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Does methanol or propanol release more heat?

  1. Oct 11, 2006 #1
    during combustion which one releases more heat and what is the rationale behind it?
    is there a website with all the bond energies between each element combination?
    Last edited: Oct 11, 2006
  2. jcsd
  3. Oct 11, 2006 #2
    Have a gen chem book handy? look in the back for the thermodynamic data on methanol, propanol, CO2, and water.

    calculate the change in enthalpy of the reaction:


    for CxH2x+1OH + O2-------> CO2 +H20
  4. Oct 11, 2006 #3
    the back of my text book has methanol at -239.1 (delta H degrees, subscript f kJ/mol) and 126.8 S degrees J/mol.K
    propanol is not listed and CO2 is -393.5 (delta H degrees, subscript f kJ/mol) and 213.78 S degrees J/mol.K
    my assignment asks me which one releases more energy on combustion. I didnt realize i had to subtract the energy consumed first
    i just found another chart in my text stating the molar enthalpies of combustion for methanol as -727 kJ/mol and propanol as -2020 kJ/mol. If this is an exothermic reaction, why are these figures both negative? Is that saying methanol releases more energy than? how are these figures calculated?
    thx for your help. it seemed like a simple question to begin with but it's really making my head spin
  5. Oct 12, 2006 #4
    Exothermic reactions will always have a negative value for the change in enthalpy of a reaction and endothermic reactions will always have a positive value. As for why, I'm not going to explain it, it is a whole course on PChem. Since propanol has a "larger" negative value it releases more heat than methanol. How is this calculated? By the equation I was talking about before.

    Balance the equations for combustion of methanol and propanol.

    CH3OH + (3/2)O2 ------> CO2+2H20

    CH3CH2CH2OH +(9/2)O2------> 3 CO2+4 H20

    Calculate the change in enthalpy for the reactions:

    Hcombustion methanol= (HfCO2 + 2HfH20)-(Hfmethanol)
    Hcombustion propanol=(3HfCO2+4HfH20)-(Hfpropanol)

    Alternatively you could simply look at the balanced equations and see that combustion of propanol should release more energy. For the combustion of methanol you have 5 molecules combusting into 6 molecules of stuff, which is in increase of only 1 molecule. In the 2nd reaction you have 11 molecules on the left combusting into 14 molecules of stuff, which is in increase of 3 molecules. You can clearly see then that the increase of entropy for the combustion of propanol is greater than the increase of entropy for the combustion of methanol, thus more energy is released when propanol combusts.
  6. Oct 13, 2006 #5
    thanks grave. thats a big help. i think it was the negative thing that was screwing me up.
    is your name a pun on shakespeare's tempest phrase: brave new world?
    why so grave? a bright optomistic guy like u?
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