Heat Of Combustion Questions (1 Viewer)

[whoiamiswhoiam]

Why is there two molar heat of combustions? Why can't they give it different names so that it will be less confusing? For example, it says that molar heat of combustion of ethanol is 1367kjmol^-1. In the equation deltaH = -mCdeltaT why is deltaH also named the molar heat of combustion?!!! So some times they give questions that totally confuse me cause they expect you to sub 1367kjmol^-1 into the equation when it shouldn't be allowed! DeltaH = imCdeltaT, should be used to determine the heat absorbed or released by the container of water being burnt and only that! Also, one other thing that makes me want to hit my head with bricks is when converting kjg^-1 into kjmol^-1. You are supposed to times it with 74 right? Why is it in some other question you now have to times the value you get for kjg^-1 with the mass of fuel (ethanol, pentanol whatever!) burnt? Why is it that Chemistry calculation's so hard to interpret at most times? It's like they make it hard to interpret purposely so it's impossible to get 100% or somewhere along those lines. Why is it every time you think you got how to do it, some question that you think you have in the bag spits on your face and says, "haha, you were doing that wrong the whole time! Learn this new way!" Then you learn that way and the cycle of trickery repeats. Argh!!! It's even worse for Physics in the same manner!

Sorry if this was more of a rant. But I'm still one hundred percent serious! Please answer the question. This is why the theory in Chemistry and Physics is so much better! At least it rarely plays head games!

 

[QZP]

Your confusion is not because chemistry is confusing, but because you lack understanding. Let me help you understand.

Enthalpy change (ΔH) has to do with the energy content of products and reactants in a reaction (ΔH = energy of products - energy of reactants). Thus, it follows that this value can be positive or negative depending on whether the reaction was exothermic (products have less energy than reactants and ΔH is negative) or, alternatively, endothermic (ΔH is positive).
You also have the formula ΔH = -mCΔT (I don't know how to derive this). This is consistent with the definition I provided above.

Now,
Heat of combustion (which is not is not represented as the same symbol of enthalpy change but instead ΔHc) is defined as the amount of heat released during combustion. This is given in positive values (it makes no sense to talk about negative heat).

Example: For a given reaction, you can have a negative change in enthalpy (exothermic) and a positive heat of combustion.

Your other question didn't make much sense to me. If you want convert the heat of combustion of a fuel from kj/g to kj/mol then you must do kj/g x (g/mol) = kj/mol. In other words, you multiply by the molar mass of the fuel. The reason we have two different units for heat of combustion is because they are useful in different contexts.

Chemistry/physics is hard because you don't seek the truth.

 

[strawberrye]

Your confusion is not because chemistry is confusing, but because you lack understanding. Let me help you understand.

Enthalpy change (ΔH) has to do with the energy content of products and reactants in a reaction (ΔH = energy of products - energy of reactants). Thus, it follows that this value can be positive or negative depending on whether the reaction was exothermic (products have less energy than reactants and ΔH is negative) or, alternatively, endothermic (ΔH is positive).
You also have the formula ΔH = -mCΔT (I don't know how to derive this). This is consistent with the definition I provided above.

Now,
Heat of combustion (which is not is not represented as the same symbol of enthalpy change but instead ΔHc) is defined as the amount of heat released during combustion. This is given in positive values (it makes no sense to talk about negative heat).

Example: For a given reaction, you can have a negative change in enthalpy (exothermic) and a positive heat of combustion.

Your other question didn't make much sense to me. If you want convert the heat of combustion of a fuel from kj/g to kj/mol then you must do kj/g x (g/mol) = kj/mol. In other words, you multiply by the molar mass of the fuel. The reason we have two different units for heat of combustion is because they are useful in different contexts.

Chemistry/physics is hard because you don't seek the truth.

This is a perfect answer, what textbooks are you working from? Sometimes working from a good textbook makes a big difference, don't despair over calculations, these skills does take some time to develop, I would recommend you persist and it will get easier and you will recognise which formulas to use and when to use it and any shortcuts to calculations gradually over time. Calculations forms an enormous part of science, you will see this much more evidently when you do choose any science courses in university, so they are an intrinsic part and involves application of theory, they are not really separate from theory as much as you may be thinking they are. Best wishes