Energy Released by Hydrocarbon Reactions

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Discussion Overview

The discussion revolves around the energy released by hydrocarbon reactions, specifically comparing methane (CH4) and octane (C8H18) in terms of their combustion energy and environmental impact. Participants explore the effectiveness of converting cooking oil into biodiesel and seek to understand the basic chemistry behind hydrocarbon combustion.

Discussion Character

  • Exploratory
  • Technical explanation
  • Conceptual clarification
  • Debate/contested

Main Points Raised

  • One participant asks for the energy released by the combustion of methane and octane, specifically in joules or calories.
  • Another participant references the heat of combustion for octane, suggesting that 1 mole of C8H18 produces approximately 5.015 MJ.
  • A participant calculates the heat of combustion for methane, stating that 1 kg of CH4 produces 50.009 MJ, while 1 kg of C8H18 produces 44.27 MJ.
  • There is a discussion about the CO2 produced per kilogram of fuel, with methane producing 2.75 kg CO2 and octane producing 3.09 kg CO2.
  • Some participants propose that methane is considered cleaner than gasoline due to its higher energy release per kilogram of CO2 produced.
  • One participant suggests that the assertion of methane being cleaner is based on the energy produced per kilogram of CO2 released, specifically citing values of 18.185 MJ for methane and 14.388 MJ for octane.

Areas of Agreement / Disagreement

Participants express differing views on the reasons why methane is considered cleaner than octane, with some supporting the idea based on energy efficiency relative to CO2 emissions. However, there is no consensus on the definitive reasons or implications of these comparisons.

Contextual Notes

The discussion includes various assumptions about combustion conditions and the specific calculations of energy release and CO2 production, which may depend on definitions and methods used. Some calculations appear to be based on approximations or specific conditions that are not fully detailed.

Who May Find This Useful

This discussion may be useful for individuals interested in the environmental impact of fuels, the chemistry of hydrocarbon combustion, and the potential benefits of biodiesel as an alternative energy source.

Stephanus
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Dear PF Forum,
First of all, I'd like to thanks PF Forum for giving me invaluable answers all this time.
Now, I have a "situation" here.
My church wants to conserve environment by changing cooking oil into biodiesel from its parishioners
I'd like to know how effective this effort is.
I have many, many questions regarding biodiesel, car gasoline, hydrocarbon, etc...
But in this post, I'd like to know these simple facts first.
1. How much energy is released by reactiing 2 moles CH4 + 5 moles O2 -> 2 moles CO2 + 8 moles H2O. How many joules (or calories equivalent) are released in this process?
2. How many joules/calories are released by this reaction? 2C8H18 + 25O2 -> 16CO2 + 18H2O. All are in moles unit and given the ideal condition where carbon monoxyde are not produced.
3. Is it true that CH4 is considered "cleaner" than C8H18?
4. If that so, why?
Is it because for ...
4A. 1824 gr CH4 produces 5016 gr CO2 and 1824 gr C8H18 produces 5632 gr CO2?
4B. Given that I don't know the answers of question 1 and 2.
That for 1824 gr CH4 produces more (is it true more?) energies than 1824gr C8H18?
4C. Still that I don't know the answers of question 1 and 2:
That for every kg CO2 produced, CH4 releases more (is it true, more?) energies than C8H18?
These questions might not have anything to do with biodiesel. But I'd like to study some basic chemistry first.
Thanks for any help.
 
Chemistry news on Phys.org
Bystander said:
https://en.wikipedia.org/wiki/Heat_of_combustion
Wow, that was fast! Thanks Bystander.
It's not I haven't googled. I have visited many links, but none gave specific answer like the link you give me.
Let me see if I understand this reaction correctly.
As stated in Wiki, the heat combustion of 1 Kg octane is 44 MJ. So 1 mole C8H18 + n O2 -> ... will produce 114 gr * 44 MJ = 5.015 MJ?
 
Stephanus said:
(114 gr /1kg)* 44 MJ = 5.015 MJ?
..., given that your calculator is working properly.
 
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Bystander said:
..., given that your calculator is working properly.
And it's about 5.06478 MJ per moles. I forgot to include 44.427 MJ/Kg. I type 44 MJ/Kg. Okay, thanks I'm convinced that I'm on the right track.
So about CH4 issue.
The heat combustion for
1 kg CH4 is 50.009 MJ
1 kg C8H18 is 44.27 MJ.
Consequently
1 kg methane will produce 2.75 kg CO2
1 kg octane will produce 3.09 kg CO2

Why is methane considered cleaner than gasoline which has higher octane ratio in it?
Is it because
A. 1 kg CH4 produces higher energy than 1 kg octane?
B. 1 kg methane produces lower CO2 than 1 kg octane?
C. Methane produces 18.185 MJ per 1 kg CO2 released, while Octane produces 14.388 MJ per 1 kg CO2 released? After doing the calculation.
I think the answer is C if we focus on lower pollution.
Is my opinion correct?
 
Stephanus said:
I think the answer is C
That's the basis for the assertion.
 
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Bystander said:
That's the basis for the assertion.
Thanks. Now, I can focus on biodiesel after some online researches.
 

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