How would you calculate delta H for the following reaction:

Question

2P + 5Cl2 -> 2PCl5

Given:
PCl3 + Cl2 -> PCl5 Delta H=-87.9 kJ
2P + 3Cl2 -> 2PCl3 Delta H=-574 kJ

I have no clue how to do this since I was absent. Can someone help?

Answer 1

Look up Hess' Law.
Multiply equation 1 by 2 and add it to equation 2. Multiply delta H for equation 1 by 2 and add to delta H for equation 2.