HW 101(b) enthalpy of combustion

[LaurenDinh_3L]

For 101 part b, why do we need to use the enthalpy of formation for c6h6 when calculating the enthalpy of the rxn if benzene is involved in a combustion rxn? Shouldn't we use the enthalpy of combustion for benzene?

[Niharika Reddy 1D]

I think the problem you're looking at is 105, not 101, but I'm not sure.

Enthalpy of combustion is the change in enthalpy per mole of substance that is burned in a combustion reaction under standard conditions. This is essentially the same as the standard enthalpy of a (combustion) reaction where one mole of substance is burned completely in the presence of oxygen gas.

You can't use the standard enthalpy of combustion of benzene to calculate the standar enthalpy of the reaction since they're both one and the same, as the reaction we are looking at is a combustion reaction. If you used the standard enthalpy of combustion of benzene, your answer for the standard enthalpy of the reaction would be just that, since the reaction is the combustion of one mole of benzene and that's what the standard enthalpy of combustion of benzene is by definition.

To calculate the standard enthalpy of combustion of benzene, which is the same thing as the standard enthalpy of this reaction (the combustion of benzene), you must use the standard enthalpies of formation of each of the products and reactants. The standard enthalpy of the reaction = the sum of the standard enthalpies of formation of all the products - the sum of the standard enthalpies of formation of all the reactants.

If you calculate this, you will notice the standard enthalpy of the reaction is the same as the standard enthalpy of combustion of benzene.