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enthalpy change

Discussion in 'DAT Discussions' started by GRAD, Feb 20, 2007.

  1. GRAD

    GRAD Member
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    Calculate the enthalpy change for following reaction:C(s) + 2H2(g)--> CH4(g)

    Bond diss energy: H-H is 436 and for C-H is 415

    Hf of C (g) is 715KJ/mol
     
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  2. Nasem

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    well... this is off the top of my heat, but I think this is how you do it:

    The C(s) is in its standard state so its formation is zero.

    Enthalpy change = sum[bonds broken] - sum[bonds formed]

    [0 + 2(436)] - [4(415)] = 872 - 1660
    = -788

    *someone double check my work plz, I might be wrong but I think Im on the right track*
     
  3. iistoogood4u

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    [715 + 2(436)] - [4(415)] = 715+872 - 1660
    Because C(s) -> C(g)
    and the Hf is 715
    thus the H rxn would be -73kj/mol
     
  4. Lonely Sol

    Lonely Sol cowgoesmoo fan!
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    I dont understand why Hf of C(s) should be added. anything in a solid stage doesnt aid in final hf.
     
  5. iistoogood4u

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    yes but going from solid to gas reqires the Hf to be added. Same concept as when u compute the total Q reqired to take an ice cube at -10 C to 110 C. You compute the energy needed to bring ice from -10 C to 0, then the H at 0 (the plateau), then 0 to 100, then the H at 100, then 100 to 110. The solid state of C is 0, but the H of formation of C gas reqires energy, a change of state.
     
  6. Lonely Sol

    Lonely Sol cowgoesmoo fan!
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    Thanks alot for the explanation
     

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