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Discussion in 'MCAT Study Question Q&A' started by Gundam00, Sep 22, 2014.

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  1. Gundam00

    Gundam00 2+ Year Member

    Jul 7, 2012
    What is the standard enthalpy of formation of white phosphorus from red phosphorus?

    ΔH-formation of oxide: -2900 kJ mol-1(red), -3020 kJ mol-1(white)

    This is from GS2 test.
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  3. justadream

    justadream 5+ Year Member

    Apr 29, 2011
    Isn't it just products - reactants?
  4. tdod

    tdod 5+ Year Member

    Oct 30, 2011
    ∆H(white) – ∆H(reactants) = -3020 – -2900 = -120

    ∆ is always [final - initial], or in this case [products - reactants]. This question is about Hess's law, which is easy to understand if thought of in terms of height. ∆H(formation) is the absolute height from the ground, but ∆H(rxn) is just the distance between two points.Think of white phosphorous as the top of the empire state building; it is 3020 feet tall; it's formation releases 3020 kJ of heat. Think of red phosphorous as the third-to-last story of the empire state building; it is 2900 feet tall its formation releases 3020 kJ of heat. The distance (∆H) between the two points is 120; when going from red to white, 120 kJ must be released.

    i hope that helps.
    Gundam00 likes this.
  5. Gundam00

    Gundam00 2+ Year Member

    Jul 7, 2012

    Hey thanks thats what i figured. But the answer choices do not list -120 as a possible answer but the explanation does have -120 that is why i was a little confused

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