[TheRationalizer]

Hi all

If I introduce energy to a substance in order to start a fire I understand that the energy causes atoms to break free of their current molecules and then bond with oxygen atoms, causing molecules like CO2.

What I don't understand though is where the energy comes from to continue this process, causing the chain reaction.  A finite amount of energy is introduced initially, as far as I know there is no energy lost from the atoms in the form of electrons etc as they bond, but energy is certainly lost in the form of radiated heat and light.  So considering so much energy is escaping from the fire where does the energy come from which sustains the fire as long as there is enough fuel + oxygen?

Thanks

[Borek]

While part of the energy is radiated away, enough is left.

[TheRationalizer]

While part of the energy is radiated away, enough is left.

I cannot make sense of it.  If there is only a finite amount of energy put into the process to start with, the process leaks energy, and the bonding of atoms doesn't release energy into the process, then it makes sense that the amount of energy available to the process will depreciate until there is no longer sufficient energy to sustain the process; however that is not the behaviour we see in a fire, which will continue to burn forever as long as we have infinite fuel + oxygen.

So this energy must be being fed back into the system from somewhere.

[DrCMS]

the bonding of atoms doesn't release energy into the process

Oh yes it does and that is where the energy comes from to sustain the fire.

eg. Propane burning is

C₃H₈ + 5O₂ 3CO₂ and 4H₂O

At the start we have:
2 C-C bonds @ 346kJ/mol
8 C-H bonds @ 411kJ/mol
5 O=O bonds @ 494kJ/mol
total = 6450 kJ/mol

after we have:
6 C=O bonds @ 799kJ/mol
8 H-O bonds @ 459kJ/mol
total = 8466 kJ/mol

That give a net of -2016kJ/mol (negative means the reaction is exothermic) this calculated value is in good agreement with the measured value of the heat of combustion of propane of 2200kJ/mol.