combustion (1 Viewer)

[chip]

wats the difference between molar heat of combustion and just plain heat of combustion and how do i work it out?

 

[jims]

i think molar heat of combustion is given as per mol (kJ/mol) while heat of combustion is just in kJ. or mayb theyre the same thing. some1 should check.

measuring is easy.
its just h = mCdT (pretend the d is a triange)
where:
h is the heat of combustion
m is the mass of water heated
C is the specific heat capacity of water
dT is the change in temperature (of the water)

divide it all by the number of moles of watever it is u combusted to get the molar heat of combustion (ie mCdT/moles)

and h is always negative because ur getting energy or something. dont forget the negative sign.

edit: i just noticed that the formula is on the chem data sheet under "Some useful formulae"

 

[chip]

ahhh ok then
is it always negative?
when i have the normal combustion and i've worked out the moles
i go combustion
-------------
moles

is that right

 

[Toodulu]

if it's negative, temp of surround env. (eg. the water) increased, it's an exothermic reaction.
and if it's positive, temp decreased, it's endothermic.
i think.
hmm, don't quote me on that, it could be the other way around.

 

[jims]

yeh it all sounds right
usually wen they ask for the heat of combustion they ask for it as kJ/mol anyway

 

[inasero]

1. yes toods is right
2. molar heat of combustion if heat energy liberated or absorbed when one mole of a chemical undergoes a given rxn....
plain 'heat of combustion' is same as above except in terms of the given quantity....so it doesnt necessarily have to be moles