Hess' Law Question (for combustion)?

Question

There is an easy equation to use for formation questions, but in combustion I get confused. How do you construct the energy cycles? Thanks!

Like how do you know which way the arrows should be pointing etc

Answer 1

Combustion is just the release of energy in the form of heat.

It is generally complex because of so many reactions but that is the basic principle.

Heat is added which initiates a reaction which relases more heat which cause more reactions.

For example in the body glucose combusts with oxygen to form carbon dioxide and water.

Answer 2

Formation - the arrows go upwards from elements at the bottom to the compounds each side of the arrow.
Combustion - the arrows go downwards from the substances each side of the arrow to products of combustion at the bottom.
With a combustion cycle, you must also add enough oxygen either side of the arrow for everything to burn completely.

Another way is:

Fomation - products minus reactants.
Combustion - reactants minus products.

Answer 3

you desire 2C2H6 on the left, so write the middle equation backwards and multiply it by applying 2: 2C2H6 ===> 2C2H4 + 2H2 / = +274 kJ you do not have C2H4 everywhere interior the combustion, so multiply the 1st equation by applying 2 so as that they are going to cancel out once you upload: 2C2H4 + 6O2 ===> 8CO2 + 4H2O / = -2646 kJ you have 6H2O interior the combustion, top area, so multiply the 0.33 equation by applying six: 6H2 + 3O2 ===> 6H2O / = -1452 kJ upload all 3 new equations jointly, cancelling out redundant words top and left, and upload up each and all the kJ .

Answer 4

Lancenigo di Villorba (TV), Italy

Hess's law derives by thermodynamic properties of "state's function". As you know, "state's function" are physico-chemical properties of matter which depend only by state's parameters defining state's equilibrium of matter.
You understand when a process interest a physico-chemical system, the latter show a change of "state's function" which depend only by initial and final state's parameters but not in reference to pathway followed.
Hess's law permit you estimate change of a "state's function" about a process if you know other two or more processes, the latters able to close a "cyclic macro-process" or retrieve a nul global change of "state's function".
Mathematical expression of this foundament is following :
d(A)a-b = d(A)a-a - d(A)b-c - d(A)c-a
where d(.) is differential operation and minute letters are conventional referee to state's equilibria (e.g. a-a means a-b-c-a and it represents cyclic process).
So, you put
d(A)a-a = 0
and you can rearrange the notations
d(A)a-b = d(A)c-b + d(A)a-c
since change of "state's function" reverse its algebraic sign when you reverse its direction pathway
(e.g. d(A)b-c + d(A)c-b = 0).
AN EXAMPLE
At stated T, P conditions, you want retrieve enthalpy's change for the following combustion :
2 C(s) + O2(g) <---> 2 CO(g) (referee a-b)
How does it?
You can write other two reaction, the followings :
C(s) + O2(g) <---> CO2(g) (referee a-c)
2 CO(g) + O2(g) <---> 2 CO2(g) (referee b-c)
What is "cyclic process"?
It is an ordinate sequence of processes which ends where itself starts...here, your sequence starts in "referee a-b", proceeds with "referee b-c" and ends with reversed "referee a-c".
What is value of enthalpy's change who you need?
Very fast and simple!
You apply overwritten relations, here they become :
Delta(H)a-b = Delta(H)a-c - Delta(H)b-c
that is like you could subtracting "referee b-c" by "referee a-c".

I hope this helps you.