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Homework Help: Energy Change Formation

  1. Nov 4, 2007 #1
    Calculate the energy change for the formation of CaF2 (s) from its elements in their standard states and the following information:

    Ca (s) + F2 (g) -> CaF2 ?
    Ca (s) -> Ca (g) = 179.3 kJ/mol
    Ca (g) -> Ca+ (ion) (g) + e- = 589.9 kJ/mol
    Ca+ (ion) (g) -> Ca2+ (ion) (g) +e- = 1145 kJ/mol
    1/2 F2 (g) -> F(g) = 79.0 kJ/mol
    F(g) + e- -> F- (ion) (g) = -328.0 kJ/mol
    CaF2 (s) -> Ca2+ (ion) (g) + 2 F- (ion) (g) = 2630 kJ/mol

    Basically, what I did was add each one up in the order listed & I got 4290 kJ/mol. However, this doesn't seem to make any sense, because I want a negative kJ/mol, or at least according to the problems in the book (my first time dealing with these problems). Any ideas what is going on?
     
  2. jcsd
  3. Nov 4, 2007 #2

    Astronuc

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    What form of an energy equation is one using? Is one getting the correct magnitude for the energy change?
     
    Last edited: Nov 4, 2007
  4. Nov 4, 2007 #3
    Think about your target equation: Ca(s) + F2(g) --> CaF2

    If you add up all the equations, they will not give you the above.

    1/2 F2 (g) -> F(g) = 79.0 kJ/mol
    F(g) + e- -> F- (ion) (g) = -328.0 kJ/mol
    CaF2 (s) -> Ca2+ (ion) (g) + 2 F- (ion) (g) = 2630 kJ/mol

    you need to manipulate these 3 equations so that you get F2 (not 1/2F2)
    and also that CaF2 is on the right hand side (what happens to the 2630 if you reverse the equation?) ..........neg kJ/mol you are looking for
     
  5. Nov 4, 2007 #4
    :) yes, I just came on to check the thread. What switched the last equation around to get -2630 kJ/mol, then I was thinking if I multiplied the first equation by 2, I'd get F2 (g) -> 2 F (g) = 158 kJ/mol. Then I'd have to multiply the next equation by 2 to get 2 F(g) + 2e- -> 2F- (ion) (g) = -656 kJ/mol. The add them all up to get -1210 kJ/mol. Or so I think ^_^.
     
  6. Nov 4, 2007 #5
    good thinking :cool:
     
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