Enthalpy change.. help.

why do we ignore some compounds while using hess cycle such as the formation of Cl2.

for example in a cycle where Oxygen is one of the reactants we ignore the enthalpy change of that since we can't find the value of oxygen being burnt in oxygen(doesn't make sense so we don't have a value for the combustion of oxygen.

but formation of $Cl_2$ should have an enthalpy value...

mind posting the question so we could have a look?

The definition of enthalpy of formation is: 'The energy change for a reaction where the constituent elements reacts to form 1 mole of a compound'. Cl2 is an element and not a compound, there are no elements which combine to form Cl2.

haha actually I'm not sure if i'm correct.

The definition of enthalpy of formation is: 'The energy change for a reaction where the constituent elements reacts to form 1 mole of a compound, all species under standard conditions'. Cl2 is an element and not a compound, there are no elements which combine to form Cl2.

can you mention the cases in which these are ignored. ok elements like Cl2 are ignored in E.formation and E. reaction. and i guess enthalpy for the compound CO or CO2 are also ignored when we have to calculate E.reaction and e.combustion are given. ?? O2 is ignored in e. combustion. am i right??
and yes the equation as something like this " CH4 + Cl2 --> CH3Cl + HCl " and E.change for formations given for all but Cl2 and we had to calculate the E.reaction

Yes, O2 doesn't have an enthalpy of combustion, you can't burn O2 in O2. Enthalpy of combustion is the energy change for a reaction in which one mole of a compound is completely combusted in plentiful oxygen, in species in their standard states, under standard conditions.