Answer
$\Delta H = -1300kJ$
Work Step by Step
Using the Hess's law:
$P_4 (s) + 3 O_2(g) --> P_4O_6(s) : \Delta H = -1640.1kJ$
$P_4 (s) + 5 O_2(g) --> P_4O_{10}(s) : \Delta H = -2940.1kJ$
1. Reverse the first equation.
$P_4O_6(s) --> P_4 (s) + 3 O_2(g) : \Delta H = +1640.1kJ$
$P_4 (s) + 5 O_2(g) --> P_4O_{10}(s) : \Delta H = -2940.1kJ$
2. Now, sum the equations:
$P_4 (s) + 5 O_2(g) + P_4O_6(s) --> P_4O_{10}(s) + P_4 (s) + 3 O_2(g) : \Delta H = -1300kJ $
*1640.1 - 2940.1 = -1300
$2 O_2(g) + P_4O_6(s) --> P_4O_{10}(s) : \Delta H = -1300kJ$
