Say I burned for example methane and propane to boil some water.

Methane has the heat of combustion of $\pu{890kJ/mol}$ and propane has the heat of combustion of $\pu{2200kJ/mol}$

What does the difference in heat of combustion tell us? In other words, propane's heat of combustion is bigger. Does that mean that methane emits less energy and therefore is less "environmentally harmful"? Is a higher heat of combustion better or a lower heat of combustion?

  • 1
    $\begingroup$ Why would releasing less energy make it less environmentally harmful? That just means you need to burn more of it. $\endgroup$
    – user253751
    Jul 3 '20 at 17:58

Note that the mentioned values are molar heats of combustion. Propane is a larger molecule than methane and it is therefore obvious that more energy per mole is released.

If we replace the molar amount by molar mass, the resulting specific heat of combustion has the opposite relation:

$$\begin{align}\text{Methane:}&&\frac{\pu{890kJ/mol}}{\pu{16g/mol}} &= \pu{55.6kJ/g}\\[0.5em] \text{Propane:}&&\frac{\pu{2200kJ/mol}}{\pu{44g/mol}}&=\pu{50kJ/g}\end{align}$$

For the enviromental aspect of $\ce{CO2}$ emissions, the relevant value is the heat of combustion per mole of carbon atoms. It is $\pu{890 kJ/mol}$ for methane, but $\pu{2200 kJ/mol}$ for propane, which is roughly $\pu{730 kJ/mol}$ for carbon. For methane, both numbers are the same, as a propane molecule contains 3 carbon atoms, while a methane molecule just 1.

So methane has a an advantage as energy source, as it has higher energy production per the same $\ce{CO2}$ emission.

PLA is partially already oxidized, therefore it has significantly lower energy content per 1 kg of produced $\ce{CO2}$, compared to PP.

PLA has an energy content of $\pu{16.3MJ/kg}$
PLA has structure [-O-CH(CH3)-CO-]n, with basic unit containing 3 C + 4 H + 2 O
$\pu{1 kg}$ of PLA contains 36/72 kg of carbon
PLA has an energy content of $\pu{16.3 * 72 / 36 MJ/kg} \simeq \pu{32.6 MJ/kg of C}$ .

PP has an energy content of $\pu{46.4MJ/kg}$
PP has structure [-CH2-CH(CH3)- ]n, with basic unit containing 3 C + 6 H
$\pu{1 kg}$ of PP contains 36/42 kg of carbon
PP has an energy content of $\pu{46.4 * 42 / 36 MJ/kg} \simeq \pu{54.1 MJ/kg of C}$ .

To get the same energy by the burning of the stuff, PP releases $54.1/32.6 \simeq 1.66$ times less $\ce{CO2}$ than PLA. Or, with the same emission of $\ce{CO2}$, PP releases 1.66 times more energy than PLA.

  • $\begingroup$ Thanks for the answer, when you say "methane has slight advantage here" what do you mean? Why is higher energy content the "environmentally better"? $\endgroup$
    – Sarah V.P
    Jul 4 '20 at 3:28
  • $\begingroup$ Lets say I am burning 2 types of plastics, PP (Polypropylene) and PLA (Polylactic acid). After some research I found out that PLA has an energy content of 16.3 MJ/kg and PP has an energy content of 46.4 MJ/kg. Which one is more favourable in terms of "environment"? $\endgroup$
    – Sarah V.P
    Jul 4 '20 at 3:34
  • $\begingroup$ Also where did you get 730kj/mol from? engineeringtoolbox.com/… - from this website it says 2200 $kj/mol$ $\endgroup$
    – Sarah V.P
    Jul 4 '20 at 3:35
  • $\begingroup$ See the answer update. $\endgroup$
    – Poutnik
    Jul 4 '20 at 5:33
  • $\begingroup$ The values that I gave is 16.3 Mj of heat produced for combusting per 1 kg of polymer. So greater energy content = more superior energy source, but now I am talking about energy content in plastic. So they would not use as a energy source right? This pdf file says something about energy recovery potential plasticseurope.org/application/files/7415/1747/5136/…. type ctrl+F and search "energy recovery potential" $\endgroup$
    – Sarah V.P
    Jul 4 '20 at 7:51

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