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Homework Help: Standard Gibbs of a reaction

  1. Nov 19, 2011 #1
    1. The problem statement, all variables and given/known data

    For the reaction N2(g) + 3H2(g) <---> 2NH3(g)

    I am supposed to determine the equilibrium constant at 298K and 1 bar. We have been given a table that states that

    for NH3(g):
    [itex]\Delta_{r}G^{\Theta}= -16.45 kJ/mol[/itex]

    I know that
    [itex]\Delta_{r}G^{\Theta}= \Sigma v G^{\Theta}_{product} - \Sigma v G^{\Theta}_{reactant}[/itex]

    where v is the stoichiometric coefficient.

    But what is confusing me is that you could also write the equation as

    1/2 N2(g) + (3/2)(g) <----> NH3(g)

    so in the first case, the answer would be

    (2*-16.45 kJ/mol) - ((1*0)+(3*0)) = -32.90 kJ/mol

    But in the second case, the answer would be

    (-16.45 kJ/mol) - (1/2*0 + 3/2 *0) = -16.45 kJ/mol

    Since both equations should be valid, what answer is correct? I don't understand which one I should choose because they both look like the should be right.
    1. The problem statement, all variables and given/known data

    2. Relevant equations

    3. The attempt at a solution
  2. jcsd
  3. Nov 20, 2011 #2


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    In the two cases, the expression for the equilibrium constant will be different. In the first case K = [NH3]^2/[N2][H2]^3 whereas in the second case K = [NH3]/[N2]^(1/2)[H2]^(3/2). This is why you always need to include a balanced reaction along with any equilibrium constant you provide.
  4. Nov 20, 2011 #3
    Hey, I had the same concern. They are both right; it depends on the stoichiometry. The professor would have to specify which one he is looking for like he did last time.
  5. Nov 20, 2011 #4
    Thanks guys! Did he specify last time? I must not have understood him. This was really confusing me, good to know that both are right! I feel better now. :)
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