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Homework Help: How to calculate heat from enthalpy of reaction

  1. Dec 8, 2018 at 4:29 PM #1
    1. The problem statement, all variables and given/known data

    The chemical reaction is given:
    2C2H2 + 5O2 -> 4CO2 + 2 H2O. ΔrH* = -1299 kJ / mol.
    How much heat is released when 50 grams of C2H2 is burned?​
    2. Relevant equations


    3. The attempt at a solution
    First, I convert mass into moles.
    M(C2H2) = 2*12+2 = 26 g/mol

    n(C2H2) = m/M = 50 g / 26 gmol-1
    n(C2H2) = 1,923 mol

    Now I say: if 1299 kJ of heat is released when 2 moles of C2H2 are burned, then for 1.923 moles of the same thing I get 1249.05 kJ of heat (energy). Am I right?

    Please be so king to comment on my answer! Let me know if I am right or wrong? Thank you very much.
    CroSinus​
     
  2. jcsd
  3. Dec 8, 2018 at 4:45 PM #2

    Borek

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    Staff: Mentor

    Logic looks OK, although you are told 50 g which suggests you should not list you answer with 6 significant digits.
     
  4. Dec 8, 2018 at 5:08 PM #3
    Thank you for your swift answer. :-)

    I do not know about digits. Should I write only 1249.1 kJ / mol?

    The confusing thing is the unit: kJ per mol. Per mol of what?
     
  5. Dec 9, 2018 at 2:52 AM #4

    Borek

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    Staff: Mentor

    Actually I have just checked and the enthalpy as listed is per mole of acetylene reacting. IMHO this is ambiguous and should be explained more precisely. Enthalpy can be given "per mole of a reacting substance" or "per mole of a reaction as written" (yes, technically there is no such thing as "mole of reaction", but it is nothing unusual to have it written like that). No wonder we were both wrong.

    The answer should contain 2 significant digits, so (already correcting the mistake we did) 2.5×103 kJ, or just 2.5 MJ. Not that it matters much, but it is in general a mistake to present the number as if it was highly accurate when it is not.
     
  6. Dec 9, 2018 at 1:15 PM #5
    Thank you once again for dealing with my chemistry problems.

    Yes, "per mole of a reaction as written", to my thinking, that is the right interpretation of the unit kJ / mol. If I want to calculate the amount of energy (heat) released in a reaction when ΔrH° is given, should I use the formula: q = ΔrH° × n(X) / stoichiometric coefficient (X)? Where X denotes either a chosen reactant, or a chosen product. I usually come across the formula: q = ΔrH° × n(X), but without the stoichiometric coefficient. So the complete formula should be written as: q = ΔrH° × n(X) / ν (X)?

    Thanks,
    CroSinus
     
  7. Dec 9, 2018 at 4:10 PM #6

    Borek

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    Staff: Mentor

    That's the problem: it is not necessarily right. Giving enthalpy of combustion per mole of acetylene completely burned is perfectly valid and unambiguous, as long as it is clearly stated.
     
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