot enough to reignite
What is the chemical equation for pyrophoric phosphorous?
What are the uses of this? (3)
What can be said about the use of phosphorous?
P4 + 5O2 –> P4O10
phosphorous pentoxide can be used as a chemical weapon
military:
- phosphorous pentoxide is a solid that forms lots of particulates and the size of these particles scramble light very effectively so useful compound for military applications as smoke screen
- can be used as tracer fire for burst of light so can see where things are going
- can be used as an incendiary to start fires
use is dangerous:
- sticks to skin - atmospheric moisture and moisture on skin will react with phosphorous pentoxide to form phosphoric acid - a polyprotic acid
- not good to inhale as smoke contains phosphoric acid and some liberated P4 that has been evaporated.
use prohibited by the chemical weapons convention however clandestine use still observed
How can white phosphorous be quenched?
P4 + CuSO4 → H3PO4 + Cu + H2SO4
copper sulphate solution (aq) to give copper (Cu), phosphoric acid (H3PO4 and sulfuric acid (H2SO4)
P4 + 10CuSO4 + 16H2O → 4H3PO4 + 10Cu + 10H2SO4
what energy is needed to go from fuel with some ratio of oxygen to get to transition state?
activation energy - relates to amount of energy needed to put in to get a fire going.
this is the kinetic component
What is heat of combustion?
How can this be worked out?
Why are products of a lower energy?
the thermodynamic component
using values of standard enthalpies (heats) of formation using Hess’s law
as lost heat to the surroundings
Define Hess’s law.
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
or
regardless of the multiple stages or steps of a reaction, the total enthalpy change for the reaction is the sum of all changes:
= enthalpies of formation for products - enthalpies of formation for reactants
this is because enthalpy content of a substance is a state function (an intrinsic property of the substance that does not depend on how the substance has been made)
What are enthalpies of formations of elements in standard state?
enthalpy of formation is change in heat in a system to form that material from its basic elements so substances that are already in this form have no energetic requirement to stay in that form.
can you calculate enthalpy change for reactions using worked examples?
what does enthalpy change of combustion tell us?
page 16 and 17
greater it is the more energy that will be released (should be negative as exothermic)
How can you calculate enthalpy of combustion from bond enthalpies?
= energy needed to break bonds - energy needed for form bonds
only include bonds that change from reactants to products
What is more reliable calculating the enthalpy of combustion from bond enthalpies or heats of formations? Why?
from heats of formation is more accurate as bond enthalpies are not constant from compound to compound
whereas standard enthalpies of formation relate to specific individual compounds
what are heats of combustion often quoted in?
why?
worked them out on a molar basis but they are often quoted in kJ kg-1
many fuels are not pure substances but mixtures of different chemicals which cannot be defined on a molecular or molar basis
