Use the thermochemical equations:

Question

1/2 H2(g)+1/2 I2(g) ...............> HI(g) delta H = -1.1 k cal
1/2 H2(g).......> H2 (g) delta H = -104.2 k cal
1/2 I2 (g)..............> I (g) delta H = +18.1 k cal

To present for the formation of hydrogen iodide gas from gaseous atomic hydrogen and gaseous atomic iodine:
H(g) +I(g) HI(g) delta H=?

Answer 1

Use Hess' Law.
Use equation 1 as is.
Add to the reverse of equation 2 (change the sign of delta H here) and add to the reverse of equation 3 (change the sign of delta H for 3 also).
You should get H + I ==> HI, then add all of the delta Hs together to arrive at a delta H for the reaction.