How do I calculate enthalpy of combustion?

How do i calculate the enthalpy of formation given the value of enthalpy of combustion?

• example calculate the enthalpy of formation for ethene given its enthalpy of combustion is -1411kjmol^-1 enthalpy of combustion of carbon is -394, and hydrogen is -286kjmol^-1

• Answer:

  ok, i get 51Kjmol^-1, i may be wrong ofc, but im pretty sure. ok, from C2H4 (ethene), to 2CO2 and 2H2O releases 1411 kJ of energy, therefore, 1411 KJ must be supplied to reverse this process. Now, the -394 and -286 kjmol^-1 enthalpies of combustion for carbon and hydrogen respectively will need to be doubled, as 2 moles of both CO2 and H2O are formed when ethene combusts. This adds to -1360KJ. therefore, from the carbon and hydrogen, to the carbon dioxide and water, to the ethene, it takes -1360 + 1411 KJ. Which results in the 51KJmol^-1 for ethene. Hope i've helped :) And hope i got it right :P but taht sounds about right :)