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Bond Enthalpy Calculations

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    Ok Hess cycles next lesson - Want to consolidate this first though, so the answer will be either exothermic or endothermic. This process should show what exactly... it doesnt show how much energy the reactants have, just the energy required to break their bonds.

    C2H6+3.5O2 = 2CO2+3H2O

    Excuse the equals, ok so this reaction is exothermic, how much so?
    Method,

    Energy in (to break bonds)
    6 * C-H (6*413=2478)
    3 * O=O (3*498) (Theres a 0.5 here, how is that bonded and calculated?)

    Energy out (create bonds)
    4 * C=O (4*805=3220)
    6 * O-H (6*464=2784)

    Questions
    I guess its then Energy out - Energy in for your answer?
    I just want to make sure i know all the main points about this, does it show how much energy a molecule has, or just how much is released/sucked in when it reacts?
    Does this method of calculating enthalpy change have name?
    Answer is always negative for exothermic reaction?

    Any other related theory welcome, AS level. Thanks
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    energy required (endo, +)
    1 x C-C
    6 x C-H
    3.5 x O=O

    energy released (exo, -)
    4 x C=O
    6 x O-H

    sum

    (1 x C-C) + (6 x C-H) - (4 x C=O) - (6 x O-H)

    as straightforward as that.
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    Can someone answer the questions :confused:
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    (Original post by LearningMath)

    Questions
    I guess its then Energy out - Energy in for your answer?
    1. I just want to make sure i know all the main points about this, does it show how much energy a molecule has, or just how much is released/sucked in when it reacts?
    2. Does this method of calculating enthalpy change have name?
    Answer is always negative for exothermic reaction?

    Any other related theory welcome, AS level. Thanks
    1. This calculation shows the energy change when 1 mole of ethane is burned in excess air or oxygen.
    A combustion is always exothermic therefore it's always a negative value.

    2. No specific name. Reaction enthalpy by the bond energy method.

    3. By definition the enthalpy change of an exothermic process is negative.
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    Thank you... Had an idea it was more complicated.... Meh oh well, cheers

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Updated: September 24, 2008
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