[chrisjohns]

For the following problem:

For the following :
    H2 (g)    ------> 2H  (g)     ?H° = +436kj/mol
    Br2  (g)  ------->  2Br   (g)   ?H° =+ 192.5 kj/mol
    H2  (g) +  Br2 (g) ------->  2HBr (g)  ?H° = - 72.4kj/mol
Calculate ?H°  for the reaction H (g)  +   Br (g)  --------->   HBr (g)

I believe we were told in class that in a multistep reaction we can cancel out the elements from the intermediate reactions. Is that correct?From the appendix in my chemistry book I see that the ?H° for HBr is -36.3kj/mol. Is it as simple as subtracting that from the +436kj/mol in the first reaction?

[Sam (NG)]

Construct a Born Haber cycle reaction, then apply Hess' Law to find it out:
(i've done most of it for you here, you just need to plug the numbers in)

[chrisjohns]

I just want to clarify. I am just plugging in figures into the equation after the second = sign. In doing the problem that way I came up with -482.4kj/mol. Does that seem right?

[xiankai]

i got -350.42kJ/mol using back-of-the-envelope calculations

(-72.4 -192.5 - 436)/2 = -350.45 kJ/mol

your answer has something wrong.