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I have a problem with understanding basic concept about enthalpy.

  • I know there is an enthalpy (heat) of formation which tells how much energy is going to be released or consumed during chemical reaction. I found information that specific heat of formation for one of the hydrocarbon jet fuel is equal to -200 BTU/lbm. How to interpret that? If all the elemental 'building blocks' of the fuel undergoes reaction of mixing of whatever there will be 200 BTU/lbm heat released?

  • There must be second 'meaning' of enthalpy because I also found example problem of burning jet fuel with air, where in the given data is information that fuel enthalpy is equal to +189 BTU/lbm (there was no information if that's heat of formation). What does it mean that fuel has enthalpy? It sits in the tank, and before burning process is no chemical reaction

How to interpret both cases? What's the difference here. Generally we talk about enthalpy of fluid. Like compressor inlet flow enthalpy is equal to XXX, compressor exit flow enthalpy = YYY. There is no chemical reaction between inlet/outlet plane so ...

Could you give me a 'dummy' type explanation?

Thank you, Lukasz from Poland


Let me paste one of my comments here:

This is how book describes combustion in jet engine:

*specific enthalpy of mix air and fuel
hIn = ( mAir * hAir + mFuel * hFuel ) / ( mAir + mFuel)

*specific enthalpy at the end of combustion process
hOut = hIn + (LHV * (mFuel/mAir) * efficiency)

Looks like that I can't understand what hFuel, and hAir mean in that case.

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  • $\begingroup$ As you said. The enthalpy of formation of a fuel is that of the (even hypothetical) reaction forming that fuel. When the fuel is burnt, the combustion reaction will have its enthalpy of reaction. Note that the enthalpy of formation is an enthalpy of reaction as well. Saying that something has enthalpy express the above, is enthalpy that can be partially used,to heat and/or to do mechanical work. $\endgroup$
    – Alchimista
    Jan 13, 2021 at 9:48
  • $\begingroup$ Sorry I don't get it. Could you clarify that a little bit more? $\endgroup$
    – Łukasz
    Jan 13, 2021 at 9:53
  • $\begingroup$ Do you have a grasp of what enthalpy is? I just tried to clarify why a fuel can be labelled with two enthalpy - just in the sense that one is its enthalpy content and the other the one released in a reaction, combustion in the case. For a grasp of what enthalpy is I have checked the Wikipedia page and it is seems easy enough. You can even stop at the introductory part and already get a picture. en.m.wikipedia.org/wiki/…. And read the physical interpretation section as well. $\endgroup$
    – Alchimista
    Jan 13, 2021 at 9:59
  • $\begingroup$ Enthalpy content is understandable, but how to connect the one released in a reaction (combustion) with heating value? $\endgroup$
    – Łukasz
    Jan 13, 2021 at 10:08
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    $\begingroup$ What do you mean by 'heating value'? $\endgroup$
    – Gert
    Jan 13, 2021 at 10:20

2 Answers 2

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Consider a generic hydrocarbon (as a model for a fossil fuel), $\text{C}_x\text{H}_{2y}$:

$$x\text{C}(s)+y\text{H}_2 (g)\to \text{C}_x\text{H}_{2y}(g,l)\tag{1}$$

$$\text{C}_x\text{H}_{2y}(g,l)+(x+\frac{y}{2})\text{O}_2(g)\to x\text{CO}_2(g)+y\text{H}_2\text{O}(l)\tag{2}$$

$(1)$ is its formation reaction, with it is associated an Enthalpy of Formation.

$(2)$ is a typical combustion reaction, with it is associated an Enthalpy of Combustion (aka Enthalpy of Oxydation).

Both are very distinct and should not be confounded.

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  • $\begingroup$ Let me add, just to help OP, that the H of reaction equals the difference between the "sum of all reagents" and the "sum of all products" enthalpies of formation. H of reaction equals the Heat of reaction if the process is carried out at substantially constant pressure. Perhaps you can add this with nice formatting. $\endgroup$
    – Alchimista
    Jan 13, 2021 at 11:02
  • $\begingroup$ @ Gert. I am still confused a little bit. Let's start with eq. (1). This is chemical description of how fuel is created from it's building blocks. So we take Carbon, adds Hydrogen2 and we get a product (say it's a fuel) and during that reaction some heat is released. And this is the number (200 BTU/lbm) from my original question. That is heat of formation. Am I right? $\endgroup$
    – Łukasz
    Jan 13, 2021 at 11:14
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    $\begingroup$ Assuming first first bullet point was correct then you are right, yes. $\endgroup$
    – Gert
    Jan 13, 2021 at 11:21
  • $\begingroup$ @ Gert. Great. So eq. (2) shows how burning process looks like. So we have fuel, add some oxygen, and we get product of combustion (CO2 + H2O). And there is some heat released during that process. And this amount of heat is described by fuel heating value (and this is characterized by positive number). Am I right? $\endgroup$
    – Łukasz
    Jan 13, 2021 at 11:30
  • $\begingroup$ @Łukasz you can have both sign depending on the field and or whatever convention is used. It is pretty common to find - for H exothermic process and + Heat for combustion process. The two are equal in absolute value (if P is constant.). It is not clear to us if you are stuck in convention/nomenclature issues or the concept behind. Please read this answer and my first comment to it all in once. $\endgroup$
    – Alchimista
    Jan 13, 2021 at 11:42
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The enthalpy of formation of a compound is found relative to some specified reference state (of temperature and pressure) for the elements comprising the compound. The enthalpy of the elements comprising the compound is taken as zero in this reference state. So the enthalpy of formation of the compound is the change in enthalpy in going by chemical reaction from an initial state of the pure elements in the reference state to the compound in the reference state; it is equal to the amount of heat that has to be added to the system such that the temperature of the product is the same as that of the elementary reactants. This heat addition (or removal in the case of a negative heat of formation) is the result of the energy required to make and break chemical bonds.

Once you know the enthalpies of formation of a wide range of elements and compounds, you can determine the heat of reaction (enthalpy of reaction, equal to heat that has to be added to convert reactants into products) of any reactions involving these elements and compounds by applying Hess' law. One example of such a reaction is combustion.

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  • $\begingroup$ This is a coincise answer that I could have wrote instead of disperse comments. I think is all OP need, unless he choose to remain stuck in that example. $\endgroup$
    – Alchimista
    Jan 13, 2021 at 14:07

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