enthalpy change?

Question

1) given the following data:
2C (s) + O2 (g) ---> 2CO (g) delta H = - 222 kJ mol -1
2CO (g) + O2 (g) ----> 2CO2(g) delta H = -566 kJ mol -1
what is the standard enthalpy f combustion of carbon?

2)the standard enthalpy changes for the combustion of decane, octane and ethene are - 6778, -5470, -1411 kJ mol -1. use these data to calculate the standard enthalpy change for the cracking of decane :
C10H22(l) ----> C8H18 (l) + C2H4 (g)

Answer 1

This problem can be solved by using the principle of Hess's Law:
http://en.wikipedia.org/wiki/Hess%27s_Law

(1) What is given:
2C(s) + O2(g) ---> 2CO(g), ∆H = -222 kJ/mol-----(1)
2CO(g) + O2(g) ---> 2CO2(g), ∆H = -566 kJ/mol---(2)
(1) + (2) and cancel identical items from both side of equation:
2C(s) + 2O2(g) ---> 2CO(g), ∆H = -788 kJ/mol-----(3)
(3)/2:
C(s) + O2(g) ---> CO(g), ∆H = -394 kJ/mol

(2) Write out the combustion equations:
C10H22 + 15.5O2 ==> 10CO2 + 11H2O, ∆H = -6778 kJ/mol---(1)
C8H18 + 12.5O2 ==> 8CO2 + 9H2O, ∆H = -5470 kJ/mol---(2)
C2H4 + 3O2 ==> 2CO2 + 2H2O, ∆H = -1411 kJ/mol---(3)
(1) - (2) - (3):
C10H22 ==> C8H18 + C2H4, ∆H = 103 kJ/mol.