thermodynamics?

Question

whats the diff between heat of combustion and heat of formation?
when calculating, which one do i have to use products minus reactants?

ex.
if the problems tells you that the reaction of burning methane has produced say like 500 kJ...then would that be heat of combustion? except you negate it?

Answer 1

i think heat of combustion is a special case of heat of formation; it's when only C, H, and O are involved. basically, a hydrocarbon molecule is broken apart to produce water.

when calculating, both are products minus reactants

oh, yeah, and for the other part of your question...yes, 500 kJ is the heat of combustion, and it would be negative because the reaction gives off this energy. the energy that is being referred to is relative to the molecules of methane you start with

Answer 2

Heat of combustion is the heat liberated when a substance burns. Whenever there is heat liberated the H will be negative.

Heat of formation is the heat change when you take the elements that make up a compound and combine them to "form" the compound.

You always use products - reactants in thermodynamics UNLESS you are using bond energies (then its reversed).

Answer 3

heat of combustion is used when any element /compound burns in presence of oxygen here H is negative (as heat is evolved )
heat of formation means heat change when any compound is formed
2)yes you have to use products minus reactants
3) CH4 +2O2 -----> CO2 + 2H2O HEAT CHANGE = -500KJ