Enthalpy of Formation

Discussion in 'DAT Discussions' started by [DDS] Samer, 09.21.14.

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  1. [DDS] Samer

    [DDS] Samer Grind don't stop. 2+ Year Member

    Joined:
    02.23.14
    Messages:
    86
    Status:
    Pre-Dental
    For which of the following reactions is ΔHrxn =ΔHf ?
    a) Na+(aq) + Cl-(aq) → NaCl(s)
    b) C(graphite) + 1/2O2(g) → CO(g) [​IMG]
    c) Na(s) + Cl(g) → NaCl(s)
    d) 1/2O2(g) + H2O(l) →H2O2(l)

    A rxn for which ΔHrxn =ΔHf *must be a formation rxn.* A formation rxn forms 1 mole of a single
    product from individual elements (eliminates choice D as H2O is a
    compound not an element) in their standard states (eliminates choices A and C as
    the standard state of sodium is Na(s), the standard state of chlorine is Cl2(g)).
    Only choice B is left to satisfy the definition of a formation rxn.

    I do not understand why C is wrong.
     
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  3. [DDS] Samer

    [DDS] Samer Grind don't stop. 2+ Year Member

    Joined:
    02.23.14
    Messages:
    86
    Status:
    Pre-Dental
    Nevermind, I figured it out. Cl(g) is not in its standard state. If it were correct it would be written as Cl2(g).
     

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